17:15 〜 18:30
[SCG46-P09] Evolution of patterns of hydraulically conductive natural fractures
during inelastic deformation of rocks
キーワード：Plastic hardening, Critically stressed fractures, Non-associated flow rule
The study is focused on spatial orientations of hydraulically conductive natural fractures existing in rock masses subjected to varied stresses. Fluid conductivity of natural fractures is a serious issue in various fields as the opportunity to predict it is essential for such problems as assessing risks in underground infrastructure engineering or unconventional hydrocarbon reservoirs development. This problem is complicated as the inner structure of rock masses is often unknown with limited sources of information regarding it.
The current study is focused on establishing the relationship between deformation of naturally fractured rock samples and spatial orientations of hydraulically conductive fractures existing in them. Only geomechanical aspect of the problem is taken into account in the current study with other factors being outside of its scope.
The study is based on the results of series of laboratory experiments. A series of triaxial tests have been carried out using cylindrical samples of naturally fractured rocks. These samples were saturated and subjected to external stresses. Typical deformation curves – interrelationships between components of stress and strain tensors – were obtained for varied conditions. These dependencies proved to be nonlinear, so the results have been consequently analyzed through the concept of non-associated plastic flow law: a mathematical model to establish stress-strain relationship has been constructed. Plastic hardening resulting into varied elastic moduli and friction angle was taken into account within this model. The constructed model made it possible to estimate the preferable directions of natural fractures propagation during inelastic stage of rock deformation as the planes of optimally oriented fractures evolve alongside with friction angle. As a result, the constructed model made it possible to predict the statistics on spatial orientations of developed natural fractures in a rock sample subjected to inelastic deformation.
Nevertheless, this statistics itself is not enough to predict fluid conductivity of the developed natural fractures. Critically stressed fractures concept was used to analyze, which of the fractures existing in the rock are hydraulically conductive, and which are not. According to this concept, hydraulic conductivity of a natural fracture is governed by the relationship between normal and shear stress acting on its surface. There is an analytical solution providing spatial orientations of all fractures that are hydraulically conductive for a given arbitrary stress tensor. This solution is valid if critically stressed fractures concept is true and hydraulic conductivity of natural fractures is governed solely by relationship between stresses. Other cases are not within the scope of the current study. Application of the mentioned solution to the obtained statistics on spatial orientations of natural fractures existing in the rock sample may provide insights regarding the changes in hydraulically conductive natural fractures patterns caused by evolution of stress-strain state of naturally fractured rock mass. Laboratory experiments were analyzed with this technique: each stage of deformation process was characterized by such factors as current spatial orientations of all developed natural fractures alongside with spatial orientations of hydraulically conductive fractures; changes in relative number and total free surface of all developed fractures and hydraulically conductive fractures and so on.
The proposed approach of treating naturally fractured rock masses may be used for solving various problems. In the current study an example of such application is considered from the perspective of hydrocarbon reservoir development: the proposed approach made it possible to predict changes in fluid conductivity of natural fractures existing in oil reservoirs during various operations related to reservoir development.
The current study is focused on establishing the relationship between deformation of naturally fractured rock samples and spatial orientations of hydraulically conductive fractures existing in them. Only geomechanical aspect of the problem is taken into account in the current study with other factors being outside of its scope.
The study is based on the results of series of laboratory experiments. A series of triaxial tests have been carried out using cylindrical samples of naturally fractured rocks. These samples were saturated and subjected to external stresses. Typical deformation curves – interrelationships between components of stress and strain tensors – were obtained for varied conditions. These dependencies proved to be nonlinear, so the results have been consequently analyzed through the concept of non-associated plastic flow law: a mathematical model to establish stress-strain relationship has been constructed. Plastic hardening resulting into varied elastic moduli and friction angle was taken into account within this model. The constructed model made it possible to estimate the preferable directions of natural fractures propagation during inelastic stage of rock deformation as the planes of optimally oriented fractures evolve alongside with friction angle. As a result, the constructed model made it possible to predict the statistics on spatial orientations of developed natural fractures in a rock sample subjected to inelastic deformation.
Nevertheless, this statistics itself is not enough to predict fluid conductivity of the developed natural fractures. Critically stressed fractures concept was used to analyze, which of the fractures existing in the rock are hydraulically conductive, and which are not. According to this concept, hydraulic conductivity of a natural fracture is governed by the relationship between normal and shear stress acting on its surface. There is an analytical solution providing spatial orientations of all fractures that are hydraulically conductive for a given arbitrary stress tensor. This solution is valid if critically stressed fractures concept is true and hydraulic conductivity of natural fractures is governed solely by relationship between stresses. Other cases are not within the scope of the current study. Application of the mentioned solution to the obtained statistics on spatial orientations of natural fractures existing in the rock sample may provide insights regarding the changes in hydraulically conductive natural fractures patterns caused by evolution of stress-strain state of naturally fractured rock mass. Laboratory experiments were analyzed with this technique: each stage of deformation process was characterized by such factors as current spatial orientations of all developed natural fractures alongside with spatial orientations of hydraulically conductive fractures; changes in relative number and total free surface of all developed fractures and hydraulically conductive fractures and so on.
The proposed approach of treating naturally fractured rock masses may be used for solving various problems. In the current study an example of such application is considered from the perspective of hydrocarbon reservoir development: the proposed approach made it possible to predict changes in fluid conductivity of natural fractures existing in oil reservoirs during various operations related to reservoir development.