Formation Pressure Testing In the Dynamic Drilling Environment

摘要

This paper introduces a new technology in which a logging while drilling (LWD) tool is used to pressure test a formation during the drilling process. The Formation Testing While Drilling (FTWD) or GeoTAP* tool described in this paper uses a testing technique similar to wireline formation testers. A probe is extended to the formation and a small sample chamber is used to pressure test the formation. The drilling environment offers many new challenges to pressure testing, however. Wireline tools require a great deal of interaction with the engineer, which is very limited in the drilling environment. Therefore, the new tool has up-link and down-link capabilities and is highly automated for test control. New data compression routines are described that enable not only some of the raw data to be transmitted, but formation permeability (mobility) and test quality estimates to be obtained in real- time. An efficient algorithm is used down-hole to analyze the data where testing parameters and selected pressure data are transmitted to the surface in real-time for continuous monitoring of the test. The drilling environment is more dynamic than is the case for wireline testers. Pressures in the borehole are constantly changing due to several factors. Invasion and the resultant supercharge effect can affect FTWD measurements. Supercharging results from mud filtrate invasion increasing the sandface pressure just behind the mudcake. Additionally it is desirable to keep the mud pumps on while testing to avoid hole deterioration and pipe sticking. Slower hydrostatic pressure changes can also occur due to drill pipe movement causing a swabbing effect. These pressure transients can be mudcake properties and invasion, these effects are simulated. A sensitivity study demonstrates the conditions that affect the stability and accuracy of the FTWD measurements. Field data from the new GeoTAP tool are presented and compared with wireline formation test (WFT) data in test wells. The new tool requires the drill pipe to stop rotating/sliding for about 7 minutes per pressure test. During this time the pumps can be turned on or off. Most of the test results were taken with the pumps on. A "pumps on" GeoTAP tool field example is also presented, and the results compared very favorably with WFT data in terms of the absolute pressure measurement, repeatability and accuracy. The field example demonstrates the robustness of the measurement with three repeat pressure tests that were taken at three different depth points where, in each case, the pressures recorded were within 1 psi. Final conclusions are drawn regarding FTWD technology and its future direction.