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Abstract
The 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.

Panna Field Overview
The Panna Field is located offshore India in the Gulf of Cambay, 95km north-west of Mumbai, as shown in FIGURE 1. It is jointly operated by BG Exploration & Production India Limited (BGEPIL), who have a 30% holding, and the Indian Government’s Oil and Natural Gas Corporation Limited (ONGC) who maintain a 40% holding. Reliance Industries Limited (RIL) are partners with a 30% holding. The field was discovered in 1977 and was estimated to have original oil in place of one billion barrels and original gas in place of 1.9 trillion cubic feet (tcf). Primary production is from the Bassein limestone which is approximately 1,750m True
Vertical Depth (TVD). It consists of two intervals, the Middle Eocene Bassein B and Early Eocene Bassein B, separated by an unconformity1.

Recently, to expand production and recovery, a program of multi-lateral infill drilling has been performed and two further wellhead platforms (PH and PJ) have been installed. The initial 18 well multi-lateral infill drilling program was completed in January 2006 and is expected to increase recovery from the field by 35 million barrels of oil (mmbbls) and 130 billion cubic feet (bcf) of gas. The two additional platforms will expand gross reserves by a further 18 mmbbls of oil and 74 bcf of gas2.

Panna Field Horizontal Well Production Challenges
The Panna Field has been developed with horizontally drilled wells through its two primary reservoir layers (Bassein A and B) since 1997. Horizontal drilling was considered the most economic development method from the outset due to the thickness of the oil column (20m) and problems of coning. These wells were successfully drilled using combinations of steerable bent housing drilling motors, adjustable gauge stabilizers and Polycrystalline Diamond Compact (PDC) bits to extend the length of and reduce the TVD variation in the horizontal sections. The history of the early horizontal well development program and improvements made in drilling
performance using the “state of the art technology” as existed in the late 1990’s is described in detail elsewhere3.