Evaluation Of Joint Development Heterogeneity Of Coal Reservoir Based On Structural Dynamics And Its Application

The heterogeneity of fracture development influences physical properties of coal reservoirs,and thereby affects the coalbed methane exploitation. The knowledge of joint developmentcharacteristics of coal reservoirs and the data acquisition are current technological challenges.Therefore, taking the Linfen area for example, the tectonic evolution being the main line in thisdissertation, based on analyses of structure development laws, field and microscopic deformationstructures, and combined with joint formation phases and tectonic rock fabric analysis, thisdissertation reveals the dynamic mechanisms of structural deformation environment, structuredeformation and the evolution. Based on influence studies of lithology, thickness, and structureon the joint development, the correlation model between joint developments of coal and rockseams, and the model of the microscopic fracture and joint density of coal seams wereestablished, respectively; the distributions of joints and microscopic fractures of coal reservoirswere revealed. Finally, based on studies of modern tectonic stress field, the control mechanism ofstructure on the pore structures, permeability and gas content of coal reservoirs were revealed.Main conclusions obtained are listed as follows.(1) The structural deformation of the study area mainly occurs after the Middle Jurassic.The Middle Yanshanian epoch with NW striking tectonic compression is the key structural epochwhich results in the main tectonic pattern of the study area. Typical structures in the study area,i.e. Zijingshan fault belt, southeastern fault-flexure belt and the Guyi-Yaoqu fault-fold belt, areall developed due to the tectonic stress in this epoch. The study area is located in the shallowcrust stress-strain environment with low temperature. The weak structural deformation is mainlybrittle deformation. The dynamic mechanisms of structural deformation laws and evolutions ofthe study area were indicated, i.e. the NS striking compressive stress in the Indosinan epoch, NWstriking compressive stress and NW striking tensile stress in the Middle Yanshanian epoch, theNNW striking compressive stress in the Late Yanshanian epoch and the NNE and NE strikingcompressive stress in the Himalayan epoch.(2) High-dip Conjugate shear joints are developed in Linfen area. The preferred jointsapproximately strike55centigrade, followed by140centigrade. The joint density increasesobviously with the weakening mechanical properties of rock seams. The joint density shows agood negative power function relation with thickness in distinct lithologies, structural types andlocations. Preferred joints form mainly due to the NWW-NW tectonic compressive stress in theMiddle Yanshanian epoch. Joint densities of damaged belts with folds and faults increaseabnormally. The influence range doesn’t generally exceed the maximum width1km. (3) Joint development orientations of the eastern outcrop coal seams, underground coalseams and coincide with that of rock seams. The preferred joint orientation on the whole studyarea strike55centigrade on average. The lithologic parameter is proposed through theestablishment of a mathematical correlation model between joint density of coal and rock seams.The ratio of coal seam joint density to the rock seam joint density (lithologic parameter) rangesfrom18.7to22.5at the same tectonic position and with the same seam thickness in the studyarea. The predicted joint density of the No.5coal seam of the Shanxi Formation ranges from10to70joints per meter. The microscopic fracture density of the No.5coal seam shows a goodlinear relationship with the macroscopic joint density; the predicted microscopic fracture densityis mainly ranging from1.60to2.40fractures per mm.(4) The maximum horizontal principal stress of the modern tectonic stress field transformsfrom the NE orientation in the north to the NEE orientation in the south. The stress trajectoryexhibits the SE protruding arc bending. The principal stress shows a linear increase withincreasing buried depth. The maximum principal stress transforms to the vertical stress at thestress conversion depth varying from500m to1100m due to the faster increase of the verticalstress. The principal stress distribution in the plane, controlled by the tectonic role, shows theoverall low-high-low-high variation laws from the east to the west. The modern tectonic stressfield simulation shows that the stress intensity shows an overall increase with increasing burieddepth, which is favorable to the coalbed methane migration from the deep position to a shallowone. The site with developed faults presents stress concentration with the stress transforming tothe tensile stress, which is favorable to the joint opening and a permeability increase.(5) An increase of tectonic deformation induces the increases in microscopic fractures, largeand medium pores and opening holes, the better connectivity, and thereby the gradual increasesin the corresponding total pore volume, porosity and median pore diameter; however, thetransitional pores, microscopic pores and specific surface areas are not affected. Geostresscharacteristics are favorable to the opening of preferred joints striking55centigrade and anincrease in coal reservoir permeability. The preferred fractures are main contributors to the coalreservoir permeability in Linfen area. The permeability of coal seam subjected to the tensilestress presents exponential increases with increasing effective vertical stress and principal stressdifferences. The west of Guyi-Yaoqu anticline axis and Puxian-Mingzhu area exhibit lessdeveloped fracture and low principal stress difference, which forms two high gas areas.