Infill wells’ underperformance in the Permian Basin has raised concerns over decreasing overall production, which can negatively impact future reserve estimates and field economics. The underperformance of infill wells is usually due to changing of the stress state of the reservoir by the parent well, resulting in asymmetric fracturing and inefficient drainage of child wells. Child well performance is often compared to that of one parent, and production variability cannot be explained or predicted. Recent publications ignore two important factors about offset wells: the effect of multiple parent wells on a single child well and the effect of down-spacing between child wells. In addition to the effects of offset wells, existing models lack a robust understanding of geological and geomechanical properties that may influence parent-child performance.
To address these issues, we present a new workflow designed to generate a depletion score by combining spatial and temporal variables, specifically well spacing and parent wells’ time on production. A Delaware Basin well-spacing database was generated that contains the lateral and vertical distance between reference wells and the nearest offset wells. Distance between reference wells and nearest offset wells was used to calculate the depletion score for each well in the dataset. The weighting of each offset well in this scoring system is proportional to time on production with respect to the reference well. The effect of geological and geomechanical properties on parent-child fracture interference is incorporated by capturing the vertical lithofacies variability, or vertical heterogeneity, of the interval between parent and child wells. The vertical distribution of each lithofacies is calculated in terms of the number of beds of specific facies with distinct geomechanical properties, such as carbonates, and the average thickness of those beds. The overall vertical heterogeneity of the reservoir is quantified by calculating the number of facies changes per foot within each well for the targeted interval.
Depletion score correlates better with, and has a higher predictive power in, child well performance in the statistical model compared to well spacing and parent time on production individually. Multiple parent wells and closely spaced infilled wells increase the depletion score and the underperformance of child wells. For wells drilled in the same bench, vertical facies variability correlates to child underperformance related to increased well interference. Higher interference could be due to the tendency of hydraulic fractures to propagate laterally rather than vertically a strongly layered bench, which would increase the interaction between parent and child wells drilled in that same bench.
These new engineering and geologic variables have more explanatory power in multivariate statistical modeling. The multivariate statistical model is trained with 460 Delaware Basin wells and predicts percent change in new well production compared to existing (same bench) wells due to changes in completion and spacing. The approach is useful in evaluating infill wells’ drilling and completion scenarios.