Unconventional Near-Critical Fluid Characterization and GOR Modeling: Wolfcamp Formation in Permian Basin
Authors: Deng J. , Tan J. , Wolfram E. , Muralidharan V. 
Published: October 2023

PVT fluid systems vary across the Delaware Basin in Texas, transitioning 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 GOR behavior of a near-critical fluid system is important for meaningful reserves estimation, optimal well spacing, and efficient completion design. GOR behavior is controlled by PVT fluid system, the initial reservoir conditions, rock-fluid properties, the effective SRV (Stimulated Rock Volume) achieved by hydraulic fracturing, the flow regimes in the dual matrix-fracture system, inter-well communications, and well drawdown strategy. The system pressure and flow regimes developed in the Trilinear system (Brown et al. 2011) of inner fracture plane, outer fracture network, and surrounding tight matrix control the GOR profile shape. The producing GOR of Wolfcamp formations in the Delaware Basin typically exhibits a long GOR transient plateau, which is controlled by the PVT fluid system, the degree of undersaturation, the Linear Flow Parameter (LFP), fracture network complexity, and the contacted OOIP in the SRV.

In this study, numerical multiphase RTA modeling was performed for Wolfcamp wells in the near-critical PVT regions. GOR remains constant during the linear transient flow regime; late-time gradual rise in GOR is controlled by LFP/OOIP ratios, which are determined by history matching the linear flow to the boundary-dominated flow curve. The long-term production forecast was accomplished using an integrated EOS (Equation of State) compositional model. The forecast captured GOR behaviors for section-level infill development, which demonstrated long period of constant GOR followed by a gradual rise in GOR. The EOS model was characterized to represent the near-critical fluid system and was tuned using the regional PVT control points. The simulation model was upscaled from the regional subsurface geomodel with facies-controlled petrophysical properties. By incorporating the HFTS II project learnings (Bessa et al. 2021), a GOHFER fracture model was built based on standard completion designs, from which the representative SRV profiles were extracted. The key rock-fluid properties, SRV and completion efficiency, and well spacing configurations were investigated through the history matching process and sensitivity analysis.

The integrated analytical and numerical modeling workflow captures the generalized GOR profiles for the Wolfcamp formation in Delaware Basin for three PVT regions: Volatile Oil, Near-Critical Fluid, and Rich Gas Condensate. It also provides a systematic approach for GOR profile construction for given PVT fluid system. Late -time rising GOR in Delaware Basin do not adversely affect the oil EUR as approximately half of the oil recovery was still achieved during rising GOR period. High pressure gradient and high degree of under-saturation is one of the main reasons for long period of constant GOR followed by gentle climb at late-time, which also provides solution gas support for oil recovery. Finally, a regional performance coefficient was proposed for ranking field development based on the PVT fluid system, the degree of under-saturation, and the completion efficiency.