Formation Evaluation with the Wireline Tester - Merits and Shortcomings

Abstract

The wireline formation tester has proved to be a valuable tool for formation evaluation, but some tests still lead to misleading interpretations while others are inconclusive. Longer flow periods and more tests are generally required for evaluation of low-permeability sands. No particular advantages or disadvantages were found for tools with large, small and/or segregated chambers. The methods of interpretation presently in use have proved to be quite accurate; however, care must be exercised to determine that the conditions required for use of these methods are satisfied and that the tool functions properly. Errors in the measurement or recording of recoveries have probably resulted in many misleading interpretations; simply stating all volumes in the same units would speed interpretation and be helpful in recognizing errors. A single chart for interpretation of formation test recoveries from all sizes of chambers is presented. The surface pressure is helpful in the detection of a leaky seal, but it should not be used to correct for losses caused by leaks. Water resistivity data may be misleading when used to predict whether a zone would produce water and hydrocarbons or water alone. The wireline formation tester is a dependable tool for obtaining pressure measurements; however, the estimation of formation permeability from pressure build-up has been found to be qualitative at best. Introduction The wireline formation tester was introduced to the industry in 1955 by the Schlumberger Well Surveying Corp. It proved so successful that thousands of tests have been run to date and most logging companies now offer this service. It is a valuable addition to the array of wireline tools currently being used for formation evaluation and is the only one, other than the side-wall sampler, that obtains physical evidence of formation contents.