Lessons Learnt on How to Do a Successful Pressure While Drilling Tests Despite Challenging Environment in Middle East

Abstract

The pressure testing while drilling (PTWD) is one of the fastest ways how to confirm the reservoir pressure and mobility when planning the advance completions. An accurate evaluation of the pore pressure is helping to avoid the formation damage caused by overbalance and to reduce the drilling risks, associated with lost circulation, drill pipe stuck, as well as hole un-stability.

Some factors affecting the quality of pretests while drilling includes highly deviated and extended-reach horizontal drains; mud systems, such as heavy mud, oil-based mud (OBM), and high solid contents. Other challenges might be associated with unknown washouts area; well losses as well as the urgency of doing the pretests directly after drilling. The mixed lithology with various shale, anhydrite, carbonate, or sandstone content might reduce the permeability and affect the PTWD program’s success. To improve the quality of the PTWD several factors need to be assessed, such as potential loss of the seal due to the washout or fractures; sufficient timing for the mud cake; tightness of the zone above or below the probe; corrective sequence of the test parameters.

There are several scenarios were evaluated to enhance the PTWD performance. First, the mud invasion across the highly permeable formation was expected to form a fast mud cake. The results show that for the cases with mobility of more than 20mD/cP the sufficient repeatable test could be acquired as soon as 2hrs with pumps-off acquisition mode. Secondly, the cases across the tight formation with <10% p.u. were evaluated. Typically, the filtrate invasion in low permeability is very slow and if the PWD acquisition was too early, the pressure tests have no repeatability. In the third scenario, the tests were acquired 4 times across the same depth point with a very low porosity zone and a few hours between each pretest. It was visible how the mud was formed and how the pressure was reduced till the actual formation pressure only after 15 hrs from drilling. To avoid misleading pressure tests, it is recommended to plot the expected fluid gradient on top of a single valid point that has more than ~10 mD/cP mobility. For the cases with lower mobility, it is recommended to obtain three low-volume pretests with short build-up times and to repeat pretests in the nearby footage. The pump-off function is recommended for all pretests. For the cases where it is critical to have a real-time curve, it is recommended to repeat the test with pumps on only after completing the pumps off test. The OBM find to be helpful in wellbore stability and helps to improve the PTWD quality.

The result of this study shows the steps on how to plan the successful PTWD acquisition despite the formation and wellbore challenges. The quality check procedure would allow us to recognize valid tests and avoid making critical decisions based on false responses.