The cause of the Lusi mud eruption remains controversial. The review by Miller and Mazzini (2017)... more The cause of the Lusi mud eruption remains controversial. The review by Miller and Mazzini (2017) firmly dismisses a role of drilling operations at the adjacent Banjarpanji-1 well and argues that the eruption was triggered by the M6.3 Yogyakarta earthquake some 254 km away. We disagree with these conclusions. We review drilling data and the daily drilling reports, which clearly confirm that the wellbore was not intact and that there was a subsurface blowout. Downhole pressure data from Lusi directly witness the birth of Lusi at the surface on the 29th of May 2006, indicating a direct connection between the well and the eruption. Furthermore, the daily drilling reports specifically state that Lusi activity was visibly altered on three separate occasions by attempts to kill the eruption by pumping dense fluid down the BJP-1 well, providing further evidence of a connection between the wellbore and Lusi. By comparison with other examples of newly initiated mud eruptions elsewhere by other earthquakes, the Yogyakarta earthquake was far away given its magnitude. The seismic energy density of the Yogyakarta earthquake was only 0.0043 J/m 3 , which is less than a quarter of the minimum 0.019 J/m 3 seismic energy density that has ever been inferred to trigger other mud eruptions. We show that the Lusi area had previously experienced other shallow earthquakes with similar frequencies and stronger ground shaking that did not trigger an eruption. Finally, the data from the BJP-1 well indicates that there was no prior hydrodynamic connection between deep overpressured hydrothermal fluids and the shallow Kalibeng clays, and that there was no evidence of any liquefaction or remobilization of the Kalibeng clays induced by the earthquake. We thus strongly favor initiation by drilling and not an earthquake.
The unusual present-day crustal stress pattern of the Australian continent has been the subject o... more The unusual present-day crustal stress pattern of the Australian continent has been the subject of scientific debate for over 25 years. The orientation of maximum horizontal present-day stress (S Hmax) in continental Australia is unlike all other major tectonic plates in that the stress pattern shows regional variability and it is not oriented sub-parallel to the direction of absolute plate motion. Previous studies on the stress pattern of Australia revealed that the complex stress pattern of the continent is controlled, at a first-order, by the superposition of plate tectonic forces exerted at the plate boundaries. However, prior analysis of the contemporary Australian crustal stress pattern have been unable to model or explain the stress pattern observed in most of eastern Australia, and has not extensively addressed the numerous smaller scale variations in stress orientation. The recent development of unconventional reservoirs in Australia has resulted in a greatly increased amount of new data for stress analysis in previously unstudied or poorlyconstrained areas in eastern Australia. In addition, stress analysis in conventional hydrocarbon, mineral and geothermal exploration in all other parts of the continent provides the opportunity to review and update the Australian Stress Map (ASM). This study presents the new release of the ASM, with a total of 2150 stress data records in Australia (increased from 594 data records in 2003). The 2016 ASM contains 1359 data records determined from the interpretation of drillingrelated stress indicators, 650 from earthquake focal mechanism solutions, 139 from shallow engineering measurements and two from geological indicators. The results reveal four distinct regional trends for the S Hmax orientation in Australia including a NNE-SSW S Hmax trend in northern and northwestern Australia, which rotates to a prevailing E-W orientation in most Western and South Australia. The orientation of S Hmax in eastern Australia is primarily ENE-WSW and swings to NW-SE in southeastern Australia. A comparison between the new ASM database and neotectonic features further confirms the role of contemporary stress in recent deformation of the Australian crust. The 2016 ASM reveals significant discrepancies between newly observed S Hmax orientations and predictions by published geomechanical-numerical models for the Australian continent. Forces generated at the boundary of Indo-Australian Plate remain the major control on the regional crustal stress pattern in continental Australia, however, the increased data density, particularly in eastern Australia, reveals numerous local perturbations of the stress field that were not previously clearly captured. Hence, the key findings of the new release are that local (intraplate) stress sources are more significant than previously recognized, particularly in eastern Australian basins, and cause substantial local scale rotations of the present-day crustal stress pattern that have not been factored into existing Australian stress models.
The future success of both enhanced (engineered) geothermal systems and shale gas production is r... more The future success of both enhanced (engineered) geothermal systems and shale gas production is reliant on the development of reservoir stimulation strategies that suit the local geo-mechanical conditions of the prospects. The orientation and nature of the in-situ stress field and pre-existing natural fracture networks in the reservoir are among the critical parameters that will control the quality of the stimulation program. This study provides a detailed investigation into the nature and origin of natural fractures in the area covered by the Moomba–Big Lake 3D seismic survey, in the southwest termination of the Nappamerri Trough of the Cooper Basin. These fractures are imaged by both borehole image logs and complex multi-traces seismic attributes (e.g. dip-steered most positive curvature and dip-steered similarity), are pervasive throughout the cube, and exhibit a relatively consistent northwest–southeast orientation. Horizon extraction of the seismic attributes reveal a strong va...
The world stress map-a freely accessible tool for geohazard assessment, Recent Geodynamics Georis... more The world stress map-a freely accessible tool for geohazard assessment, Recent Geodynamics Georisk and
The spatio-temporal changes of the stress state in a geothermal reservoir are of key importance f... more The spatio-temporal changes of the stress state in a geothermal reservoir are of key importance for the understanding of induced seismicity and planning of injection and depletion strategies. In particular the poro-elastic effects on the stress state due to re-injection or depletion of water are of interest for both geothermal projects and hydrocarbon exploitation. In addition to the conventionally used effective stress concept, poro-elasticity affects the stress tensor components differently as a function of changes in pore pressure. Here, we provide an analytical base for the long-term changes of the 3D stress tensor components as a function of pore pressure changes. Results indicate that for a constant rate of injection or depletion the coupling between pore pressure and all stress tensor components depends on the location in the reservoir with respect to the reinjection/depletion point as well as the time since the beginning of pore pressure changes. Our systematic analysis suggests that poro-elastic stress changes can even locally modify the given tectonic stress regime. Furthermore, the results predict that localized changes of maximum shear stress can lead to different fracture orientations than those expected when poro-elastic effects are not considered. These results indicate a need for 3D geomechanical-numerical studies of more realistic reservoir settings in order to study the 3D effects of pore pressure/stress coupling. Our generic 3D geomechanical-numerical study shows that less than two years of production of a single well changes shear stresses by 0.2 MPa. Thus, in reservoirs with decades of production shear stress change can reach sufficiently high values to reactivate pre-existing faults or even generate new fractures with unexpected orientations.
The future success of both enhanced (engineered) geothermal systems and shale gas production reli... more The future success of both enhanced (engineered) geothermal systems and shale gas production relies significantly on the development of reservoir stimulation strategies that suit the local stress and mechanical conditions of the prospects. The orientation and nature of the in-situ stress field and pre-existing natural fracture networks in the reservoir are amongst the critical parameters controlling the success of any stimulation program. This work follows an initial study showing the existence of natural fractures in the area covered by the Moomba-Big Lake 3D seismic survey, in the SouthWestern termination of the Nappamerri Trough of the Cooper Basin in South Australia. The fractures, imaged both by borehole image logs and seismic attributes (including Most Positive Curvature, Ant tracking of Dip Deviation, and Variance), are pervasive across the seismic survey, and present a relatively constant NW-SE orientation. The density of the fractures, as visible on horizon extractions of attributes, is however spatially variable. A high density of fractures is found in the vicinity of the fault planes and tight antiforms. We compare apparent fractures from different seismic attributes (seismic fractures) with faults interpreted from well data and on vertical seismic sections. Results indicate that some seismic fractures are small faults with small offsets up to 3 ms. Other seismic fractures are actual fractures striking parallel to nearby faults. Analysis shows that under present day stress orientation and magnitudes, fractures striking NW-SE and NE-SW are more susceptible to stimulation, and are more likely to open for fluid flow.
Abu Dhabi International Petroleum Conference and Exhibition, 2012
ABSTRACT Pore pressure is a key parameter in controlling the well in terms of reservoir fluid pre... more ABSTRACT Pore pressure is a key parameter in controlling the well in terms of reservoir fluid pressure. An accurate estimation of pore pressure yields to better mud weight proposition and pressure balance in the bore hole. Current well known methods of pore pressure prediction are mainly based on the differences between the recorded amount and normal trend in sonic wave velocity, formation resistivity factor (FRF), or d-exponent (a function of drilling parameters) in overpressured zone. The majority of the techniques are based on the compaction of specific formation type which need localization or calibration. They occasionally fail to proper response in carbonate reservoirs. In this research, a new method for calculating the pore pressure has been obtained using the compressibility attribute of reservoir rock. In the case of overpressure generation by undercompaction (which is the case in most of the reservoirs), pore pressure is depended on the changes in pore space which is a function of rock and pore compressibility. In a simple way, pore space decreases while the formation undergoes compaction and this imposes pressure on the fluid which fills the pores. Generally, rock compressibility has minor changes over a specific formation, but even this small amount must be considered. Thus, the statistical tools should be used to distribute the compressibility over the formation. Therefore, based on the bulk and pore compressibility achieved from the special core analysis (SCAL) or well logs in one well, the pore pressure in the other locations of a formation could be predicted. Introduction Rock mass as well as the pore space is always under the several stresses and the current underground state of estate is what has been defined by Terzaghi[1] indicating that a body of soil is in a state of plastic equilibrium if every part of it is on the verge of failure. This means that the moment of rotation with respect to all dimensions must set to zero. That implies with the fact that the rock stays under the condition with the most stable situation of the stress state. These are the facts about the current state of the rock. However, until this stage of stability, mineral maturation and rock type deformation are results of burying during the deposition. Thus, bulk and pores of the rock under the overburdern stress will tend towards the deformation until the rock reaches the most stable condition. In other words, Overburden pressure which is mostly the main stress tensor in the depth of oil and gas reservoirs, leads to the compaction of the formation. Consequently, if the rock is surrounded by any type of impermeable layers, which means no way of escaping for pore fluid, a portion of the stress will be imposed to the fluid and the pore pressure will rise. This process can be modelled in in terms of stress-strain analysis whereas this research has focoused on a phenomenon which is known as the compressibility mechanism. The amount of excess stress which is exerted on the pore fluid is called "pore pressure??.
Geomechanics is the discipline dealing with the prediction and management of rock deformation and... more Geomechanics is the discipline dealing with the prediction and management of rock deformation and failure. Geomechanical problems resulting from the change of stress in the rock induced by drilling and production make many hydrocarbon production projects challenging. Examples of geomechanical problems include wellbore stability and fracturing of the formation during drilling that may lead to financial loss due to losses, kicks, stuck pipe, extra casing strings, and sidetracks. Other problems are due to reservoir stress changes occurring during production such as reservoir compaction, surface subsidence, formation fracturing, casing deformation and failure, sanding, reactivation of faults, and bedding parallel slip. The impact of geomechanical problems on field development is best examined early in the life of the field, when there is an opportunity for proactive planning. The interaction between complex geology, well trajectories, and stress changes due to drilling or production is a complex 3D problem, and is best solved using a 3D approach. A mechanical Earth model (MEM) consists of a 3D description of pore pressure, stress, and mechanical properties linked to a 3D framework model consisting of surfaces such as formation tops and faults, and allows drilling risks and geomechanical problems to be identified and contingency plans for handling those risks to be developed. Formation strength and in-situ stress are the key components of the MEM critical to well engineering design. Technical approaches to geomechanics can be roughly classified as empirical, analytical, or numerical. The empirical approach is based on application of best practices through company or industry standards, without much consideration for the underlying mechanisms at work. This approach can be very useful, though, and is often applied. Analytical geomechanical models aim to describe the deformation and stress change with mathematical equations (constitutive relations) and represent the subsurface with simplified geometries, depletion patterns, and mechanical properties. Examples of analytical geomechanics include depletion of disk-shaped or spheroid-shaped reservoirs in an infinite linear elastic medium. For many problems, analytical models provide a good first-order approximation. With the recent advances in computational power and memory capability, numerical methods have become more popular: They allow inclusion of the natural (geologic) variation in geologic structure, depletion, and mechanical property variation at scales of tens to hundreds of meters. However, any geomechanical model needs calibration to in-situ data before it can be used as a predictive tool. If calibration is not done, or not done properly, geomechanical model predictions will be imprecise and inaccurate. An important component of the workflow is the estimation of geomechanical properties from well-log data using suitably calibrated transforms. Static and dynamic moduli differ due to differences in the time scale over which the deformation is applied (frequency dependence or dispersion) and due to differences in strain magnitude. The main contribution to dispersion arises from the presence of fluids in the pore space, and depends on whether the time required for pore-pressure equalization is long or short compared to the period of the elastic wave. The dynamic moduli therefore depend on whether seismic, sonic, or ultrasonic frequencies are used. In the low-frequency limit, the pore fluid can be considered to be in pore-pressure equilibrium, and Gassmann's equations can be used to determine the
The World Stress Map Project compiles a global database of contemporary tectonic stress informati... more The World Stress Map Project compiles a global database of contemporary tectonic stress information of the Earth's crust. Early releases of the World Stress Map Project demonstrated the existence of first-order (plate-scale) stress fields controlled by plate boundary forces and second-order (regional) stress fields controlled by major intraplate stress sources such as mountain belts and zones of widespread glacial rebound. The 2005 release of the World Stress Map Project database provides, for some areas, high data density that enables us to investigate third-order (local) stress field variations, and the forces controlling them such as active faults, local inclusions, detachment horizons, and density contrasts. These forces act as major controls on the stress field orientations when the magnitudes of the horizontal stresses are close to isotropic. We present and discuss examples for Venezuela, Australia, Romania, Brunei, western Europe, and southern Italy where a substantial increase of data records demonstrates some of the additional factors controlling regional and local stress patterns.
... Oliver Heidbach, Friedemann Wenzel and Phillip Fleckenstein. ... The WSM has provided key ins... more ... Oliver Heidbach, Friedemann Wenzel and Phillip Fleckenstein. ... The WSM has provided key insights into the state of plate-scale and regional stress fields in the earth's crust and revealed that these are primarily controlled by forces exerted at plate boundaries, in particular mid ...
The World Stress Map (WSM) project is a global compilation of information on the contemporary cru... more The World Stress Map (WSM) project is a global compilation of information on the contemporary crustal stress field from a wide range of stress indicators. The WSM database release 2008 contains 21,750 stress data records that are quality-ranked using an updated and refined quality-ranking scheme. Almost 17,000 of these data records have A-C quality and are considered to record the orientation of maximum horizontal compressional stress SH to within ±25°. As this is almost a triplication of data records compared with the first WSM database release in 1992, we reinvestigate the spatial wavelength of the stress patterns with a statistical analysis on a global 0.5° grid. The resulting smoothed global stress map displays both; the mean SH orientation that follows from the maximum smoothing radius for which the standard deviation is b25° and a countour map that displays the wavelength of the stress pattern. This smoothed global map confirms that long wavelength stress patterns (N2000 km) exist for example in North America and NE Asia. These have been used in earlier analyses to conclude that the global stress pattern is primarily controlled by plate boundary forces that are transmitted into the intraplate region. However, our analysis reveals that rather short wavelength of the stress pattern b200 km are quite frequent too, particularly in western Europe, Alaska and the Aleutians, the southern Rocky Mountains, Basin and Range province, Scandinavia, Caucasus, most of the Himalayas and Indonesia. This implies that local stress sources such as density contrasts and active fault systems in some areas have high impact in comparison to plate boundary forces and control the regional stress pattern.
The present-day state of stress in Western Europe is considered to be controlled by forces acting... more The present-day state of stress in Western Europe is considered to be controlled by forces acting at the plate boundaries. It is assumed that the Alpine orogen only influence the regional pattern of present-day stress in Western Europe within the Alps themselves. We examine the present-day maximum horizontal stress orientation in the Molasse Basin in the Alpine foreland in order to investigate the possible influence of the Alps on the far-field stress pattern of Western Europe. Four-arm caliper and image logs were analysed in 137 wells, in which a total of 1348 borehole breakouts and 59 drilling-induced fractures were observed in 98 wells in the German Molasse Basin. The borehole breakouts and drilling-induced fractures reveal that stress orientations are highly consistent within the Molasse Basin and that the present-day maximum horizontal stress orientation rotates from N-S in southeast Germany (002°N±19°) to approximately NNW-SSE in Tectonics southwest Germany and the Swiss Molasse Basin (150°N±24°). The present-day maximum horizontal stress orientation in the Molasse Basin is broadly perpendicular to the strike of the Alpine front, indicating that the stress pattern is probably controlled by gravitational potential energy of Alpine topography rather than by plate boundary forces. The present-day maximum horizontal stress orientations determined herein have important implications for the production of hydrocarbons and geothermal energy in the German Molasse Basin, in particular that hydraulically-induced fractures are likely to propagate N-S and that wells deviated to the north or south may have reduced wellbore instability problems.
The vertical or lithostatic stress is an important factor in tectonic and geomechanical studies a... more The vertical or lithostatic stress is an important factor in tectonic and geomechanical studies and is commonly used in the prediction of pore pressures and fracture gradients. However, the vertical stress is not always calculated in-situ and the approximation of 1.0 psi/ft (22.63 MPa/km) is often used for the vertical stress gradient. Vertical stress has been determined in 24 fields in the Baram Basin, Brunei, using density log and checkshot velocity survey data. The Baram Basin shows a variation in vertical stress gradient between 18.3-24.3 MPa/km at 1500 m depth below the surface. This variation has a significant effect on in-situ stress related issues in field development such as wellbore stability and fracture stimulation. The variation is caused by a bulk rock density change of 2.48-2.07 g/cm 3 from the hinterland of the delta to its front. Differential uplift and erosion of the delta * Corresponding author 2 hinterland and undercompaction associated with overpressure are the interpreted causes of the density and hence vertical stress variation.
Differences in fluids origin, creation of overpressure and migration are compared for end member ... more Differences in fluids origin, creation of overpressure and migration are compared for end member Neogene fold and thrust environments: the deepwater region offshore Brunei (shale detachment), and the onshore, arid Central Basin of Iran (salt detachment). Variations in overpressure mechanism arise from a) the availability of water trapped in pore-space during early burial (deepwater marine environment vs arid, continental environment), and b) the depth/temperature at which mechanical compaction becomes a secondary effect and chemical processes start to dominate overpressure development. Chemical reactions associated with smectite rich mud rocks in Iran occur shallow (w1900 m, smectite to illite transformation) causing load-transfer related (moderate) overpressures, whereas mechanical compaction and inflationary overpressures dominate smectite poor mud rocks offshore Brunei. The basal detachment in deepwater Brunei generally lies below temperatures of about 150 C, where chemical processes and metagenesis are inferred to drive overpressure development. Overall the deepwater Brunei system is very water rich, and multiple opportunities for overpressure generation and fluid leakage have occurred throughout the growth of the anticlines. The result is a wide variety of fluid migration pathways and structures from deep to shallow levels (particularly mud dykes, sills, laccoliths, volcanoes and pipes, fluid escape pipes, crestal normal faults, thrust faults) and widespread inflationarytype overpressure. In the Central Basin the near surface environment is water limited. Mechanical and chemical compaction led to moderate overpressure development above the Upper Red Formation evaporites. Only below thick Early Miocene evaporites have near lithostatic overpressures developed in carbonates and marls affected by a wide range of overpressure mechanisms. Fluid leakage episodes across the evaporites have either been very few or absent in most areas. Locations where leakage can episodically occur (e.g. detaching thrusts, deep normal faults, salt welds) are sparse. However, in both Iran and Brunei crestal normal faults play an important role in the transmission of fluids in the upper regions of folds.
The cause of the Lusi mud eruption remains controversial. The review by Miller and Mazzini (2017)... more The cause of the Lusi mud eruption remains controversial. The review by Miller and Mazzini (2017) firmly dismisses a role of drilling operations at the adjacent Banjarpanji-1 well and argues that the eruption was triggered by the M6.3 Yogyakarta earthquake some 254 km away. We disagree with these conclusions. We review drilling data and the daily drilling reports, which clearly confirm that the wellbore was not intact and that there was a subsurface blowout. Downhole pressure data from Lusi directly witness the birth of Lusi at the surface on the 29th of May 2006, indicating a direct connection between the well and the eruption. Furthermore, the daily drilling reports specifically state that Lusi activity was visibly altered on three separate occasions by attempts to kill the eruption by pumping dense fluid down the BJP-1 well, providing further evidence of a connection between the wellbore and Lusi. By comparison with other examples of newly initiated mud eruptions elsewhere by other earthquakes, the Yogyakarta earthquake was far away given its magnitude. The seismic energy density of the Yogyakarta earthquake was only 0.0043 J/m 3 , which is less than a quarter of the minimum 0.019 J/m 3 seismic energy density that has ever been inferred to trigger other mud eruptions. We show that the Lusi area had previously experienced other shallow earthquakes with similar frequencies and stronger ground shaking that did not trigger an eruption. Finally, the data from the BJP-1 well indicates that there was no prior hydrodynamic connection between deep overpressured hydrothermal fluids and the shallow Kalibeng clays, and that there was no evidence of any liquefaction or remobilization of the Kalibeng clays induced by the earthquake. We thus strongly favor initiation by drilling and not an earthquake.
The unusual present-day crustal stress pattern of the Australian continent has been the subject o... more The unusual present-day crustal stress pattern of the Australian continent has been the subject of scientific debate for over 25 years. The orientation of maximum horizontal present-day stress (S Hmax) in continental Australia is unlike all other major tectonic plates in that the stress pattern shows regional variability and it is not oriented sub-parallel to the direction of absolute plate motion. Previous studies on the stress pattern of Australia revealed that the complex stress pattern of the continent is controlled, at a first-order, by the superposition of plate tectonic forces exerted at the plate boundaries. However, prior analysis of the contemporary Australian crustal stress pattern have been unable to model or explain the stress pattern observed in most of eastern Australia, and has not extensively addressed the numerous smaller scale variations in stress orientation. The recent development of unconventional reservoirs in Australia has resulted in a greatly increased amount of new data for stress analysis in previously unstudied or poorlyconstrained areas in eastern Australia. In addition, stress analysis in conventional hydrocarbon, mineral and geothermal exploration in all other parts of the continent provides the opportunity to review and update the Australian Stress Map (ASM). This study presents the new release of the ASM, with a total of 2150 stress data records in Australia (increased from 594 data records in 2003). The 2016 ASM contains 1359 data records determined from the interpretation of drillingrelated stress indicators, 650 from earthquake focal mechanism solutions, 139 from shallow engineering measurements and two from geological indicators. The results reveal four distinct regional trends for the S Hmax orientation in Australia including a NNE-SSW S Hmax trend in northern and northwestern Australia, which rotates to a prevailing E-W orientation in most Western and South Australia. The orientation of S Hmax in eastern Australia is primarily ENE-WSW and swings to NW-SE in southeastern Australia. A comparison between the new ASM database and neotectonic features further confirms the role of contemporary stress in recent deformation of the Australian crust. The 2016 ASM reveals significant discrepancies between newly observed S Hmax orientations and predictions by published geomechanical-numerical models for the Australian continent. Forces generated at the boundary of Indo-Australian Plate remain the major control on the regional crustal stress pattern in continental Australia, however, the increased data density, particularly in eastern Australia, reveals numerous local perturbations of the stress field that were not previously clearly captured. Hence, the key findings of the new release are that local (intraplate) stress sources are more significant than previously recognized, particularly in eastern Australian basins, and cause substantial local scale rotations of the present-day crustal stress pattern that have not been factored into existing Australian stress models.
The future success of both enhanced (engineered) geothermal systems and shale gas production is r... more The future success of both enhanced (engineered) geothermal systems and shale gas production is reliant on the development of reservoir stimulation strategies that suit the local geo-mechanical conditions of the prospects. The orientation and nature of the in-situ stress field and pre-existing natural fracture networks in the reservoir are among the critical parameters that will control the quality of the stimulation program. This study provides a detailed investigation into the nature and origin of natural fractures in the area covered by the Moomba–Big Lake 3D seismic survey, in the southwest termination of the Nappamerri Trough of the Cooper Basin. These fractures are imaged by both borehole image logs and complex multi-traces seismic attributes (e.g. dip-steered most positive curvature and dip-steered similarity), are pervasive throughout the cube, and exhibit a relatively consistent northwest–southeast orientation. Horizon extraction of the seismic attributes reveal a strong va...
The world stress map-a freely accessible tool for geohazard assessment, Recent Geodynamics Georis... more The world stress map-a freely accessible tool for geohazard assessment, Recent Geodynamics Georisk and
The spatio-temporal changes of the stress state in a geothermal reservoir are of key importance f... more The spatio-temporal changes of the stress state in a geothermal reservoir are of key importance for the understanding of induced seismicity and planning of injection and depletion strategies. In particular the poro-elastic effects on the stress state due to re-injection or depletion of water are of interest for both geothermal projects and hydrocarbon exploitation. In addition to the conventionally used effective stress concept, poro-elasticity affects the stress tensor components differently as a function of changes in pore pressure. Here, we provide an analytical base for the long-term changes of the 3D stress tensor components as a function of pore pressure changes. Results indicate that for a constant rate of injection or depletion the coupling between pore pressure and all stress tensor components depends on the location in the reservoir with respect to the reinjection/depletion point as well as the time since the beginning of pore pressure changes. Our systematic analysis suggests that poro-elastic stress changes can even locally modify the given tectonic stress regime. Furthermore, the results predict that localized changes of maximum shear stress can lead to different fracture orientations than those expected when poro-elastic effects are not considered. These results indicate a need for 3D geomechanical-numerical studies of more realistic reservoir settings in order to study the 3D effects of pore pressure/stress coupling. Our generic 3D geomechanical-numerical study shows that less than two years of production of a single well changes shear stresses by 0.2 MPa. Thus, in reservoirs with decades of production shear stress change can reach sufficiently high values to reactivate pre-existing faults or even generate new fractures with unexpected orientations.
The future success of both enhanced (engineered) geothermal systems and shale gas production reli... more The future success of both enhanced (engineered) geothermal systems and shale gas production relies significantly on the development of reservoir stimulation strategies that suit the local stress and mechanical conditions of the prospects. The orientation and nature of the in-situ stress field and pre-existing natural fracture networks in the reservoir are amongst the critical parameters controlling the success of any stimulation program. This work follows an initial study showing the existence of natural fractures in the area covered by the Moomba-Big Lake 3D seismic survey, in the SouthWestern termination of the Nappamerri Trough of the Cooper Basin in South Australia. The fractures, imaged both by borehole image logs and seismic attributes (including Most Positive Curvature, Ant tracking of Dip Deviation, and Variance), are pervasive across the seismic survey, and present a relatively constant NW-SE orientation. The density of the fractures, as visible on horizon extractions of attributes, is however spatially variable. A high density of fractures is found in the vicinity of the fault planes and tight antiforms. We compare apparent fractures from different seismic attributes (seismic fractures) with faults interpreted from well data and on vertical seismic sections. Results indicate that some seismic fractures are small faults with small offsets up to 3 ms. Other seismic fractures are actual fractures striking parallel to nearby faults. Analysis shows that under present day stress orientation and magnitudes, fractures striking NW-SE and NE-SW are more susceptible to stimulation, and are more likely to open for fluid flow.
Abu Dhabi International Petroleum Conference and Exhibition, 2012
ABSTRACT Pore pressure is a key parameter in controlling the well in terms of reservoir fluid pre... more ABSTRACT Pore pressure is a key parameter in controlling the well in terms of reservoir fluid pressure. An accurate estimation of pore pressure yields to better mud weight proposition and pressure balance in the bore hole. Current well known methods of pore pressure prediction are mainly based on the differences between the recorded amount and normal trend in sonic wave velocity, formation resistivity factor (FRF), or d-exponent (a function of drilling parameters) in overpressured zone. The majority of the techniques are based on the compaction of specific formation type which need localization or calibration. They occasionally fail to proper response in carbonate reservoirs. In this research, a new method for calculating the pore pressure has been obtained using the compressibility attribute of reservoir rock. In the case of overpressure generation by undercompaction (which is the case in most of the reservoirs), pore pressure is depended on the changes in pore space which is a function of rock and pore compressibility. In a simple way, pore space decreases while the formation undergoes compaction and this imposes pressure on the fluid which fills the pores. Generally, rock compressibility has minor changes over a specific formation, but even this small amount must be considered. Thus, the statistical tools should be used to distribute the compressibility over the formation. Therefore, based on the bulk and pore compressibility achieved from the special core analysis (SCAL) or well logs in one well, the pore pressure in the other locations of a formation could be predicted. Introduction Rock mass as well as the pore space is always under the several stresses and the current underground state of estate is what has been defined by Terzaghi[1] indicating that a body of soil is in a state of plastic equilibrium if every part of it is on the verge of failure. This means that the moment of rotation with respect to all dimensions must set to zero. That implies with the fact that the rock stays under the condition with the most stable situation of the stress state. These are the facts about the current state of the rock. However, until this stage of stability, mineral maturation and rock type deformation are results of burying during the deposition. Thus, bulk and pores of the rock under the overburdern stress will tend towards the deformation until the rock reaches the most stable condition. In other words, Overburden pressure which is mostly the main stress tensor in the depth of oil and gas reservoirs, leads to the compaction of the formation. Consequently, if the rock is surrounded by any type of impermeable layers, which means no way of escaping for pore fluid, a portion of the stress will be imposed to the fluid and the pore pressure will rise. This process can be modelled in in terms of stress-strain analysis whereas this research has focoused on a phenomenon which is known as the compressibility mechanism. The amount of excess stress which is exerted on the pore fluid is called "pore pressure??.
Geomechanics is the discipline dealing with the prediction and management of rock deformation and... more Geomechanics is the discipline dealing with the prediction and management of rock deformation and failure. Geomechanical problems resulting from the change of stress in the rock induced by drilling and production make many hydrocarbon production projects challenging. Examples of geomechanical problems include wellbore stability and fracturing of the formation during drilling that may lead to financial loss due to losses, kicks, stuck pipe, extra casing strings, and sidetracks. Other problems are due to reservoir stress changes occurring during production such as reservoir compaction, surface subsidence, formation fracturing, casing deformation and failure, sanding, reactivation of faults, and bedding parallel slip. The impact of geomechanical problems on field development is best examined early in the life of the field, when there is an opportunity for proactive planning. The interaction between complex geology, well trajectories, and stress changes due to drilling or production is a complex 3D problem, and is best solved using a 3D approach. A mechanical Earth model (MEM) consists of a 3D description of pore pressure, stress, and mechanical properties linked to a 3D framework model consisting of surfaces such as formation tops and faults, and allows drilling risks and geomechanical problems to be identified and contingency plans for handling those risks to be developed. Formation strength and in-situ stress are the key components of the MEM critical to well engineering design. Technical approaches to geomechanics can be roughly classified as empirical, analytical, or numerical. The empirical approach is based on application of best practices through company or industry standards, without much consideration for the underlying mechanisms at work. This approach can be very useful, though, and is often applied. Analytical geomechanical models aim to describe the deformation and stress change with mathematical equations (constitutive relations) and represent the subsurface with simplified geometries, depletion patterns, and mechanical properties. Examples of analytical geomechanics include depletion of disk-shaped or spheroid-shaped reservoirs in an infinite linear elastic medium. For many problems, analytical models provide a good first-order approximation. With the recent advances in computational power and memory capability, numerical methods have become more popular: They allow inclusion of the natural (geologic) variation in geologic structure, depletion, and mechanical property variation at scales of tens to hundreds of meters. However, any geomechanical model needs calibration to in-situ data before it can be used as a predictive tool. If calibration is not done, or not done properly, geomechanical model predictions will be imprecise and inaccurate. An important component of the workflow is the estimation of geomechanical properties from well-log data using suitably calibrated transforms. Static and dynamic moduli differ due to differences in the time scale over which the deformation is applied (frequency dependence or dispersion) and due to differences in strain magnitude. The main contribution to dispersion arises from the presence of fluids in the pore space, and depends on whether the time required for pore-pressure equalization is long or short compared to the period of the elastic wave. The dynamic moduli therefore depend on whether seismic, sonic, or ultrasonic frequencies are used. In the low-frequency limit, the pore fluid can be considered to be in pore-pressure equilibrium, and Gassmann's equations can be used to determine the
The World Stress Map Project compiles a global database of contemporary tectonic stress informati... more The World Stress Map Project compiles a global database of contemporary tectonic stress information of the Earth's crust. Early releases of the World Stress Map Project demonstrated the existence of first-order (plate-scale) stress fields controlled by plate boundary forces and second-order (regional) stress fields controlled by major intraplate stress sources such as mountain belts and zones of widespread glacial rebound. The 2005 release of the World Stress Map Project database provides, for some areas, high data density that enables us to investigate third-order (local) stress field variations, and the forces controlling them such as active faults, local inclusions, detachment horizons, and density contrasts. These forces act as major controls on the stress field orientations when the magnitudes of the horizontal stresses are close to isotropic. We present and discuss examples for Venezuela, Australia, Romania, Brunei, western Europe, and southern Italy where a substantial increase of data records demonstrates some of the additional factors controlling regional and local stress patterns.
... Oliver Heidbach, Friedemann Wenzel and Phillip Fleckenstein. ... The WSM has provided key ins... more ... Oliver Heidbach, Friedemann Wenzel and Phillip Fleckenstein. ... The WSM has provided key insights into the state of plate-scale and regional stress fields in the earth's crust and revealed that these are primarily controlled by forces exerted at plate boundaries, in particular mid ...
The World Stress Map (WSM) project is a global compilation of information on the contemporary cru... more The World Stress Map (WSM) project is a global compilation of information on the contemporary crustal stress field from a wide range of stress indicators. The WSM database release 2008 contains 21,750 stress data records that are quality-ranked using an updated and refined quality-ranking scheme. Almost 17,000 of these data records have A-C quality and are considered to record the orientation of maximum horizontal compressional stress SH to within ±25°. As this is almost a triplication of data records compared with the first WSM database release in 1992, we reinvestigate the spatial wavelength of the stress patterns with a statistical analysis on a global 0.5° grid. The resulting smoothed global stress map displays both; the mean SH orientation that follows from the maximum smoothing radius for which the standard deviation is b25° and a countour map that displays the wavelength of the stress pattern. This smoothed global map confirms that long wavelength stress patterns (N2000 km) exist for example in North America and NE Asia. These have been used in earlier analyses to conclude that the global stress pattern is primarily controlled by plate boundary forces that are transmitted into the intraplate region. However, our analysis reveals that rather short wavelength of the stress pattern b200 km are quite frequent too, particularly in western Europe, Alaska and the Aleutians, the southern Rocky Mountains, Basin and Range province, Scandinavia, Caucasus, most of the Himalayas and Indonesia. This implies that local stress sources such as density contrasts and active fault systems in some areas have high impact in comparison to plate boundary forces and control the regional stress pattern.
The present-day state of stress in Western Europe is considered to be controlled by forces acting... more The present-day state of stress in Western Europe is considered to be controlled by forces acting at the plate boundaries. It is assumed that the Alpine orogen only influence the regional pattern of present-day stress in Western Europe within the Alps themselves. We examine the present-day maximum horizontal stress orientation in the Molasse Basin in the Alpine foreland in order to investigate the possible influence of the Alps on the far-field stress pattern of Western Europe. Four-arm caliper and image logs were analysed in 137 wells, in which a total of 1348 borehole breakouts and 59 drilling-induced fractures were observed in 98 wells in the German Molasse Basin. The borehole breakouts and drilling-induced fractures reveal that stress orientations are highly consistent within the Molasse Basin and that the present-day maximum horizontal stress orientation rotates from N-S in southeast Germany (002°N±19°) to approximately NNW-SSE in Tectonics southwest Germany and the Swiss Molasse Basin (150°N±24°). The present-day maximum horizontal stress orientation in the Molasse Basin is broadly perpendicular to the strike of the Alpine front, indicating that the stress pattern is probably controlled by gravitational potential energy of Alpine topography rather than by plate boundary forces. The present-day maximum horizontal stress orientations determined herein have important implications for the production of hydrocarbons and geothermal energy in the German Molasse Basin, in particular that hydraulically-induced fractures are likely to propagate N-S and that wells deviated to the north or south may have reduced wellbore instability problems.
The vertical or lithostatic stress is an important factor in tectonic and geomechanical studies a... more The vertical or lithostatic stress is an important factor in tectonic and geomechanical studies and is commonly used in the prediction of pore pressures and fracture gradients. However, the vertical stress is not always calculated in-situ and the approximation of 1.0 psi/ft (22.63 MPa/km) is often used for the vertical stress gradient. Vertical stress has been determined in 24 fields in the Baram Basin, Brunei, using density log and checkshot velocity survey data. The Baram Basin shows a variation in vertical stress gradient between 18.3-24.3 MPa/km at 1500 m depth below the surface. This variation has a significant effect on in-situ stress related issues in field development such as wellbore stability and fracture stimulation. The variation is caused by a bulk rock density change of 2.48-2.07 g/cm 3 from the hinterland of the delta to its front. Differential uplift and erosion of the delta * Corresponding author 2 hinterland and undercompaction associated with overpressure are the interpreted causes of the density and hence vertical stress variation.
Differences in fluids origin, creation of overpressure and migration are compared for end member ... more Differences in fluids origin, creation of overpressure and migration are compared for end member Neogene fold and thrust environments: the deepwater region offshore Brunei (shale detachment), and the onshore, arid Central Basin of Iran (salt detachment). Variations in overpressure mechanism arise from a) the availability of water trapped in pore-space during early burial (deepwater marine environment vs arid, continental environment), and b) the depth/temperature at which mechanical compaction becomes a secondary effect and chemical processes start to dominate overpressure development. Chemical reactions associated with smectite rich mud rocks in Iran occur shallow (w1900 m, smectite to illite transformation) causing load-transfer related (moderate) overpressures, whereas mechanical compaction and inflationary overpressures dominate smectite poor mud rocks offshore Brunei. The basal detachment in deepwater Brunei generally lies below temperatures of about 150 C, where chemical processes and metagenesis are inferred to drive overpressure development. Overall the deepwater Brunei system is very water rich, and multiple opportunities for overpressure generation and fluid leakage have occurred throughout the growth of the anticlines. The result is a wide variety of fluid migration pathways and structures from deep to shallow levels (particularly mud dykes, sills, laccoliths, volcanoes and pipes, fluid escape pipes, crestal normal faults, thrust faults) and widespread inflationarytype overpressure. In the Central Basin the near surface environment is water limited. Mechanical and chemical compaction led to moderate overpressure development above the Upper Red Formation evaporites. Only below thick Early Miocene evaporites have near lithostatic overpressures developed in carbonates and marls affected by a wide range of overpressure mechanisms. Fluid leakage episodes across the evaporites have either been very few or absent in most areas. Locations where leakage can episodically occur (e.g. detaching thrusts, deep normal faults, salt welds) are sparse. However, in both Iran and Brunei crestal normal faults play an important role in the transmission of fluids in the upper regions of folds.
Uploads
Papers by Mark Tingay