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The operator has been active in Panna field for several years drilling a main horizontal and two laterals through a window milled in pre-existing 9–5/8″ casing. The laterals vary in length between 800m to 1600m, depending on the carbonate reservoir's constraints. However, interbedded formations with contrasting hardness had the potential to cause stick-slip and lateral vibration. Initially, the operator utilized RSS/MWD to geosteer the BHA within the target formation. However, previously chosen PDC bits caused severe lateral vibrations, stick slip and BHA whirl that resulted in multiple downhole tool failures. It appeared new PDC technology was required. An in-depth study revealed the BHA was exposed to severe stick-slip and lateral vibrations. There were also regular instances of BHA whirl and severe acceleration and deceleration caused by stick-slip. To enhance overall bit/BHA stability, a new-style 8–1/2″ PDC was implemented with innovative depth-of-cut control (DOCC) technology and enhanced stability features. BHA design was also reviewed and minor changes were implemented. The result was a significant reduction in stickslip, lateral vibration and BHA whirl. The new PDC/RSS set a new national Indian record for the longest 8–1/2" offshore section making 3956m of hole in a single run. The bit has since successfully drilled several multilateral drainholes in a single run proving the reliability of the new PDC design and enabling technology. The authors will present the vibration signature of a standard PDC versus the new one with DOCC. The authors will discuss proper RSS/PDC bit selection and demonstrate the benefit of DOCC/PDC on RSS when applied in high angle/horizontal wells. Introduction The Panna field1–3 is located in Arabian Sea about 95 km west of Mumbai, India (Figure 1). It is surrounded by other fields including Mukta, Neelam, Tapti and Bombay High. Together they constitute an important oil producing area of India. The Panna field is jointly owned by BG Exploration & Production India Limited (BGEPIL), who have a 30% holding, the Indian Government's Oil and Natural Gas Corporation Limited (ONGC) who maintain a 40% holding and Reliance Industries Limited (RIL) are partners with a 30% share. The field was discovered in 1977 and is estimated to have approximately one billion bbls of oil and 1.9 trillion cubic feet (tcf) of gas in place. Panna field produces from predominantly two carbonate reservoirs, including the upper A-zone (Oligocene) and lower B-zone (Middle Eocene). Overburden consists of argillaceous sediments of Chinchini, Tapti, Mahim and Bombay limestone followed by Alternations that lies directly on the A-zone reservoir (Figure 2). The Alternations horizon is between 70–100m thick and consists of thin interbedded shale and limestone formations of varying hardness. BGEPIL's current drilling campaign involves re-entering wells through a window milled in pre-existing 9–5/8″ casing to drill multiple fishbone laterals (Figure 3) through the reservoir section utilizing a rotary closed loop (RCLS) drilling system integrated with a resistivity, density and neutron porosity formation evaluation (FE) package (Figure 4). Application Challenges The initial horizontal wells in the Panna drilling campaign utilized steerable motors with LWD tools. However, this BHA experienced sliding difficulties that limited the extent of the horizontal reach. There were also drilling dynamics issues that caused severe tool damage on several occasions. Directional requirements/formation mineralogy while drilling the open-hole sidetrack in the 8–1/2″ horizontal section required the bit to deliver optimum dogleg severity without having an aggressive gauge as this may initiate hole spiraling and compromise wellbore quality and operational efficiency.

TX 75083-3836 U.S.A., fax 1.972.952.9435. AbstractThe Panna Field is located offshore West India and is operated by BG Exploration and Production India Limited. The reservoir is a tight limestone which is drained using multilateral wells. There are significant drilling challenges. Firstly, the well trajectories are complex to reach the reservoir and the laterals are three dimensional profiles requiring precise steering control to maintain position in the productive zones. Secondly, total fluid losses to the fractured limestone are frequently encountered. Thirdly, high levels of vibration and high drilling torque threaten the integrity of the drillstring and ultimately limit the reach to which the lateral sections can extend. A rotary closed loop drilling system had been use to successfully drill the wells, but the desire to improve gross drilling performance, reduce the risk to drillstring and extend the reach of the laterals prompted introduction of a new drilling system which was being tested in the North Sea. This new system integrates a high performance drilling motor with high speed rotary closed loop system to improving overall drilling performance in challenging areas, reduce stress on the drillstring and enable wells to extend past prior reach limits.On the first well; rate of penetration increased by 46%, the distance drilled per BHA increased by over 300% and the longest ever lateral in the field was successfully drilled. Based on the performance noted on the initial well, the new system was used on a further six Panna tri-laterals -completing the drilling campaign ahead of schedule.This paper discusses challenges drilling to and through the Panna field reservoir with the reasoning behind mobilization of the new drilling system to the area at an early phase of its introduction. The overall results of drilling the tri-laterals are then presented.

Planning for the deep vertical exploration well, in shallow waters off the West Coast of India commenced more than a year ahead of drilling operations. The well was targeted to 4500m to explore deep objectives in a four-way closure. Bottom hole pressure was expected to be in excess of 12000 psi and formation temperatures were predicted to be higher than 400°F (205°C). A major challenge was mobilizing special high pressure equipment to an area with limited oilfield infrastructure. The successful drilling and formation evaluation of the well has validated the efficacy of the design philosophy, highlighted the critical importance of mud hydraulics, HP/HT rheology, temperature modeling, real time detection of pressure transition zones, and will significantly impact future exploratory drilling operations in the area.

This paper was prepared for presentation at the 1999 SPE/IADC Middle East Drilling Technology Conference held in Abu Dhabi, UAE, 8–10 November 1999.