A new practical method for reserve estimation and production forecast of gas wells exhibiting gas–water two-phase flow

This paper addresses the challenge of reserve estimation and production forecasting for gas wells affected by gas–water two-phase boundary dominated flow (BDF). Traditional production data analysis models for gas reservoirs often assume water as an immobile phase, a premise invalid in cases such as tight gas sandstone and during flowback periods of multi-fractured horizontal wells where formation water flow is significant. The study focuses on the nonlinearity in the governing flow equations due to pressure-dependent phase properties and saturation-dependent relative permeabilities in gas–water systems. We propose an approximate method to estimate saturation–pressure relationships during reservoir depletion, crucial for evaluating gas and water phase mobilities. Additionally, a density-based production data analysis method is developed for reserve estimation and production forecasting in scenarios involving gas–water two-phase flow. The proposed models were validated against finely gridded numerical simulations with controlled input parameters. The effectiveness of these formulations for calculating original gas-in-place (OGIP) and original water-in-place (OWIP) is demonstrated through a field case from the Sulige tight gas reservoir. This study introduces novel approaches that extend beyond existing literature, providing specific, quantifiable improvements in saturation–pressure relationship evaluation and gas and water phase mobility evaluations during BDF.