Decline-curve analysis with decline rate for improving reserves estimation in gas reservoirs

Abstract

Over decades, the rate decline analysis has been the industry's bedrock for reserve estimation. The Arps method has gained credibility since its introduction in 1945, particularly in oil reservoirs. However, gas reservoirs present challenges due to handling increasingly deeper geologic horizons with higher pressure and low-permeability formations, rock compaction, and overpressured reservoirs, among other challenges. This study shows a novel decline-curve analysis involving the decline rate rather than the flow rate. The proposed methodology applies to production data from a gas reservoir with varying operating conditions during the boundary-dominated flow (BDF) period. The method does not involve any pseudo-functions.The proposed method is independent of the variation of flow rate and pressure. The PVT data (viscosity, compressibility, z-factor) aids the construction of a type-curve involving the viscosity-compressibility ratio with the recovery factor without any reservoir-specific information. The decline rate with adjustment for changing operating conditions leads to a single decline-curve plot involving the viscosity-compressibility ratio (λ) and dimensionless pressure (pD) without any correlating parameter. Two synthetic examples verified the proposed approach, and two field datasets validated it. Overall, the proposed method strengthens the reserve estimation with an independent process for meeting corporate and regulatory requirements in conventional and tight-gas reservoirs.