A 3D analytical model has been developed to improve the analysis of the transient pressure response of a reservoir to wireline formation testing (WFT). The mathematical model was used to evaluate the validity of current interpretation techniques and to investigate the sensitivity of transient pressure behavior to wet/bore parameters.
WFT is an openhole-logging technique where data are used to determine initial reservoir pressure; vertical pressure distribution along an open hole, fluid contacts, and formation permeability. Formation permeability is deduced from the pressure behavior observed during the test.1–8 WFT pressure data are typically noisy, and interpretation is based on simplified flow configurations; thus, until now, only a qualitative estimate of formation permeability from WFT data has been possible.
WFT consists of one or two drawdown periods and a buildup period. A WFT tool typically has a probe, a pressure gauge, and one or two pretest chambers. Fig. 1 gives a schematic of the WFT tool. The test is performed after the packer on the tool squeezes the mudcake out and properly isolates the formation. The probe is inserted into the formation through the mudcake, and formation fluid is withdrawn until the first chamber is full; the second chamber, when present, is filled at a higher flow rate. Then, the fluid withdrawal is stopped.
During the drawdown and subsequent buildup periods, the pressure at the probe is recorded by a pressure transducer. Fig. 2 illustrates the flow-rate schedule and the expected pressure behavior at the probe for a WFT tool.
In recent years, considerable effort has been directed to obtaining the formation permeability from WFT data. The use of the WFT pressure data for permeability evaluation was first proposed by Moran and Finklea.6 Later, Stewart and Wittmann7 extended this study.