Investigation of Wellhead Pressure Decline and Inflow Performance Relationships in Mature Gas Wells of OML 13, Cross River State, South South Nigeria

Wellhead pressure decline in mature gas wells is a diagnostic indicator of reservoir depletion and inflow performance deterioration that requires careful analysis to distinguish between reservoir-driven and near-wellbore damage-related production decline. This study investigates wellhead pressure decline trends and constructs inflow performance relationships for mature gas wells in OML 13, Cross River State. Monthly production and wellhead pressure data for twelve gas wells over a ten-year operating period from 2012 to 2022 were compiled and analysed. Backpressure deliverability tests conducted in 2014 and 2020 provided the basis for constructing and comparing inflow performance relationship curves at two distinct reservoir pressure states. The C and n deliverability coefficients were determined from log-log regression of test data, and the Absolute Open Flow potential at each test date was compared to identify wells where deliverability decline exceeds predictions based on reservoir pressure alone. For six of the twelve wells, deliverability decline rates exceed the reservoir pressure-based expectation, suggesting accumulation of liquid loading in the tubing as a compounding decline mechanism. Plunger lift installation is evaluated as a liquid unloading strategy, with nodal analysis confirming its technical viability for the identified candidate wells. Recommendations are made for a field-wide liquid unloading programme and improved wellbore surveillance protocols. Keywords: wellhead pressure decline, inflow performance, mature gas wells, liquid loading, Cross River State.