Abstract
Most of interpretation and analysis procedures developed for pressure transients acquired by multi-probe and packer-probe wireline formation testers (WFTs) are used to conduct are based on the slightly compressible fluid of constant viscosity and compressibility. Hence, these interpretation and analysis procedures apply for oil and water bearing formations. There is a concern that the interpretation/analysis methods based on the assumption of slightly compressible fluid may not be applicable in the case of testing a single-layer or a multi-layer gas zone(s) with the effects of nonlinear gas properties including non-Darcy flow for multi-probe or packer-probe wireline formation testers. In the literature, to the best of our knowledge, there is no a comprehensive study investigating the validity of the above stated assumption for the interpretation of WFT pressure transient data in gas zones.
In this work, variety of cases considered for investigating the effect (or sensitivity) of non-linear gas flow on the pressure transients from multi-probe and packer-probe wireline formation testers (WFTs). These effects include gas gravity, variation of gas viscosity and compressibility with pressure, non-Darcy flow, position of active (flowing) and observation probes, mechanical skin and radius of skin (or invaded) zone, and reservoir heterogeneity in the vertical direction. A three-dimensional r-θ-z single-phase-gas fully-implicit finite-difference model for a limited-entry vertical well has been developed for the purpose of this investigation. The results show that for multi-probe wireline testers, the sink (or the flowing) and horizontal probe pressure responses are highly affected by the effects of the non-Darcy flow and invaded zone, while the vertical probe pressures are mainly influenced by the properties of the uninvaded zones with non significant non-Darcy flow effect. For packer-probe testers, similar results are obtained. Both synthetic cases are presented to confirm the theory and procedures developed in this work.
