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Pore fluid pressure effects in anisotropic rocks: mechanisms of induced seismicity and weak faults
71A 942081 Fractal and length analysis of fractures during brittle to ductile changes Velde, B; Moore, D; Badri, A; Ledesert, B
942085 Model for simulation of the failure of brittle rocks Li, C; Nordlund, E Proc EUROCK'93, Lisbon, 21-24 June 1993 V1, P143-150.
J Geopl~ys Res V98, NB7, July 1993, Pl1935-11940
Publ Rotterdam: A A Balkema, 1993
Four deformation experiments were carried out on Barre granite specimens at confining pressures 0.5-5kbar. The fractures were mapped and separated by orientation into classes of shear, tension, relaxation, and decompression origin, using image analysis techniques. Fracture length distributions were determined. The fracture sets were also subject to fractal analysis using the Cantor's dust method. Difference in fractal dimensions of individual sets tends to decrease with increasing confining pressure. Difference in fracture style in fracture fields can be described in terms of fractal and linear dimensions. Change in style as deformation moves from brittle towards ductile can be measured.
A model has been developed for failure of brittle rocks under compression,based on analysis of behaviour of microcracks and shear faulting. In the pre-failurc stage the model is based on closure of initial microcracks and growth of tensile wing cracks from their tips. Post-failure behaviour is simulated by considering a shear band formed by microcrack coalescence. Nonlinear and hysteretic behaviour can be described. Simulations are compared with experimental data and effects of material and crack parameters on deformation examined.
942082 Joint pattern development: effects of subcritical growth and mechanical crack interaction Olson, J E
J Geophys Res V98, NB7, July 1993, P12251-12265 The development of fracture networks is examined on the basis of subcritical growth theory. Equilibrium crack geometries from the propagation of multiple fractures are generated using a fracture mechanics model. Propagation velocity is seen to exert a controlling effect on fracture length distribution and spacing. Velocity is modelled as proportional to the n-th power of mode I stress intensity. For n = 1, numerous closely spaced fractures of similar length are generated and many enechelon arrays arise from interaction. For increasing values of n,fewer, more widely spaced fractures occur, clustered in narrow zones. Patterns arise in minutes for n = 1, up to hundreds of years for large values of n. 942083 Pore fluid pressure effects in anisotropic rooks: mechanisms of induced seismicity and weak faults Chen Q; Nut, A
Pure Appl Geophys 11139, N3/4, 1992, P463-479 Pore fluid properties, pressure, and flow are rccognised as playing an important role in many crustal processes. An anisotropic effective stress law is derived and it is proposed that deviatoric effects of pore fluid pressure in anisotropic rocks both affect effective stress and cause variation in shear stress. Such variation can lead to instability, trigger earthquakes, and provide the mechanism for failure of weak crustal faults at low shear stresses. Potential areas of application of these observations are described. 942084 Examination of a cored hydraulic fracture in a deep gas well Warpinski, N R; Lorenz, J C; Branagan, P T; Myal, F R; Gall, B L
SPE Prod Facil I/8, N3, Aug 1993, P150-158 Because of difficulties of access, few hydraulic fractures have been accurately studied and the conventional geometry is assumed that from idealised fracture models. A hydraulic fracture stimulation at depth 2164m in Mesaverde strata was cored in a deviated well. Two fracture intervals both containing multiple fractures were found. Implications for hydraulic fracturing models and analysis include: higher pressure drops in fractures; shorter fracture lengths; bedding-abetted containment; possible tip-zone process effects; questionable proppant transport theory;large amounts of residue; and multiple fracture zone behaviour with large offset.
942086 Field verification of fracture height growth associated with a • restricted access completion Spafford, S D; Saulsberry, J L; Shraufnagel, R A Proc 1993 International Coalbed Methane Symposium,
Birmingham, Alabama, 17-21 May 1993 V1, P139-144. Publ Tuscaloosa: University of Alabama, 1993 The restricted access completion treatment is presented, by which multiple coal seams in a coal group are effectively stimulated with only one seam perforated. A suitable stress profile and absence of barrier beds to vertical growth are necessary for its success. Cost savings and improved fracture length by minimising multiple parallel fractures and excessive height growth are possible. A case of its use is presented and equipment for stimulation and verification of its success is illustrated. 942087 Proposal for a probabilistic model on the stress-strain relation of rock and its application to some rocks Tano, H Proc International Symposium o n Assessment and Prevention
of Failure Phenomena in Rock Engineering, Istanbul, 5-7 April 1993 P245-250. Publ Rotterdam: A A Balkema, 1993 A model for the damage probability of rock materials is proposed. The effective cross sectional area of a specimen is related to a probability function of strain under uniaxial compression. The function is calculated from the change in compliance of the specimen as a result of microfracturing and corresponds to the probability of damage under progressive failure. In general, this function is in an arbitrary form and cannot give analytical values of ultimate or residual strength. The Weibull function however is suitable and fits the stress strain curve for some rocks. It is demonstrated that the general probability function for progressive damage is roughly similar to a composite Weibull function. 942088 Investigations on the stress state near the neighbouring fractures by boundary elements method Savchenko, S N; Kozyrev, A A Proc International Symposium on Assessment and Prevention
of Failure Phenomena in Rock Engineering, Istanbul, 5-7 April 1993 P251-256. Publ Rotterdam: ,4 A Balkema, 1993 Stress distribution around two neighbouring parallel fractures in a rock mass is examined as a function of their orientation relative to compressive forces acting at infinity. The fractures can be coplanar or not. The orientation of the fractures at which maximum values of tensile stress arise, conditions where potential for fracture propagation is greatest, are determined.
942085 Model for simulation of the failure of brittle rocks Li, C; Nordlund, E Proc EUROCK'93, Lisbon, 21-24 June 1993 V1, P143-150.
J Geopl~ys Res V98, NB7, July 1993, Pl1935-11940
Publ Rotterdam: A A Balkema, 1993
Four deformation experiments were carried out on Barre granite specimens at confining pressures 0.5-5kbar. The fractures were mapped and separated by orientation into classes of shear, tension, relaxation, and decompression origin, using image analysis techniques. Fracture length distributions were determined. The fracture sets were also subject to fractal analysis using the Cantor's dust method. Difference in fractal dimensions of individual sets tends to decrease with increasing confining pressure. Difference in fracture style in fracture fields can be described in terms of fractal and linear dimensions. Change in style as deformation moves from brittle towards ductile can be measured.
A model has been developed for failure of brittle rocks under compression,based on analysis of behaviour of microcracks and shear faulting. In the pre-failurc stage the model is based on closure of initial microcracks and growth of tensile wing cracks from their tips. Post-failure behaviour is simulated by considering a shear band formed by microcrack coalescence. Nonlinear and hysteretic behaviour can be described. Simulations are compared with experimental data and effects of material and crack parameters on deformation examined.
942082 Joint pattern development: effects of subcritical growth and mechanical crack interaction Olson, J E
J Geophys Res V98, NB7, July 1993, P12251-12265 The development of fracture networks is examined on the basis of subcritical growth theory. Equilibrium crack geometries from the propagation of multiple fractures are generated using a fracture mechanics model. Propagation velocity is seen to exert a controlling effect on fracture length distribution and spacing. Velocity is modelled as proportional to the n-th power of mode I stress intensity. For n = 1, numerous closely spaced fractures of similar length are generated and many enechelon arrays arise from interaction. For increasing values of n,fewer, more widely spaced fractures occur, clustered in narrow zones. Patterns arise in minutes for n = 1, up to hundreds of years for large values of n. 942083 Pore fluid pressure effects in anisotropic rooks: mechanisms of induced seismicity and weak faults Chen Q; Nut, A
Pure Appl Geophys 11139, N3/4, 1992, P463-479 Pore fluid properties, pressure, and flow are rccognised as playing an important role in many crustal processes. An anisotropic effective stress law is derived and it is proposed that deviatoric effects of pore fluid pressure in anisotropic rocks both affect effective stress and cause variation in shear stress. Such variation can lead to instability, trigger earthquakes, and provide the mechanism for failure of weak crustal faults at low shear stresses. Potential areas of application of these observations are described. 942084 Examination of a cored hydraulic fracture in a deep gas well Warpinski, N R; Lorenz, J C; Branagan, P T; Myal, F R; Gall, B L
SPE Prod Facil I/8, N3, Aug 1993, P150-158 Because of difficulties of access, few hydraulic fractures have been accurately studied and the conventional geometry is assumed that from idealised fracture models. A hydraulic fracture stimulation at depth 2164m in Mesaverde strata was cored in a deviated well. Two fracture intervals both containing multiple fractures were found. Implications for hydraulic fracturing models and analysis include: higher pressure drops in fractures; shorter fracture lengths; bedding-abetted containment; possible tip-zone process effects; questionable proppant transport theory;large amounts of residue; and multiple fracture zone behaviour with large offset.
942086 Field verification of fracture height growth associated with a • restricted access completion Spafford, S D; Saulsberry, J L; Shraufnagel, R A Proc 1993 International Coalbed Methane Symposium,
Birmingham, Alabama, 17-21 May 1993 V1, P139-144. Publ Tuscaloosa: University of Alabama, 1993 The restricted access completion treatment is presented, by which multiple coal seams in a coal group are effectively stimulated with only one seam perforated. A suitable stress profile and absence of barrier beds to vertical growth are necessary for its success. Cost savings and improved fracture length by minimising multiple parallel fractures and excessive height growth are possible. A case of its use is presented and equipment for stimulation and verification of its success is illustrated. 942087 Proposal for a probabilistic model on the stress-strain relation of rock and its application to some rocks Tano, H Proc International Symposium o n Assessment and Prevention
of Failure Phenomena in Rock Engineering, Istanbul, 5-7 April 1993 P245-250. Publ Rotterdam: A A Balkema, 1993 A model for the damage probability of rock materials is proposed. The effective cross sectional area of a specimen is related to a probability function of strain under uniaxial compression. The function is calculated from the change in compliance of the specimen as a result of microfracturing and corresponds to the probability of damage under progressive failure. In general, this function is in an arbitrary form and cannot give analytical values of ultimate or residual strength. The Weibull function however is suitable and fits the stress strain curve for some rocks. It is demonstrated that the general probability function for progressive damage is roughly similar to a composite Weibull function. 942088 Investigations on the stress state near the neighbouring fractures by boundary elements method Savchenko, S N; Kozyrev, A A Proc International Symposium on Assessment and Prevention
of Failure Phenomena in Rock Engineering, Istanbul, 5-7 April 1993 P251-256. Publ Rotterdam: ,4 A Balkema, 1993 Stress distribution around two neighbouring parallel fractures in a rock mass is examined as a function of their orientation relative to compressive forces acting at infinity. The fractures can be coplanar or not. The orientation of the fractures at which maximum values of tensile stress arise, conditions where potential for fracture propagation is greatest, are determined.