Formation Testing While Drilling, a New Era in Formation Testing

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 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. 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 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. Introduction The ability to record formation pressure during the drilling process has been sought after for many years. Patents dating back as far as drill stem testing (DST) have claimed to achieve this goal. 1,2,3 In this same time period (1950's) wireline testing was also emerging.4,5 Drill stem testing evolved into tools connected to the drill stem and used for testing the formation shortly after drilling. Now WFT and DST tools consist of a complex set of downhole hardware and services with an array of options, but the fact remains that wireline testing services have displaced much of the DST market. Now LWD is displacing many of the traditional wireline logging tools. The wireline tester is one of the most challenging of the services to commercialize into the LWD market. First and foremost, for a FTWD tool to be successful it must withstand the rigors of the LWD environment. The same probe packer concept that has been highly successful in WFT tools since the 1950's was implemented in the GeoTAP FTWD service presented in this paper. Field testing started in early 2002 and helped to harden and refine the prototype design. In early 2003 the GeoTAP tool was deployed as a worldwide commercial service. In most cases WFT tools perform pressure tests in a matter of minutes. Because of this, WFT tools are highly interactive tools. That is to say, the operator and observers are constantly changing the test plan based on the results just obtained. In fact, it is now possible to have a team of experts observe a WFT in action using remote monitoring software over the Internet in real-time. Most LWD tool strings rely on mud-pulse telemetry with a limited bandwidth. Achieving this high level of interactivity is another of the challenges in the LWD environment. The GeoTAP service employs new approaches to analyzing the pressure transients in real-time. By characterizing the pressure data downhole and transmitting a select set of pressure data and curve parameters, it is possible to recreate the pressure curve that graphically represents the entire pressure transient. This visual representation is supplemented with selected pressure points and a statistical evaluation, so the quality of the pressure test can be assessed in real-time.

Challenges in Drilling in Campos Basin

“…4), that have been employed, sometimes to pressure-test a formation during the drilling process. 8 With a FTWD tool, it is possible to obtain direct pore pressure measurements as a well is drilled with accuracy and precision comparable to that of wireline formation testers. A FTWD tool also minimizes the rig time, risk, and cost that would otherwise be incurred by running wireline or pipeconveyed formation test tools to acquire critical reservoir pressure and mud density information.…”

Formation Tester While Drilling Experience in Caspian Development Projects

“…Similar to these wireline tools, it uses a donut-shaped rubber pad to form a seal around a metal snorkel. 5 When this probe configuration is pressed against the wall of a wellbore, the snorkel penetrates the mudcake and contacts the formation to make a pressure measurement. The external elastomeric pad has been designed to withstand a drawdown pressure differential of 6000 psi.…”

“…The tool is capable of operating while the rig pumps are active (i.e., pumps-on) with no apparent detrimental effect on data quality. 5,6 Having the pumps on is a preferred mode of operation because it reduces the potential of differential sticking and keeps the wellbore and mud system conditioned. If a mud motor is used, the preferred mode of operation is "pumps-off" due to the relatively high vibration motion from the mud motor stator/rotor interaction while pumping.…”

New Drilling Optimization Technologies Make Drilling More Efficient

“…5 Also, LWD imaging tools 6 , and Seismic While Drilling (SWD) 7 , which have significantly improved the quality of the optimization services.…”