Robust and Simple Graphical Solution For Wireline Formation Tests: Combined Drawdown and Buildup Analyses
Authors: Kasap E. , Huang K. , Shwe T. , Georgi D. 
Published: December 2007

Wireline Formation Testers (WFT) can provide valuable, cost-effective information on undisturbed reservoir pressure (P*), vertical pressure gradients, in addition to formation fluid samples, and an estimate of near-wellbore permeability. Various analysis techniques have been borrowed from the well testing studies and adapted to analyze WFT-measured drawdown and buildup data.

Spherical-flow analysis utilizes early-time data and usually gives a reliable estimate of permeability. For P* determination, cylindrical-flow analysis is preferred because it focuses on late-time buildup data. However, the cylindrical-flow analysis has its drawbacks. Late-time data crucial for cylindrical-flow analysis, especially in low-permeability formations, but long testing periods are not desirable because of potential tool "sticking" problems. Even on long tests, the cylindrical-flow period may not occur or may not be detectable on WFTs. When it does occur, permeability estimates derived from the cylindrical-flow period may be incorrect and their validity is difficult to judge.

We introduce a new analysis technique that simplifies the interpretation of WFT pressure-transient data. The theory is based on the geometric factor concept, which is equally valid for drawdown, stabilized flowrate, and buildup data. The technique is less sensitive to data quality than other methods, does not require long buildup periods for low-permeability formation testing, and can be implemented with a simple graph of pressure vs. formation flowrate, from which both near-wellbore permeability and P* are readily determined.

Three numerical simulation cases are set to test the theories of spherical, cylindrical, stabilized drawdown and the newly introduced pressure vs. flowrate analysis techniques. Occurrence of these flow regimes are tested by comparing their predicted slopes with the slopes of simulation run data. Conventional and new analysis techniques are applied to two field-measured data sets and the results are compared.

Background

A wireline formation test is initiated when a probe from the tool is set against the formation. A measured volume of fluid is then withdrawn from the formation through the probe. The test continues with a buildup until the pressure stabilizes. Pressure in the tool is continuously monitored throughout the test.

WFTs provide formation fluid samples and produce high-precision pressure and permeability profiles and vertical pressure gradients, which, in turn, can be used to locate fluid contacts. Wireline formation testing is much faster compared with the regular pressure transient testing. Total drawdown time for a formation test is just a few seconds and buildup times vary from less than a second (for permeability of hundreds of mD) to half a minute (for permeability of less than 0.1 mD), depending on the ratio between drawdown volume and formation permeability. Wireline formation testing can also generate detailed profiles of formation heterogeneity (degree of damage to the formation, existence of thin shale laminations, and natural or induced fractures). Because the investigated volume can be small (a few cm3), the details of reservoir heterogeneity on a fine scale are given with better spatial resolution than is possible with conventional pressure transient tests. Furthermore, WFTs may be preferable to laboratory core permeability measurements because WFTs are conducted at reservoir in-situ conditions and provide formation fluid samples.

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