Original Development of a Geomechanics-Constrained Decline Curve Analysis Framework for Compaction-Driven Productivity Impairment in High-Porosity Turbidite Reservoirs, Deepwater Nigeria

Productivity decline in high-porosity deepwater turbidite reservoirs is driven by a combination of reservoir pressure depletion, water encroachment, and compaction-induced permeability reduction, and the contributions of compaction to total productivity decline cannot be distinguished using conventional Arps decline curve analysis, which is not formulated to resolve geomechanical effects on reservoir flow properties. This study develops an original decline curve analysis framework that incorporates geomechanics-based compaction constraints to separate and quantify compaction-driven productivity impairment in high-porosity turbidite reservoirs of deepwater Nigeria. The theoretical framework derives modified Arps-type decline equations in which the decline exponent and initial decline rate are expressed as functions of effective stress-dependent permeability, using an original compaction permeability function parameterised from laboratory compaction tests on unconsolidated turbidite sand samples from deepwater Nigerian cores. The framework is inverted using a Levenberg-Marquardt non-linear least squares algorithm to simultaneously estimate conventional Arps decline parameters and compaction correction factors from field production data. Validation on three deepwater Nigerian fields with independent compaction measurements from 4D seismic confirms that the geomechanics-constrained framework recovers compaction permeability reduction factors within 12 percent of independently measured values. Application to production decline diagnostics demonstrates that compaction accounts for 18 to 34 percent of total productivity decline in fields with initial porosities exceeding 32 percent. Keywords: decline curve analysis, compaction, permeability reduction, turbidite reservoir, deepwater Nigeria.