Explicit Determination of Reserves for Variable-Bottomhole-Pressure Conditions in Gas Rate-Transient Analysis

Abstract

Summary Current rate-transient-analysis tools for gas wells producing under boundary-dominated-flow (BDF) conditions largely rely on the deployment of the Arps empirical decline models (Arps 1945), or liquid-based analytical models rewritten in terms of pseudofunctions. Recently, Stumpf and Ayala (2016) demonstrated that, contrary to common practice, decline exponents (b) used in Arps’ hyperbolic equations when applied to gas-well analysis can be rigorously estimated before any field-production data are collected. This determination is solely dependent on gas pressure/volume/temperature (PVT) properties and prevailing constant-bottomhole-pressure (BHP) specification for volumetric, single-phase gas-flow conditions. In the study, we extend that work to a more-realistic variable-BHP condition, which is the most common production-specification condition, in terms of the ratio of changing BHP to average reservoir pressure. The decline exponent (b) is thus rederived, and it is shown that under such conditions, variable BHP hyperbolic decline coefficients become solely dependent on fluid PVT properties and take their largest possible magnitude compared with constant-BHP production. Step-by-step analysis procedures are presented that enable explicit and straightforward estimation of original gas in place (OGIP) and other reservoir properties by universal-type-curve and straight-line analysis. Finally, several cases using simulated and field data are discussed in detail to validate the capabilities of the proposed approach.