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Summary Because of interwell interference, the completion and production of infill wells in unconventional reservoirs often change established production profiles for parent wells and lead to infill-well production lower than expected. Parent-well injection has been used in some fields in an attempt to reduce interwell interference. However, mixed responses were received from these attempts, and few modeling studies have been presented to investigate the mechanisms of the mixed responses. This study investigates the effects of subsequent injection in parent wells with legacy production on interwell interference using a data set from Eagle Ford Shale. A numerical-modeling work flow is presented for the characterization of poroelastic behaviors of multiphase-fluid diffusivity and rock deformation using the finite-element method and multifracture propagation using the displacement discontinuity method. It solves for the spatial-temporal evolutions of pore pressure and in-situ stress because of parent-well production and injection and models the fracture propagation during infill-well completion on the basis of updated heterogeneous in-situ stresses. Thus, the approach obtains the interwell fracture network comprising parent-well fractures and fractures from infill-well completion and captures fracture hits, which are necessary for the analysis of the injection effectiveness. Numerical results indicate that subsequent injections in parent wells make infill-well fractures grow more transversely, denoting improved completion qualities of infill wells. Also, the required subsequent injection volume leading to transverse infill-well fractures is positively correlated with the volume of legacy production in parent wells. In addition to subsequent injection volume, locations of perforation clusters along the infill well are another key parameter affecting the associated interwell interference. Results show that it is easier to generate fracture hits after infill-well completion, when perforation-cluster locations along the infill wellbore are identical to those along parent wellbores. In contrast, certain infill-wellbore perforation-cluster locations different from those in parent wellbores guarantee transverse infill-well fractures and avoid fracture hits during/after infill-well completion. On the basis of the numerical results in this specific study, when infill-well perforation cluster locations are properly placed, the volume of parent-well subsequent injection should be at least 76.9% of the total depleted liquid volume during the legacy production of parent wells for subsequent injection to be effective in avoiding fracture hits. This value is on a case-by-case basis and should not be generalized. The contribution of this work lies in its analyses of the mixed performance by parent-well subsequent injection in the reduction of interwell interference using a reservoir-geomechanics/fracturing modeling work flow.

Summary Tightly spaced horizontal wells are widely used in the development of unconventional resources. The effectiveness of this strategy is largely affected by interwell fracturing interference, indicated by interwell fracture geometry and fracture hits, because interwell interference affects both the parent- and infill-well production. This work proposes a reservoir/geomechanics/fracturing modeling work flow for understanding the interference mechanism and quantifying effects of parent-well fracture geometry, differential stress, and the design of infill-well completion on interwell fracturing interference. Reservoir models are constructed for the analysis of Eagle Ford scenarios. The numerical work flow involves a finite-element model that fully couples reservoir flow and geomechanics, and a complex multifracture propagation model coupling rock mechanics and fluid flow in wellbore and fractures. The work flow characterizes the temporal-spatial evolution of pressure and stress caused by legacy-parent-well production. The fracture model is used to simulate the complex fracture geometry created by infill-well completion, on the basis of an updated heterogeneous reservoir stress state. The resulting fracture geometry quality is quantified by the occurrence of fracture hits and the relative growth of fractures in longitudinal and transverse directions. Nonuniform fracture geometries lead to more-complex stress changes, induced by depletion, rather than by uniform fracture geometries along parent wells. A smaller in-situ differential stress results in stronger stress reorientation that is caused by parent-well depletion, which induces longitudinal fractures along infill wells, and greatly reduces stimulated reservoir volume (SRV) and initial well performance of infill wells. A larger in-situ differential stress induces less stress reorientation and is more likely to lead the fractures to propagate toward pre-existing fractures, generate fracture hits, and affect the production of parent wells. The quantification study in the sensitivity analysis indicates that differential stress and the infill-well completion design have the most significant influences on interwell interference. This study suggests optimal infill-well completion designs for Eagle Ford scenarios. The study also provides insights for an infill-well completion design in unconventional reservoirs developed by tightly spaced horizontal wells, in terms of how to adjust field operational schedules to avoid fracture hits, and change the complexity of the interwell fracture networks.

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