Enhancing Gas-Reservoir Characterization by Integrating a Reliable Formation-Testing Permeability Method Into the Workflow

Abstract

Permeability is one of the most important parameters in determining the productivity index and potential reserves of a producing interval. Very few instruments, however, are available for use on discovery and appraisal wells that can make this measurement. Although cores can be used, this process always leaves some uncertainty concerning the in-situ permeability and its variation across the interval. Moreover, the results are often not available as they are needed to assist in decision making. Nuclear magnetic resonance (NMR) logging tools can provide a continuous log of permeability; however, a core study is typically needed to calibrate the log. Formation testers can determine the mobility (permeability/viscosity), but the viscosity must be estimated to determine the permeability. This paper presents methods for estimating the viscosity and then deriving permeability from the analysis of formation tester pretests. The required viscosity estimate could be obtained by considering the properties of the mud system used when drilling, given that pretest’s radius of investigation is limited to the invaded zone surrounding the wellbore. The fluid occupying the pore space in this region must be considered as mud filtrate, especially in permeable zones that are of greatest potential. After obtaining the viscosity estimate and expressing the results of the formation pretest analysis in terms of permeability, we are able to validate them by comparing them with the actual core data. We can also use them to quantify the total reservoir flow capacity (kh) by calibrating the NMR permeability log or by up-scaling the results obtained at several depth points across the zone. Total reservoir-flow capacity (kh) can then be used to accurately forecast production capacity. In addition, this result could already compare or be validated with the total kh result obtained from a long extended drill-stem test (DST) or production test in each zone if they are performed. In this paper, we address the steps involved in this process; field examples are included to validate all of these approaches. The primary goal of this study is to increase the use and confidence of the formation tester’s results.