Stress reorientation in low permeability reservoirs
Roussel, Nicolas Patrick
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The acknowledgement of the existence of stress changes in the reservoir due to production from a propped-open fracture has resulted in the development of a new concept: oriented or altered-stress refracturing. By initiating a secondary fracture perpendicular to the initial fracture, refracturing makes it possible to access higher pressurized regions of the reservoir, thus improving the productivity of the well. The redistribution of stresses around a fractured vertical well has two sources: (a) opening of propped fracture (mechanical effects) and (b) production or injection of fluids in the reservoir (poroelastic effects). The coupling of both phenomena is numerically modeled to quantify the extent and timing of stress reorientation around fractured production wells. Guidelines and type-curves are established that allow an operator to choose the timing of the refracture operation in the life of the well, and evaluate the potential increase in well production after refracturing. The selection of candidate wells for refracturing is often very difficult based on the information available at the surface. We propose a systematic methodology, based on dimensionless groups, that allows a field engineer to evaluate a well's potential for refracturing from an analysis of field production data and other reservoir data commonly available. This analysis confirms the crucial role played by stress reorientation in the success of refracturing operations. Another topic of interest is the multi-stage fracturing of horizontal wells. The opening of a propped transverse fracture causes a reorientation of stresses in its neighborhood, which in turn affects the direction of propagation of subsequent fractures. This phenomenon, often referred to as stress shadowing, can negatively impact the efficiency of each fracturing stage. By calculating the trajectory of multiple transverse fractures, we offer some insight on the completion designs that will (a) minimize fracture spacing without compromising the efficiency of each fracturing stage and (b) effectively stimulate natural fractures in the vicinity of the created fracture. In addition, a novel detection method of mechanical interference between multiple transverse fractures is established, based on net fracturing pressure data measured in the field, to calculate the optimum fracture spacing for a specific well.