Pre-bored pressuremeter tests were conducted in the Opalinus Clay at Mont Terri Rock Laboratory. ... more Pre-bored pressuremeter tests were conducted in the Opalinus Clay at Mont Terri Rock Laboratory. The objective of the tests was to quantify the in-situ rock stiffness at multiple pressure levels. Tests in two boreholes by different types of pressuremeter probes both demonstrate high quality of field measurement. Both stress and strain dependencies of pressuremeter shear modulus are derived and assessed under undrained condition. It can be shown from the variation of pressuremeter shear modulus with stress applied at the borehole wall that the rock stiffness is gradually recovered as the load increases. This observed stress dependency may be further explained through a constitutive study of stiffness evolution under the stress/strain path relevant to pressuremeter testing.
ABSTRACT: Understanding the elastic stiffness evolution of the host rock under stress is importan... more ABSTRACT: Understanding the elastic stiffness evolution of the host rock under stress is important in the prediction of ground deformation during the construction and operation of a radioactive waste repository. The relationship established between dynamic elastic properties and stress also allows for an accurate interpretation of seismic survey and borehole sonic logging data in the field. In this work, the dynamic elastic properties of Opalinus Clay are determined using axial ultrasonic surveys at varying stress steps in triaxial tests. The evolutions of the P- and S-wave velocities are investigated along different stress paths, respectively with constant mean stress (CMS) and constant radial stress (CRS). For CMS testing, a cycle of triaxial compression and extension is performed. It is shown that P-wave velocity gradually reduced at the stage of triaxial compression and reversing the stress path in triaxial extension does not fully recover the velocity changes. The onset of spec...
Low permeability, naturally fractured reservoirs such as coal seam gas (CSG, coalbed methane or C... more Low permeability, naturally fractured reservoirs such as coal seam gas (CSG, coalbed methane or CBM) and shale gas reservoirs generally require well stimulation to achieve economic production rates. Coupling hydraulic fracturing and micro-proppant or graded particle injections (GPI) can be a means to maximise hydrocarbon recovery from these tight, naturally fractured reservoirs, by maintaining or improving cleat or natural fracture conductivity. This paper presents a summary of the National Energy Resources Australia (NERA) project "Converting tight contingent CSG resources: Application of graded particle injection in CSG stimulation" - which assessed the application of micro-proppants, providing guidance on key considerations for GPI application to CSG reservoirs. Over the last decade, laboratory research and modelling have shown the benefits of the application of GPI to keep pre-existing natural fractures and induced fractures open during production of coal reservoirs wi...
Understanding the elastic stiffness evolution of Opalinus Clay is necessary for predicting ground... more Understanding the elastic stiffness evolution of Opalinus Clay is necessary for predicting ground deformation during the construction and operation of radioactive waste repositories. The elastic stiffness is stress dependent, and it degrades when a deviatoric stress/strain threshold is exceeded. A series of multi-staged triaxial tests is used to examine the elastic stiffness evolution of the Opalinus Clay at pre- and post-peak stages. With eight material parameters, a phenomenon-based constitutive model is proposed to account for stress-dependent elastic stiffness, damage, and plasticity in this rock type. The model is further implemented in a finite element (FE) code to predict borehole response during drilling unloading and pressuremeter testing using three proposed stress-dependent stiffness functions, respectively, in the minimum principal stress, the mean stress, and the full stress tensor. The use of the latter function predicts stress-induced anisotropy and explains the nonlinear variations of the elastic stiffness observed in both laboratory and in-situ tests. The FE analysis using the model parameters calibrated from the triaxial tests in this study, however, does not predict the unloading-induced damage in the borehole nearfield as indicated by the borehole ultrasonic survey. Possible reasons are provided concerning the representativeness of the laboratory data and the simplified borehole model for in-situ conditions. The impact of the borehole damage on the pressuremeter measurement is studied using the modified damage and failure parameters. The results show that the measured pressuremeter shear modulus is overall reduced and cannot be fully recovered under pressuremeter loading. Despite the limitation of reproducing the realistic damage behavior of Opalinus Clay using a continuum approach, this study effectively quantifies laboratory and in-situ measurements under a simple constitutive framework. An effective constitutive model is developed to assess the elastic stiffness evolution of Opalinus Clay in a multi-stage triaxial test. The model is implemented in a finite element code to capture the nonlinearity of borehole response in Opalinus Clay during pressuremeter testing. The unloading-induced borehole damage can potentially affect the interpretation of the modulus measured in a pressuremeter test. An effective constitutive model is developed to assess the elastic stiffness evolution of Opalinus Clay in a multi-stage triaxial test. The model is implemented in a finite element code to capture the nonlinearity of borehole response in Opalinus Clay during pressuremeter testing. The unloading-induced borehole damage can potentially affect the interpretation of the modulus measured in a pressuremeter test.
ABSTRACT The use of geostatistics has become an important concept to characterize uncertainty in ... more ABSTRACT The use of geostatistics has become an important concept to characterize uncertainty in reservoir simulations. Only recently has the concept been applied to geomechanics and coupled reservoir geomechanical simulations. The work presented here describes how the Geological Strength Index (GSI) is used to represent the fractured reservoir properties of a coalseam reservoir, and link changes in flow properties with stress changes as well as to strength and deformation. The approach is demonstrated by using a spherical geostatistical model to create 11 GSI realizations and one constant GSI model.Two years of primary methane production and then 3 months CO2 storage were simulated using each of the 12 models as inputs. Much higher cumulative gas and gas production rate profiles were observed for models with a mean GSI below 80, than the 80GSI model. All models above 80 GSI show much lower cumulative gas and gas production rates than the 80GSI model. The same trend held true for the CO2 injection component of the operation. Stress path profiles for three monitoring points in the reservoir showed large variability, but similar paths, whereas monitoring points in the overlying caprock showed very low variability for each realization.
The objective of this study was to evaluate a series of mud systems and additives typically used ... more The objective of this study was to evaluate a series of mud systems and additives typically used in coalbed methane drilling in terms of formation of an instantaneous filter cake, ability of the coal reservoir to rid itself of the filter cake during production, and overall impact on coal permeability. To achieve this, a series of laboratory tests were conducted initially using artificially cleated gypstone rock (to simulate coal). This was followed by the use of large-diameter coal cores, which, unfortunately, did not allow for the tests to be done under in-situ confining stress conditions. The three mud systems tested against coal (Xantham Gum, HEC and Na-CMC) did not have a negative impact on coal permeability, in contrast to previous laboratory data that showed large decreases. Two fluid loss control additives, which have been used successfully in drilling clastic and carbonate rocks, were also tested using a non-ionic polymer mud system. During simulated drilling, these additives (FLC 2000™ and Q-Stop) were very effective in building a thin filter cake on the coal surface almost instantaneously, to the point that no solids were detected in the downstream fluid accumulator. During simulated production, a small pressure drop was sufficient to remove the filter cake. Coal permeability (to water) returned to its original (pre-test) value, which suggested that there was no permanent permeability damage caused by the two additives. When coal-derived fines were added to the drilling mud in another experiment using the same coal, the near wellbore coal permeability was reduced by 87.5%, indicating severe damage to the cleat system and in agreement with previously reported laboratory data. Following the very good performance of FLC 2000™ and Q-Stop in the laboratory tests, these two additives were then used in field applications. Their presence in the drilling fluid resulted in the successful drilling of 953 m and 1400 m of total horizontal length in the deep Mannville coals in Alberta (at True Vertical Depth of 1400 m and 1150 m, respectively). No borehole instability problems were encountered during drilling of the two horizontal wellbores. The monitored mud losses were low in both cases, with the horizontal well #2 experiencing lower mud loss possibly as a result of the absence of large fractures encountered along the horizontal path. Horizontal well #1 remained stable, which allowed sufficient time to insert a production liner.
A methodology is proposed to quickly assess the stability risk for horizontal wells drilled in co... more A methodology is proposed to quickly assess the stability risk for horizontal wells drilled in coal seams. The first task involves the estimation of strength and deformation parameters of the coal by indirect methods, such as the use of core description to assign a Geological Strength Index (GSI). The second task involves the use of a finite element analysis called FLAC to investigate factors such as the depth of fluid penetration while drilling the horizontal well and the stability of the wellbore during simulated production. These two factors are important because coal fines generated by the drill bit action are carried by the drilling fluid into the cleat system of the coal, thereby plugging the permeability pathways, and causing formation damage. On the other hand, a wellbore that may become unstable during pressure drawdown would require a perforated or slotted production liner. In this study, the conditions of drilling and production necessary for a stable horizontal well drilled into a Mannville coal seam at 750–800 m in the central Alberta Plains, were studied. Modeling showed that the depth of fluid penetration would be at least 2.2 m if there is no filter cake formation while drilling underbalanced. Drilling fluid penetration would be minimal if a filter cake is formed under overbalanced conditions. Furthermore, FLAC analysis showed that drilling a smaller diameter hole (5 cm) would be preferable because this size results in a stable wellbore during production compared to the standard 15-cm diameter hole.
... These investigations took place under non-trivial stress paths. 2. Background. CBM in Alberta... more ... These investigations took place under non-trivial stress paths. 2. Background. CBM in Alberta has been identified as a major resource that can add to Canada's energy economy by filling part of the gap left by declining conventional gas reserves. ...
The effects of fractures or joints on geomechanical properties of coal when transitioning from sm... more The effects of fractures or joints on geomechanical properties of coal when transitioning from small samples to larger field scales have long been recognized. Two geomechanical characterization methodologies for of a coal seam for reservoir engineering are presented which attempt to account for these scale effect. The first is a well established empirical rock mass classification method, the Geological Strength
Proceedings of the SPE/AAPG/SEG Asia Pacific Unconventional Resources Technology Conference, 2019
Establishing bulk rock properties in friable material such as coal is difficult simply because re... more Establishing bulk rock properties in friable material such as coal is difficult simply because retrieval of a sufficient sample is challenging particularly because fractured/cleated coal disintegrates in the coring process. This paper describes the use of synthetic rock with embedded simple discrete natural fracture (DFN) systems to establish key rock mechanical properties in synthetic rocks with varying DFN complexity and varying degrees of depletion. The ultimate goal of the work aims to inform late-life technology choices in the depleted CSG reservoirs. To achieve this, we measured the deformation behaviour of the printed intact matrix and the printed interface (fracture) and expanded the rock mass equivalent continuum theory by Huang et al (1995) to related P32 (Area of Fractures/Volume or Rock) to Young's modulus and Bulk modulus. 3D printed synthetic right cylinder rock specimens with zero, one and two fractures were printed in a vertical building and horizontal building d...
The Synthetic Rock Mass (SRM) approach combines the Bonded Particle Model for rock with a Discret... more The Synthetic Rock Mass (SRM) approach combines the Bonded Particle Model for rock with a Discrete Fracture Network and a Smooth Joint model to numerically examine the behaviour of rock masses. The SRM has been shown to predict the behaviour of a rock mass, however the Smooth Joint model has not been rigorously tested against simple laboratory observations. In this work, numerical experiments have been carried out to determine if the PFC2D Smooth Joint model adequately simulates the observed behaviours of one, two and three flaws embedded in a rock-like material under uniaxial compression. Numerical experiments were also completed to examine the influence of loading rate and sample resolution on the fracturing behaviour. RÉSUMÉ La stratégie Synthetic Rock Mass (SRM) est un combinaison du Bonded Particle Model avec Discrete Fracture Network en utilisant le Smooth Joint Model pour analyser numeriquement le comportement des masses rocheuses. SRM prevoit le comportement des masses roche...
This paper presents the result of five pressuremeter tests conducted in deep three clay shale for... more This paper presents the result of five pressuremeter tests conducted in deep three clay shale formations at a thermal operation site. Pressuremeter loading was imposed parallel to the bedding plane under an approximately undrained condition and test data were analyzed using the axisymmetric cavity expansion theory. In parallel with triaxial test results, the analyses revealed several constitutive characteristics of the Westgate clay shale, in particular, the limited nonlinearity and the stress/strain-path dependency of shear modulus. A procedure is proposed to correct the data including multi-azimuth radial displacement measurements in the caliper plane and the anisotropic response of the borehole can be studied. Two aspects of azimuthal anisotropy are assessed - 1) in-plane anisotropy of borehole stiffness and 2) the anisotropy in expansion after borehole plastic yielding. The latter provides the implication of anisotropic in-situ horizontal stresses, demonstrated by both experimen...
Pre-bored pressuremeter tests were conducted in the Opalinus Clay at Mont Terri Rock Laboratory. ... more Pre-bored pressuremeter tests were conducted in the Opalinus Clay at Mont Terri Rock Laboratory. The objective of the tests was to quantify the in-situ rock stiffness at multiple pressure levels. Tests in two boreholes by different types of pressuremeter probes both demonstrate high quality of field measurement. Both stress and strain dependencies of pressuremeter shear modulus are derived and assessed under undrained condition. It can be shown from the variation of pressuremeter shear modulus with stress applied at the borehole wall that the rock stiffness is gradually recovered as the load increases. This observed stress dependency may be further explained through a constitutive study of stiffness evolution under the stress/strain path relevant to pressuremeter testing.
ABSTRACT: Understanding the elastic stiffness evolution of the host rock under stress is importan... more ABSTRACT: Understanding the elastic stiffness evolution of the host rock under stress is important in the prediction of ground deformation during the construction and operation of a radioactive waste repository. The relationship established between dynamic elastic properties and stress also allows for an accurate interpretation of seismic survey and borehole sonic logging data in the field. In this work, the dynamic elastic properties of Opalinus Clay are determined using axial ultrasonic surveys at varying stress steps in triaxial tests. The evolutions of the P- and S-wave velocities are investigated along different stress paths, respectively with constant mean stress (CMS) and constant radial stress (CRS). For CMS testing, a cycle of triaxial compression and extension is performed. It is shown that P-wave velocity gradually reduced at the stage of triaxial compression and reversing the stress path in triaxial extension does not fully recover the velocity changes. The onset of spec...
Low permeability, naturally fractured reservoirs such as coal seam gas (CSG, coalbed methane or C... more Low permeability, naturally fractured reservoirs such as coal seam gas (CSG, coalbed methane or CBM) and shale gas reservoirs generally require well stimulation to achieve economic production rates. Coupling hydraulic fracturing and micro-proppant or graded particle injections (GPI) can be a means to maximise hydrocarbon recovery from these tight, naturally fractured reservoirs, by maintaining or improving cleat or natural fracture conductivity. This paper presents a summary of the National Energy Resources Australia (NERA) project "Converting tight contingent CSG resources: Application of graded particle injection in CSG stimulation" - which assessed the application of micro-proppants, providing guidance on key considerations for GPI application to CSG reservoirs. Over the last decade, laboratory research and modelling have shown the benefits of the application of GPI to keep pre-existing natural fractures and induced fractures open during production of coal reservoirs wi...
Understanding the elastic stiffness evolution of Opalinus Clay is necessary for predicting ground... more Understanding the elastic stiffness evolution of Opalinus Clay is necessary for predicting ground deformation during the construction and operation of radioactive waste repositories. The elastic stiffness is stress dependent, and it degrades when a deviatoric stress/strain threshold is exceeded. A series of multi-staged triaxial tests is used to examine the elastic stiffness evolution of the Opalinus Clay at pre- and post-peak stages. With eight material parameters, a phenomenon-based constitutive model is proposed to account for stress-dependent elastic stiffness, damage, and plasticity in this rock type. The model is further implemented in a finite element (FE) code to predict borehole response during drilling unloading and pressuremeter testing using three proposed stress-dependent stiffness functions, respectively, in the minimum principal stress, the mean stress, and the full stress tensor. The use of the latter function predicts stress-induced anisotropy and explains the nonlinear variations of the elastic stiffness observed in both laboratory and in-situ tests. The FE analysis using the model parameters calibrated from the triaxial tests in this study, however, does not predict the unloading-induced damage in the borehole nearfield as indicated by the borehole ultrasonic survey. Possible reasons are provided concerning the representativeness of the laboratory data and the simplified borehole model for in-situ conditions. The impact of the borehole damage on the pressuremeter measurement is studied using the modified damage and failure parameters. The results show that the measured pressuremeter shear modulus is overall reduced and cannot be fully recovered under pressuremeter loading. Despite the limitation of reproducing the realistic damage behavior of Opalinus Clay using a continuum approach, this study effectively quantifies laboratory and in-situ measurements under a simple constitutive framework. An effective constitutive model is developed to assess the elastic stiffness evolution of Opalinus Clay in a multi-stage triaxial test. The model is implemented in a finite element code to capture the nonlinearity of borehole response in Opalinus Clay during pressuremeter testing. The unloading-induced borehole damage can potentially affect the interpretation of the modulus measured in a pressuremeter test. An effective constitutive model is developed to assess the elastic stiffness evolution of Opalinus Clay in a multi-stage triaxial test. The model is implemented in a finite element code to capture the nonlinearity of borehole response in Opalinus Clay during pressuremeter testing. The unloading-induced borehole damage can potentially affect the interpretation of the modulus measured in a pressuremeter test.
ABSTRACT The use of geostatistics has become an important concept to characterize uncertainty in ... more ABSTRACT The use of geostatistics has become an important concept to characterize uncertainty in reservoir simulations. Only recently has the concept been applied to geomechanics and coupled reservoir geomechanical simulations. The work presented here describes how the Geological Strength Index (GSI) is used to represent the fractured reservoir properties of a coalseam reservoir, and link changes in flow properties with stress changes as well as to strength and deformation. The approach is demonstrated by using a spherical geostatistical model to create 11 GSI realizations and one constant GSI model.Two years of primary methane production and then 3 months CO2 storage were simulated using each of the 12 models as inputs. Much higher cumulative gas and gas production rate profiles were observed for models with a mean GSI below 80, than the 80GSI model. All models above 80 GSI show much lower cumulative gas and gas production rates than the 80GSI model. The same trend held true for the CO2 injection component of the operation. Stress path profiles for three monitoring points in the reservoir showed large variability, but similar paths, whereas monitoring points in the overlying caprock showed very low variability for each realization.
The objective of this study was to evaluate a series of mud systems and additives typically used ... more The objective of this study was to evaluate a series of mud systems and additives typically used in coalbed methane drilling in terms of formation of an instantaneous filter cake, ability of the coal reservoir to rid itself of the filter cake during production, and overall impact on coal permeability. To achieve this, a series of laboratory tests were conducted initially using artificially cleated gypstone rock (to simulate coal). This was followed by the use of large-diameter coal cores, which, unfortunately, did not allow for the tests to be done under in-situ confining stress conditions. The three mud systems tested against coal (Xantham Gum, HEC and Na-CMC) did not have a negative impact on coal permeability, in contrast to previous laboratory data that showed large decreases. Two fluid loss control additives, which have been used successfully in drilling clastic and carbonate rocks, were also tested using a non-ionic polymer mud system. During simulated drilling, these additives (FLC 2000™ and Q-Stop) were very effective in building a thin filter cake on the coal surface almost instantaneously, to the point that no solids were detected in the downstream fluid accumulator. During simulated production, a small pressure drop was sufficient to remove the filter cake. Coal permeability (to water) returned to its original (pre-test) value, which suggested that there was no permanent permeability damage caused by the two additives. When coal-derived fines were added to the drilling mud in another experiment using the same coal, the near wellbore coal permeability was reduced by 87.5%, indicating severe damage to the cleat system and in agreement with previously reported laboratory data. Following the very good performance of FLC 2000™ and Q-Stop in the laboratory tests, these two additives were then used in field applications. Their presence in the drilling fluid resulted in the successful drilling of 953 m and 1400 m of total horizontal length in the deep Mannville coals in Alberta (at True Vertical Depth of 1400 m and 1150 m, respectively). No borehole instability problems were encountered during drilling of the two horizontal wellbores. The monitored mud losses were low in both cases, with the horizontal well #2 experiencing lower mud loss possibly as a result of the absence of large fractures encountered along the horizontal path. Horizontal well #1 remained stable, which allowed sufficient time to insert a production liner.
A methodology is proposed to quickly assess the stability risk for horizontal wells drilled in co... more A methodology is proposed to quickly assess the stability risk for horizontal wells drilled in coal seams. The first task involves the estimation of strength and deformation parameters of the coal by indirect methods, such as the use of core description to assign a Geological Strength Index (GSI). The second task involves the use of a finite element analysis called FLAC to investigate factors such as the depth of fluid penetration while drilling the horizontal well and the stability of the wellbore during simulated production. These two factors are important because coal fines generated by the drill bit action are carried by the drilling fluid into the cleat system of the coal, thereby plugging the permeability pathways, and causing formation damage. On the other hand, a wellbore that may become unstable during pressure drawdown would require a perforated or slotted production liner. In this study, the conditions of drilling and production necessary for a stable horizontal well drilled into a Mannville coal seam at 750–800 m in the central Alberta Plains, were studied. Modeling showed that the depth of fluid penetration would be at least 2.2 m if there is no filter cake formation while drilling underbalanced. Drilling fluid penetration would be minimal if a filter cake is formed under overbalanced conditions. Furthermore, FLAC analysis showed that drilling a smaller diameter hole (5 cm) would be preferable because this size results in a stable wellbore during production compared to the standard 15-cm diameter hole.
... These investigations took place under non-trivial stress paths. 2. Background. CBM in Alberta... more ... These investigations took place under non-trivial stress paths. 2. Background. CBM in Alberta has been identified as a major resource that can add to Canada's energy economy by filling part of the gap left by declining conventional gas reserves. ...
The effects of fractures or joints on geomechanical properties of coal when transitioning from sm... more The effects of fractures or joints on geomechanical properties of coal when transitioning from small samples to larger field scales have long been recognized. Two geomechanical characterization methodologies for of a coal seam for reservoir engineering are presented which attempt to account for these scale effect. The first is a well established empirical rock mass classification method, the Geological Strength
Proceedings of the SPE/AAPG/SEG Asia Pacific Unconventional Resources Technology Conference, 2019
Establishing bulk rock properties in friable material such as coal is difficult simply because re... more Establishing bulk rock properties in friable material such as coal is difficult simply because retrieval of a sufficient sample is challenging particularly because fractured/cleated coal disintegrates in the coring process. This paper describes the use of synthetic rock with embedded simple discrete natural fracture (DFN) systems to establish key rock mechanical properties in synthetic rocks with varying DFN complexity and varying degrees of depletion. The ultimate goal of the work aims to inform late-life technology choices in the depleted CSG reservoirs. To achieve this, we measured the deformation behaviour of the printed intact matrix and the printed interface (fracture) and expanded the rock mass equivalent continuum theory by Huang et al (1995) to related P32 (Area of Fractures/Volume or Rock) to Young's modulus and Bulk modulus. 3D printed synthetic right cylinder rock specimens with zero, one and two fractures were printed in a vertical building and horizontal building d...
The Synthetic Rock Mass (SRM) approach combines the Bonded Particle Model for rock with a Discret... more The Synthetic Rock Mass (SRM) approach combines the Bonded Particle Model for rock with a Discrete Fracture Network and a Smooth Joint model to numerically examine the behaviour of rock masses. The SRM has been shown to predict the behaviour of a rock mass, however the Smooth Joint model has not been rigorously tested against simple laboratory observations. In this work, numerical experiments have been carried out to determine if the PFC2D Smooth Joint model adequately simulates the observed behaviours of one, two and three flaws embedded in a rock-like material under uniaxial compression. Numerical experiments were also completed to examine the influence of loading rate and sample resolution on the fracturing behaviour. RÉSUMÉ La stratégie Synthetic Rock Mass (SRM) est un combinaison du Bonded Particle Model avec Discrete Fracture Network en utilisant le Smooth Joint Model pour analyser numeriquement le comportement des masses rocheuses. SRM prevoit le comportement des masses roche...
This paper presents the result of five pressuremeter tests conducted in deep three clay shale for... more This paper presents the result of five pressuremeter tests conducted in deep three clay shale formations at a thermal operation site. Pressuremeter loading was imposed parallel to the bedding plane under an approximately undrained condition and test data were analyzed using the axisymmetric cavity expansion theory. In parallel with triaxial test results, the analyses revealed several constitutive characteristics of the Westgate clay shale, in particular, the limited nonlinearity and the stress/strain-path dependency of shear modulus. A procedure is proposed to correct the data including multi-azimuth radial displacement measurements in the caliper plane and the anisotropic response of the borehole can be studied. Two aspects of azimuthal anisotropy are assessed - 1) in-plane anisotropy of borehole stiffness and 2) the anisotropy in expansion after borehole plastic yielding. The latter provides the implication of anisotropic in-situ horizontal stresses, demonstrated by both experimen...
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Papers by Nathan Deisman