Time Dependent Depletion of Parent Well and Impact on Well Spacing in the Wolfcamp Delaware Basin
Authors: Defeu Cyrille , Garcia Ferrer Giselle , Ejofodomi Efe , Shan Dan , Alimahomed Farhan 
Published: October 2018

Abstract
Parent-child relationship is becoming a topic of high interest in the Permian Basin as more infill wells are being drilled at various times after the parent well has been produced. This paper uses an advanced modelling workflow to determine the impact of parent depletion on infill well spacing at various periods of the parent well production.
As the parent well is being produced, constant well spacing based on virgin condition becomes problematic because pressure depletion around the well leads to change in stress magnitude and orientations. This change in reservoir conditions, is critical for planning infill well.
Parent well depletion results in potential negative impact including: –Asymmetric fracture propagation from the child well into the depleted area around the parent well–Potential detrimental fracturing hits to the parent well
These effects would potentially impair the production performance of both parent and infill wells, further reducing the overall pad efficiency of the pad completions.
Parent well behavior is simulated using an unconventional fracture model (UFM), and the model is calibrated with available treating data. The resulting hydraulic fracture uses an advanced unstructured gridding algorithm that accounts for a fine complex fracture network along the lateral. A high-resolution, numerical reservoir simulator that combines the unstructured grid, rock physics, and reservoir fluid data is then used to match historical production data. The reservoir pressure depletion profile at various timesteps (6, 12, 24, and 36 months) is used as an input to calculate the resulting stress field state via a finite element model. The resulting updated geomechanical properties are used to simulate the infill well hydraulic fracture geometries at various spacing; subsequent unstructured grids are created and used to forecast production. Results are then compared to quantify the impact of depletion. –Initial reservoir pressure and horizontal stress reduce progressively with increasing time of production of the parent well; the average minimum stress change in the stimulated area reaches 18% decrease after 36 months of parent production.–Hydraulic fractures of infill wells grow preferentially towards the adjacent depleted area, reducing fracture extension in virgin rock by more than 60%.–Parent well depletion impacts fracture geometry and production performance of child wells.–Wells closer to the parent are more affected with increasing depletion time; these wells see up to 50% in production reduction as compared to the parent well.–At larger well spacing, little impact is observed due to limited interference between wells.–To help mitigate the impact of parent depletion on infill wells, an innovative spacing scheme that consists of using varying spacing on infill wells closest to the depleted parent well can be used. For this study and with current reservoir properties and completion design, if the parent well has been produced for less than 12 months, infill wells should be placed a least 750 ft away from the parent and at least 900 ft away for parent production beyond 1 year.