A Two-Phase Transfer Function for Simulation of Fractured Ultratight Gas-Condensate Reservoirs

Abstract

In modeling of fractured ultratight gas-condensate reservoirs it is particularly important to represent liquid dropout, and its impact on fluid flow behavior, accurately. This, in turn, allows for design and optimization of reservoir production strategies, to mitigate potential production loss below the dewpoint pressure. Unfortunately, the traditional transfer functions used to describe the mass transfer between matrix/fracture segments in dual-porosity models (DPM’s) do not explicitly represent the complex physics of the inherent multiphase problem, which may complicate the application of existing simulators. Therefore, the objective of this work is to develop a two-phase transfer function that improves the modeling of gas-condensate systems via a DP representation. Commercial simulators typically offer a simple matrix/fracture mass transfer model to perform calculations in DP systems: The default shape factors are based on a pseudo-steady state (PSS) approximation, and for multiphase problems, the transfer function of Kazemi et al. (1976) is commonly used. Without sub-gridding, the traditional transfer functions cannot represent the extended transient behavior observed in ultratight fractured reservoirs. Furthermore, in compositional modeling of gas-condensate reservoirs, the averaging of matrix block properties masks the compositional changes near the matrix/fracture interface, that is critical to prediction of production behavior below the dewpoint. To overcome the limitations of the current models, we introduce a new transfer function that includes 1) a time-dependent shape factor and 2) a modified representation of the two-phase flow. We utilize the correction factor introduced by Zhang et al. (2022) allowing the shape factors to vary with time. For two-phase problems, instead of evaluating the mass transfer at an average pressure and composition, we propose a novel approach that evaluates the transfer function at conditions that reflect/approximate the pressure and composition/saturation gradients within the matrix block. By approximating the fluid state near the matrix/fracture interface, we represent the liquid buildup that dictates the fluid mobility more accurately. The open-source MATLAB Reservoir Simulation Toolbox (MRST) was used as a platform for the work presented in this paper: The existing single-porosity (SP) compositional model of MRST was first extended and validated (with the analytical solution for the single-phase pressure diffusion equation) to allow for DP modeling and simulation of condensate systems. We present calculation results for a single-block DPM, representative of a fractured ultratight gas-condensate reservoir, to demonstrate the limitations of the traditional mass transfer modeling approach. We then introduce the new transfer function and compare all calculation results with fine-grid SP reference models. In this work, we consider two condensate fluid descriptions: 1) a 4-component analog rich gas, and 2) a realistic 24-component fluid description from a gas-condensate reservoir. We demonstrate that the new transfer function represents the physical mechanisms at play more accurately and provides for a substantial improvement over the traditional formulation in terms of the amounts and compositions of the produced phases. The proposed method is motivated by the physics of the problem and requires no sub-gridding. The enhanced simulation of gas-condensate systems enables improved decision making in reservoir management and supports optimization of field development plans. In addition, the improved calculation of the surface streams, obtained from the new method, provides for a more accurate design of field surface facilities.