Developing C-EOS Model and Thermal Maturity Driven Inverted Compositional Gradient for Wolfcamp Formation in Permian Basin

Abstract The Wolfcamp formation reservoir fluid systems and pressure-volume-temperature (PVT) properties vary dramatically across the Texas Delaware Basin. The fluids transition from black oil in eastern Loving County to volatile oil in the center of northern Loving County, shifting to near-critical fluid or rich gas condensate on the western part of Reeves County. Understanding the gas/oil ratio (GOR) and condensate-to-gas ratio (CGR) behaviors of near-critical fluid and liquid-rich gas condensate system are crucial to effectively estimate reserves, optimize well spacing, and design efficient completions. Meanwhile, the regional thermal maturity profiles indicated by vitrinite reflectance (VRo%) values range between 0.9 and 1.2. The highest GORs are observed in the western part, influenced by the basin structural inversion and heating source from the southwest, which increases maturity levels in the current structural highs. A notable correlation exists between the C1 mol% and the recombination GOR, and thermal maturity indicator vitrinite reflectance VRo%. Moreover, significant GOR variations with depth are observed across multiple codeveloped Wolfcamp benches. Scientific data and hydraulic fracture models have confirmed significant vertical fractures growth connecting multiple zones with different producing GORs. Therefore, building a common equation of state (EOS) model (C-EOS) is critical in capturing PVT fluid behavior and vertical composition gradient changes for fieldwide codevelopment scenarios. The C-EOS models are tuned and validated by matching laboratory PVT data (saturation pressure, phase density and viscosity, mole fraction, GOR, relative oil volume and liquid dropouts) through regression on C7+ pseudocomponents, critical temperature/pressure, acentric factor, and binary interaction coefficient. The composition gradient is applied to represent a generic PVT system, which is initialized with a lighter component downdip (gas condensate with higher C1 mol%), and heavier fluid updip (near-critical fluids) associated with vertical thermal maturity gradient. A regression model has been developed that can predict GOR based on thermal maturity indicators such as vitrinite reflectance, total organic carbon (TOC), and the composition of PVT fluids (C1 and C7+ mol%). In this study, the long-term production is forecasted by constructing a C-EOS compositional model to represent various GOR/CGR for codeveloped Wolfcamp benches. The forecast captures long-term CGR behaviors for section-level infill development. This behavior demonstrates an early time constant CGR when the system pressure remains above the saturation pressure, followed by a gradual declining profile with pressure depletion. The C-EOS model with thermal maturity driven inverted compositional gradient workflow captures the generalized GOR/CGR profiles for Wolfcamp formations in the Texas Delaware Basin with various PVT regions: volatile oil, near-critical fluid, and rich gas condensate. The field-level EOS-based compositional simulation models help estimate the long-term condensate yield forecast for liquid-rich PVT regions at a specific development well spacing.