As Permian basin activity continues to grow, questions on the accuracy of EUR predictions continue to grow as well. Citing an increase in initial GORs, recent articles have concluded that oil wells will reach their economic limit sooner than expected. GOR variations, both temporal and spatial, can be due to processes that occur on both production and geologic scales: reservoir depletion, uplift and unroofing, source rock maturity, etc. In the Delaware basin, the geology and the drilling history combine to yield an overall increasing GOR trend for wells completed in the Wolfcamp A in the past five years. A nonlinear multi-variate model predicts higher 6-month GOR in the western part of the basin, agreeing with existing studies citing more mature source rocks due to the higher geothermal gradient of the Diablo Platform. As Wolfcamp A drilling has been marching westward, the wells have been increasingly exposed to reservoirs with higher proportions of solution gas. Thus, little can be inferred about reservoir management with respect to GOR trends until the larger geological overprint is removed. We focus on a core study area with similar predicted 6-month GOR and the earliest drilling history. While similar, 6-month GOR does reflect the regional trend and increases in this area from SE to NW. For infill wells in the study area, our geological model predicts 6-month produced GOR to +/- 2000 scf/bbl, so we do not see a reason to call upon interference as a cause for initial GOR changes. For all the wells in the study area, we see a trend of flatter decline for the past several years with EUR significantly higher in 2017 wells. Several factors likely contribute to this trend: longer completion intervals, both completion intensity and type have been steadily evolving, and moving to regions with higher solution gas should mean more favorable fluid viscosity and reservoir energy.
The Permian basin has been a hotbed of activity, with huge potential in previously untapped reservoirs. As exploratory and development drilling activity continues to grow, questions on the accuracy of estimated ultimate recover (EUR) predictions continue to linger. Citing an increase in initial gas-oil ratios (GOR), recent articles have concluded that oil wells will reach their economic limit sooner than expected. This has led to further scrutiny of empirical forecasting methods for Permian basin oil wells. To understand the effect of GOR on recovery, we focus on the Wolfcamp A of the Delaware basin and examine both the geological conditions and the well spacing history which could influence initial GOR.
