A comprehensive study on three-dimensional stress evolution in unconventional reservoirs and implications for infill drilling and completion

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He, Jia Joanna

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With increasing energy demand around the globe comes an increase in infill well drilling activities. Ideally, infill wells are placed in the undrained region of the unconventional reservoir, producing from where the horizontal parent wells were unable to produce. Geomechanics play a critical role in infill well operations, because production alters the initial in-situ stresses abiding to the theory of poroelasticity, and the change will inevitably impact child fracture growth and hydraulic fracturing efficiency. Despite the numerous existing literature on the topic of poroelastic stress change and infill well strategies, numerical studies on the effects of well and fracture designs on stress change are limited, and it is even more rare to seek literature regarding the stochastic of hydraulic fracture and natural fracture distributions when modeling stress evolution. Therefore, the purpose of this research is to provide a comprehensive sensitivity study on the most prominent well and fracture designs, and the effects of stochastic fracture property distributions on stress reorientation in unconventional reservoirs. A three-dimensional (3D) reservoir model is built with fluid flow and geomechanics iteratively coupled. The first part of this study characterized hydraulic fractures using the local grid refinement (LGR) approach, and the infill region is in between a pair of horizontal wells. Effects of well spacing, minimum bottom-hole pressure (BHP), and cluster spacing on stress reorientation are investigated. Stress evolution in space and time are observed, as well as stress reversal at the infill region. The stress reversal onset time is most susceptible to changes in the BHP, followed by cluster spacing and well spacing. The second and third parts of this study involve a multi-layer reservoir, and the hydraulic and natural fractures are characterized by the Embedded Discrete Fracture Model (EDFM). The parent well is in the middle layer of the reservoir, and there are two prospective locations for the infill well, where one is in the top layer, and another in the producing layer. Hydraulic fracture properties and natural fracture properties are stochastically distributed in the effort to identify and quantify their impacts on in-situ stress evolution. The simulation results show that stress reversal only occurred in the producing layer, while stress reorientation reached a peak value in the top layer before commencing to return to its initial state. The stochastic distributions of hydraulic fracture half-length and height demonstrate highest level of influence on stress reorientation in the producing and top layers, respectively. Natural fractures in general accelerates stress rotation; natural fracture length presents to be the most influential property on stress change. Specific infill well fracturing times are provided on a case-by-case scenario. Overall, this work is based on the theory of poroelasticity in the hope to extend on the current knowledge of flow-induced stress alteration. It provides a detailed investigation on several factors that will affect in-situ stresses. The highlights of the simulation outputs should shed light on infill well strategies


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