An efficient numerical method using simple structured grids for accurate modelling of shale production with complex hydraulic fractures and natural fracture networks is very important to evaluate the impacts of frac hits. In this study, we applied a powerful non-intrusive EDFM (Embedded Discrete Fracture Model) method in conjunction with a traditional reservoir simulator with structured grids to simulate shale gas production under different degrees of frac hits. Through the non-intrusive EDFM method, natural fractures that are connected with primary hydraulic fractures can be easily embedded into the structured grids without the need of unstructured grids and local grid refinement. We built a field-scale shale gas reservoir model with a parent/child horizontal well set and a stochastic distribution of inter well natural fractures. We performed a short term (3-year) production simulation with a flowing bottomhole pressure constraint that has a similar decline trend to the field observation. The gas flow rate and cumulative gas production for the three designed frac hits scenarios are plotted and compared. In addition, pressure distributions in matrix and fractures, and drainage volumes for the three designed scenarios after 1 year and 3 years of production are visualized and compared. Understanding the impact of inter well frac hits on well performance in shale reservoirs with the non-intrusive EDFM method provides significant insights for the spacing/drilling job optimization of unconventional resources.

1. Introduction

With the extensive development of unconventional reservoirs, companies are experimenting with different spacing schemes. Progressively tighter spacings between parent and child wells are widely practiced to hopefully maximize hydrocarbon recoveries, but the problems of fracture hits (or shortened as "frac hits") arises. Frac hits is the phenomenon of which newly created hydraulic fractures from child well develop communications with existing fractures, either with natural fractures or hydraulic fractures from parent well (Frohne and Mercer, 1984; Lawal et al., 2013; Jia et al., 2017). This problem presents a tremendous challenge to the infill drilling design and can reduce hydrocarbon recovery by at least 20% (Jacobs, 2019). In addition, extensive frac hits studies on hundreds of wells within various formations were carried out and the results demonstrated the severity of frac hits remarkably (Gupta et al., 2020). Furthermore, analytical work from perspective of geomechanics, regarding the impacts of frac hits has also been previous studied (Pei et al., 2020). In this work, the complexity and significance of natural fractures is pointed out but simplified due to the stochastic nature of these background fractures. To add another layer of complexity, a field study in Williston Basin showed that frac hits have different effects on parent/child wells from the same pad (Abivin et al., 2020). All the above-mentioned works imply the difficult characteristics and profound influences of frac-hit problem.

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