Abstract
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 realtime. 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. This environment is modeled using a finite element simulator to show the effects these dynamics might have on pressure measurements. The results show that even in this highly dynamic state it is possible to obtain reliable pressure measurements.
Field data from the new GeoTAP tool is 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 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.
