The numerical RTA workflow proposed by Bowie and Ewert (2020) has been widely adopted in the industry the last couple of years. It was originally introduced to solve inherent problems associated with complex superposition and multiphase flow effects observed in liquid-rich shale wells. This paper outlines the importance of numerical RTA also for dry gas wells. The simple, fully-penetrating planar fracture model proposed is a useful numerical symmetry element model that provides the basis for the work presented in this paper. Results are given for simulated and field data from the SPE data repository.
The linear flow parameter (LFP) is modified to include porosity ((equation)). The original (surface) gas in place (OGIP) represents the contacted pore volume after completion. With a known (a) well geometry, (b) fluid initialization (PVT and water saturation), (c) relative permeability relations, and (d) bottomhole pressure (BHP) time variation (above and below saturation pressure), three fundamental relationships exist in terms of LFP’ and contacted original (surface) gas in place (OGIP). Numerical reservoir simulation is used to define these relationships, providing the foundation for numerical RTA, namely that wells: (1) with the same value of LFP’, the gas and water surface rates will be identical during infinite-acting (IA) behavior; (2) with the same ratio LFP’/OGIP, producing water-gas-ratio (WGR) will be identical for all times, IA and boundary dominated (BD); and (3) with the same values of LFP’ and OGIP, rate performance of gas and water be identical for all times, IA and BD. These observations lead to an efficient, semi-automated process to perform rigorous RTA, assisted by a symmetry element numerical model.
The numerical RTA workflow proposed by Bowie and Ewert solves the inherent problems associated with complex superposition and PVT effects involving time and spatial changes in pressure, compositions and PVT properties, saturations, and complex phase mobilities, also for dry gases. The numerical RTA workflow decouples fluid initialization data (PVT, initial saturations and relative permeabilities) from well geometry and petrophysical properties (L, xf, h, nf, φ, k), providing a rigorous yet efficient and semi-automated approach to define production performance for many wells.
Contributions include a technical framework to perform numerical RTA for dry gas wells. A suite of key diagnostic plots associated with the workflow is provided, with synthetic and field examples used to illustrate the application of numerical simulation to perform rigorous RTA. Semi-analytical models, time, and spatial superposition (convolution), pseudopressure and pseudotime transforms are not required.
