Estimation of Skin Factor for Wells in Tight Gas Reservoirs

Abstract

Skin factor is a common measure of formation damage and is an important input parameter for reservoir simulation of tight gas (TG) reservoirs. We explored, with reservoir simulation, production from TG reservoirs without hydraulic fracturing and incorporated a new model for drilling induced skin. We undertook an extensive sensitivity study to estimate the TG reservoir’s skin factor. In the study, we developed a three-concentric-layered damaged zone parameterization. We explored different borehole sizes and found that the skin factor decreases when formation permeability anisotropy increases, assuming the same horizontal invasion depth and the same damaged permeability ratio. A near-wellbore reservoir simulation model was selected for our application. In the study, we analyzed the effect of the filter cake during over-balanced drilling operation with water-based mud and its effect on the gas production. The distributions of water saturation and effective gas permeability in the example have been calculated during invasion and after production. The total skin factor is calculated based on averaging method. We found the filter cake has a minor effect on the depth of invasion due to the low effective permeability of the formation. Core testing of mud invasion/production are needed for verification.