Formation Pressure Testing In the Dynamic Drilling Environment

摘要

This paper introduces a new technology in which a loggingwhile drilling (LWD) tool is used to pressure test a formationduring the drilling process. The Formation Testing WhileDrilling (FTWD) or GeoTAP* tool described in this paper usesa testing technique similar to wireline formation testers. Aprobe is extended to the formation and a small samplechamber is used to pressure test the formation. The drillingenvironment offers many new challenges to pressuretesting, however.Wireline tools require a great deal of interaction with theengineer, which is very limited in the drilling environment.Therefore, the new tool has up-link and down-link capabilitiesand is highly automated for test control. New datacompression routines are described that enable not only someof the raw data to be transmitted, but formation permeability(mobility) and test quality estimates to be obtained in realtime.An efficient algorithm is used down-hole to analyze thedata where testing parameters and selected pressure data aretransmitted to the surface in real-time for continuousmonitoring of the test.The drilling environment is more dynamic than is the casefor wireline testers. Pressures in the borehole are constantlychanging due to several factors. Invasion and the resultantsupercharge effect can affect FTWD measurements.Supercharging results from mud filtrate invasion increasingthe sandface pressure just behind the mudcake. Additionallyis desirable to keep the mud pumps on while testing to avoidhole deterioration and pipe sticking. Slower hydrostaticpressure changes can also occur due to drill pipe movementcausing a swabbing effect. These pressure transients can bemudcake properties and invasion, these effects are simulated.A sensitivity study demonstrates the conditions that affect thestability and accuracy of the FTWD measurements.Field data from the new GeoTAP tool are presented andcompared with wireline formation test (WFT) data in testwells. The new tool requires the drill pipe to stoprotating/sliding for about 7 minutes per pressure test. Duringthis time the pumps can be turned on or off. Most of the testresults were taken with the pumps on. A "pumps on" GeoTAPtool field example is also presented, and the results comparedvery favorably with WFT data in terms of the absolutepressure measurement, repeatability and accuracy. The fieldexample demonstrates the robustness of the measurement withthree repeat pressure tests that were taken at three differentdepth points where, in each case, the pressures recorded werewithin 1 psi. Final conclusions are drawn regarding FTWDtechnology and its future direction.