Estimating Thickness-Independent Horizontal and Vertical Permeabilities With Packer/Probe Wireline Formation Testers

Abstract

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper IPTC 13912, ’A Novel Analysis Procedure for Estimating Thickness- Independent Horizontal and Vertical Permeabilities From Pressure Data at an Observation Probe Acquired by Packer/ Probe Wireline Forma tion Testers,’ by M. Onur, SPE, Istanbul Technical University, and P.S. Hegeman, SPE, I.M. Gok, SPE, and F.J. Kuchuk, SPE, Schlumberger, pre pared for the 2009 International Petroleum Technology Conference, Doha, Qatar, 7-9 December. The paper has not been peer reviewed. A spherical-flow cubic-analysis procedure has been developed for estimating horizontal and vertical permeabilities, kvh and kv, respectively, solely from pressure-transient data acquired at an observation probe of a dual-packer/probe wireline formation tester (WFT) for any wellbore-inclination angle. The procedure is based on an analytical spherical-flow equation and a conventional spherical-flow-analysis graph of the observed drawdown or build-up pressures. The procedure does not require knowledge of the formation thickness, h, or observing the radial-flow regime at the probe. Introduction Over the last 4 decades, wireline formation testing has become widely accepted for reservoir-pressure pro-filing, fluid sampling and identification, interval-pressure-transient tests (IPTTs), and in-situ stress testing. IPTTs conducted with packer/probe formation testers provide dynamic permeability and anisotropy information with high vertical resolution along the wellbore. Permeability is one of the most important parameters in reservoir management and well performance. Permeability and permeability anisotropy (kv /kh ratio) strongly affect all reservoir-displacement processes. Thus, it is increasingly important to determine these values as operators shift their focus from primary recovery to secondary and tertiary recovery. Fig. 1 shows a schematic of a packer/probe interference test (usually referred to as an IPTT) in a deviated well in an anisotropic formation that is bounded vertically and is laterally infinite. The schematic represents a general configuration that is valid for all inclination angles, θw, of the well-bore. For instance, if θw=0°, the IPTT is considered conducted in a vertical well, whereas if θw=90°, the IPTT is considered conducted in a horizontal well. In these tests, a dual packer is set against the formation to serve as a flow source. The pressure transients are measured at both the packer interval and the observation probe. Any pressure change at the observation probe caused by flow from the packer interval clearly indicates communication within the formation between the two locations. Interpretation of packer and probe data provides permeability values in both the vertical and horizontal direction. Furthermore, the near-wellbore heterogeneity can be characterized from IPTTs. Interpretation of packer/probe IPTT data begins with an independent analysis of each pressure data set. The first step is to identify the flow regimes on the basis of conventional pressure-derivative analysis. Then, initial estimates of parameters such as spherical and horizontal permeabilities are obtained from special straight-line (semilog and spherical) analyses based on flow regimes identified from the log-log diagnostic graphs of the draw-down/buildup and/or deconvolved responses. Construction of a formation model from existing openhole-log data is essential for the interpretation. Log data provide porosity and rock compressibility, while fluid analysis provides fluid compressibility and viscosity. All available data (including log, core, and pretests) are used for parameter initialization of the formation model before nonlinear-regression analysis. Then nonlinear-regression analysis is performed to optimize the key parameters such as horizontal and vertical permeabilities, skin, and wellbore- or tool-storage coefficient. The final step of the analysis involves verification and sensitivity analyses of the history matches of the pressure data.