COUPLED DIAGNOSIS OF WELL DELIVERABILITY DECLINE IN MASSIVE OIL RESERVOIRS: QUANTIFYING NEAR-WELLBORE DAMAGE, DEPLETION, AND WATER-ENCROACHMENT EFFECTS

Abstract

Massive oil accumulations, particularly those hosted by thick carbonate bodies with bottom-water support and strong vertical heterogeneity, often exhibit a progressive loss of well deliverability that cannot be attributed to a single mechanism. The observed decline is commonly the coupled outcome of reservoir-pressure depletion, stress-sensitive permeability reduction, increasing multiphase flow resistance during water encroachment or gas liberation, and an additional near-wellbore pressure loss represented by the skin factor. This study develops an integrated diagnostic framework for separating and quantifying these effects without relying on unverified field data. A transparent parametric model is constructed from radial inflow theory, the classical skin concept, Corey-type oil relative permeability, and a pressure-dependent permeability term. The framework is evaluated through six representative scenarios and a local sensitivity analysis. The calculations show that a moderate positive skin of 4 reduces the single-phase geometric productivity component to approximately 64% of its clean-well value, whereas an increase in near-wellbore water saturation from 0.22 to 0.36 reduces the oil relative-permeability component to approximately 54%. When depletion, viscosity change, water encroachment and skin act simultaneously, normalized productivity falls to about 0.30 in the coupled-moderate scenario and to about 0.11 in the coupled-severe scenario. The results demonstrate that diagnosing productivity decline only from rate history can misidentify the dominant mechanism: similar production losses may originate from substantially different combinations of formation damage and saturation redistribution. A practical workflow is therefore proposed that links pressure-transient interpretation, production logging, water-cut evolution, fluid sampling, scale/fines diagnostics and stimulation history to a mechanism-based intervention decision. The study provides a reproducible engineering basis for distinguishing reversible near-wellbore impairment from reservoir-scale depletion and water-encroachment effects in massive oil reservoirs.