Formation Pressure while Drilling, a valuable alternative in difficult drilling environment in Northern Germany
Authors: Bildstein Joachim , Strobel Joachim 
Published: December 2007

Highly deviated wells, either horizontal or slanted, are difficult to evaluate with respect to the acquisition of formation pressure data through wireline based technology.

In Northern Germany, the wireline based approach has proven to be very critical and unfortunately often not successful. Tight gas reservoirs at high temperatures and different pressure regimes cause tools to fail or to get stuck and on the other hand high porosity high permeability oil reservoirs can not be logged because of unstable clay formation. Pipe Conveyed Logging technology provides an alternative, however it is time and cost intensive as well as failure prone.Overall, this task has proven to be demanding with respect to data quality and rig time.

The introduction of LWD technology based Formation Pressure While Drilling (FPWD) tools provides an attractive alternative to the existing wireline technology.

This paper describes the successful use of LWD based formation pressure devices in slanted wells targeting high permeable oil bearing sandstones.

Another successful use has been in made in a field with a highly deviated, high temperature (155 C°/310 degF) well targeting low porosity, low permeability gas bearing sandstones below 5000 m (~16.400 ft) depth.

FPWD data from both fields together with validation data will be presented. Additionally, the results from mobility calucations will be compared to wireline and core values.

Introduction

Formation Pressure and Mobility Data are critical pieces of information when it comes to the evaluation of any reservoir, which has been penetrated by well. The acquisition of data was historically done by means of Wireline Formation Tester (WFT) or through Drill Steam Tests (DST). In difficult drilling environment, especially deviated wells, unstable formations or HPHT reservoirs, the use of wirline tools bears the risk of leaving the site without the required information or cost and time consuming operations. Formation Testing while Drilling (FTWD) technology offers a new method to obtain the data under these circumstances. The tools have to adapt to the more dynamic drilling environment as well as meet the hight data quality standards of wireline based systems. This paper presents three field examples in which LWD tools have been used to optain pressure and mobility information during the drilling process.

In a horizontal well environment with high permeable sandstones, the development of huge breakouts shortly after drilling has been completed, prevented the use of wireline technique. Previous attemps to aquire pressure data through Drillpipe Conveyed Tools, resulted in incomplete operations or non valid data.

In all cases, previous wells in similar environments had a longterm record of non- obtained information related to wellbore conditions and extensive lost time related to operational problems. This past experience was the major drive to open the door to the new technology.

Available Technology

The market for FTWD is developing rapidly and the following information on available technology can only represent the technology used in the presented field examples. With a growing acceptance and proven experience of the FTWD tools, Service Companies are quick to respond to client needs and start to offer a range of tools for different borehole sizes. Sampling is not available yet, however this is only a question of time. The reference list contains information regarding published papers for the available services.

The GeoTAP* (*Mark of Halliburton) tool design is similar to many probe style wireline tools, where a donut shaped rubber pad forms a seal around a metal snorkel (see Fig.1). The GeoTAP tool operation is pre-programmed (at surface) with a timed sequence of events (see Fig. 2). The time to operate is about 7 min and the timed events can be altered by sending a downlink signal to the tool.The preferred mode of operation is with pumps on, though operation is possible with the pumps turned off. When the pumps are on, basic test data is pulsed in real time to surface at a 6 bit/sec rate. A summary with fit coefficients is send at the end of the test. The tool allows a 10 or 30 cc pretest at a drawdown rate of 1–2 cc/sec. If an error is detected or the system loses power, the probe is automatically retracted.