Lower Silurian marine shale play in Sichuan Basin, Southwestern China has complicated geological features that involve highly heterogeneous and anisotropic rock properties, pre-existing natural fractures at multiple length scales, and faulting system. To produce gas from this shale play, we have encountered casing deformations that affect the implementation of multi-stage hydraulic fracturing stimulation program. During a multi-stage hydraulic fracturing stimulation, the stimulation may trigger significant variation of the in-situ stresses at any stage, which may cause casing deformations and the failure of the following fracturing stimulation.
In this paper, we will present multi-disciplinary efforts to develop a quantitative understanding of the causes based on 6 casing deformation events. The diagnostic process starts with a large-scale pre-existing natural fracture mapping and faulting system from available seismic and log data. Next, we applied stress analysis and stimulated fracture network to evaluate the risk of casing deformation. As a conclusion, the main causes of the casing deformations are the shear slips of the large-scale natural fractures and the hydraulic fracture propagation from adjacent wells based on micro-seismic analysis and impression block with coiled tubing.
We have further developed a numerical model that couples rock deformation, hydraulic fracture propagation and its interaction with natural fractures. With the consideration of the stress shadow created by hydraulic fracture propagation, the proposed numerical model has been applied to predict the variation of the formation stresses. This numerical model has been applied to predict the shear slips of natural fractures/faults during hydraulic fracturing stimulation. The simulation studies show that the orientation of natural faults, the distance between hydraulic fracture and natural fractures, net pressure, and wall friction coefficient of fractures are the main factors, which needs to be considered and optimized to avoid casing deformations.
Based on this comprehensive study and diagnostics, we have proposed a guideline for hydraulic fracturing stimulation design and implementation in this region to prevent casing deformation. The consequent results are very encouraging in the prevention of casing deformation during multi-stage hydraulic fracturing stimulations.