Rate transient analysis (RTA) is commonly used to determine stimulated reservoir volume (SRV) oil in place (OIP) from a flowing material balance (FMB) plot with the identification of the end of linear flow. However, basinwide integrated reservoir analysis of multi-fractured horizontal wells (MFHWs) in the Permian Basin has consistently suggested that the actual SRV OIP values are always greater than the ones estimated from RTA, especially the wells with higher water cuts. The objective of this paper is to propose a practical and effective method to improve the RTA-estimated SRV OIP through an adjustment that accounts for water production.
Using numerical simulation, several synthetic models were created in which the drainage volume and different saturations are known. Simulation results were analyzed in RTA to compare the FMB-derived SRV OIP against the known values for each model. The result shows that the accuracy of SRV OIP estimated from RTA decreases with increasing water-oil ratio (WOR). The basis of the governing equation for the RTA FMB plot was then revisited, and a new equation was derived from fundamental reservoir engineering equations using assumptions that are generally more applicable to the Permian oil wells based on the overall performance observed.
The derived equation calls for water adjustment to the SRV OIP estimated from single-phase FMB, and the adjustment factor is comprised of variables that can be easily determined from production. This method was validated by comparing the adjusted FMB-derived SRV OIP from analytical model with the numerical simulation values. The water correction generally increases SRV OIP estimated from single-phase FMB by 2X to 4X for Permian wells. With drainage height determined from fracture modeling and other diagnostic tools, the fracture half-lengths calculated from the adjusted SRV OIP are showing better alignment with field studies. However, it is still considerably less than the actual lateral drainage, particularly around the landing zones, due to the rectangular shape assumption in analytical models. This matter is outside of scope of this paper and will be addressed in a separate paper.
The water-adjustment method offers a quick and easy way to improve the estimation of SRV OIP and fracture geometries derived from RTA. While well spacing is an economic decision and often requires an iterative optimization process, the fracture half-length calculated based on the water-corrected SRV OIP defines the minimum well-to-well distance for development that will help mitigate overcapitalization due to overly tight lateral spacing.