Lessons Learnt in Optimizing Formation Pressure Testing While Drilling in Tight and Low Mobility Formations Across Western Offshore, India
Authors: Telang Viraj , Deshpande Chandrashekhar , Shukla Pradeep 
Published: November 2015

There is increased confidence in LWD data because of the availability of new technology. Acquiring the maximum amount of data during drilling is standard practice when drilling highly deviated or horizontal wells to counteract high rig operating costs. In some cases, LWD services are completely replacing wireline services. Until just a few years ago, the wireline formation tester was the most common service used for formation pressure testing. Today, because of well complexity, the use of wireline testers is limited. High risks of tool sticking, invasion effects and additional rig days are some of the important factors giving LWD formation pressure testers a leading edge over its wireline counterpart.

Baker Hughes' formation pressure testing-while-drilling tool, TesTrakTM, was deployed for ONGC, India in March 2014 as a part of the new LWD contract. The LWD formation testing tool was used in more than 50 wells, displaying exemplary performance with a sealing efficiency of more than 95%. Equipped with state-of-the art-technology, the LWD formation pressure testing tool saved a huge amount of rig time. The tool delivers real-time answers for wellbore connectivity, pressure depletion, reservoir compartmentalization, fluid identification and drainage. In addition to reservoir characterization, the tool also aided perforation decisions.

The Mumbai High field comprises of two blocks: Mumbai High North (MHN) and Mumbai High South (MHS). The blocks are divided by a shale barrier that is used to assist in independent exploitation of reserves at the north and south fields of Mumbai High. Various oil and gas reservoirs, namely, (from top to bottom), L-I, L-II, L-III, L-IV and L-V, basal clastics and fractured basement are present on the Mumbai High project field. L-II and LIII are primarily the limestone oil reservoirs of Miocene age, and are further classified into several layers. [S.K. Mitra et.al., ONGC, Increased Oil Recovery From Mumbai High Through ESP Campaign; Offshore Technology Conference and Kharak Singh et.al. Mumbai High Redevelopment – Geo-Scientific Challenges and Technological Opportunities; 5th Conference & Exposition on Petroleum Geophysics][5,6,7] The FPWD service was run extensively in the Panna-Bassein-Heera block as well. This block located east of Mumbai High/Platform and south of Surat Depression has three distinct north-south to northwest-southeast trending tectonic units which lose their identity in Miocene. The western block is a composite high block dissected by a number of small grabens. The Central graben is a syn-sedimentary sink during Paleogene and Early Neogene. The eastern block is a gentle eastward rising homocline. The target zone is Bassein and all the drain holes in the field are drilled and completed in this zone. (From the information available on dgh official website)[8] Pressure tests are often recorded in the Bassein and Mukta formations.

While testing in high-mobility formations, the tests were completed in a shorter time because of the controlled drawdown rate maintained by the tool. But, against tight, supercharged or low-mobility formations, it took more time to obtain stabilized pressure readings. Rugose hole conditions and dynamic mud losses, often observed in this field, made pressure testing even more challenging. To optimize testing in such formations, a detailed data set encompassing the testing parameters for the tool, formation type and other important observations was compiled after every job performed. With meticulous pre-job planning and real-time log analysis, test points were chosen in each zone to be tested. A detailed study of the data set aided in the selection of the test type and selection of parameters while testing in the same zone or zones with a similar log response. The lessons learnt from the previous formation pressure testing experience, coupled with efficient real- time monitoring from the base and good communication with the rig team and base team during the job, enabled testing time to be reduced significantly, thus saving rig time and simultaneously giving the data before the BHA is pulled out of hole.

This paper presents the lessons learnt from the LWD formation pressure testing experience in India, the results obtained from various jobs, the accuracy of the data, the readings across various layers in Mumbai Offshore Basin (Mumbai High North and South and Neelam- Heera) and suggests best practices developed for the selection of optimized test parameters to increase accuracy, reduce testing time and reduce rig time without compromising data quality.