Enhancing Gas-Reservoir Characterization by Integrating a ReliableFormation-Testing Permeability Method into the Workflow

摘要

Permeability is one of the most important parameters in determining the productivity index and potential reserves of a producinginterval. 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 variationacross the interval. Moreover, the results are often not available as they are needed to assist in decision making. Nuclear magneticresonance (NMR) logging tools can provide a continuous log of permeability; however, a core study is typically needed tocalibrate the log. Formation testers can determine the mobility (permeability/viscosity), but the viscosity must be estimated todetermine the permeability. This paper presents methods for estimating the viscosity and then deriving permeability from theanalysis of formation tester pretests. The required viscosity estimate could be obtained by considering the properties of the mud system used when drilling, giventhat pretest’s radius of investigation is limited to the invaded zone surrounding the wellbore. The fluid occupying the pore space inthis 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, weare able to validate them by comparing them with the actual core data. We can also use them to quantify the total reservoir flowcapacity (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 couldalready compare or be validated with the total kh result obtained from a long extended drill-stem test (DST) or production test ineach 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. Theprimary goal of this study is to increase the use and confidence of the formation tester’s results.