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Abstract
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.