Abstract
Based on the comprehensive analysis of wellbore characteristics in a deep shale gas field, the in-situ stress state of the shale reservoir was assessed in this study for the Longmaxi formation in the Dingshan area, Southwestern China. The data obtained from hydraulic fracturing, drilling-induced fractures, and in-situ core testing were used to determine the magnitude and direction of the maximum principal horizontal stress in five wells. Besides, hydraulic fracturing and cross-multipole array acoustic logging (XMAC) were employed to determine the vertical variation of the in-situ stress. Based on the logging interpretation and mechanical test results, the spatial distribution of rock mechanical parameters in the Dingshan area was assessed by the amplitude variation versus offset (AVO) seismic inversion. A 3D heterogeneous mechanical inversion model was realized via the ANSYS and CATIA3D finite element software packages, providing the area in-situ stress field simulation. The depth, fault strike, and position change effects on the main stress, horizontal stress difference, and horizontal stress difference coefficient were numerically simulated. The results show that the maximum principal stress azimuth was mainly concentrated in the NE20°-NE80° sector. Moreover, the development zone of natural fractures was related to the area with the highest principal stress differences. The maximum principal stress variation in the study area was mainly in the compressive range from −135 to −45 MPa, gradually increasing from east to west and south to north. The stress type mainly depended on the depth, fault strike, and rock mechanical parameters, while the stress difference and stress difference coefficient near the fault structure were relatively small. This study’s findings are considered instrumental in improving the borehole stability, determining the casing setting point, and optimizing the well location in deep shale reservoirs with similar geological conditions.
Highlights
The rapid progress of horizontal staged fracturing and similar technologies provided breakthrough solutions for the global commercial development of shale gas
The in-situ stress is the internal stress existing in the crust rock mass, which is the force per unit area inside the medium caused by the vertical and horizontal movements of the crust and other factors
The in-situ stress state is described by the stress tensor, which includes three orthogonal principal stresses, and each principal stress has a direction and size (Rajabi et al, 2016)
Summary
The rapid progress of horizontal staged fracturing and similar technologies provided breakthrough solutions for the global commercial development of shale gas. The stress field simulation required the following steps: 1) determining the structural elements (such as faults, anticlines, and synclines) and strata geometry (e.g., elevation, thickness, etc.); 2) determining the spatial distribution of mechanical parameters (such as Poisson’s ratio and the Young modulus) of the rocks at different positions; 3) determining the load state and related boundary conditions of the geological body.
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