Abstract

Since discovery of oil in deep Eocene formation of Oil India Limited's (OIL) Assam field in 1998, OIL identified the deep Eocene target as strategically important for the future. Kopili shales are potentially serious problem in Eocene wells. These are non reactive, pressured shales. While drilling these shale sections OIL faced lot of problems such as excessive cavings leading to well washouts, frequent held up and reaming during drilling and round trips, stuck pipes, high torque, drag and hole fill up in almost all round trips. These problems led to poor hole condition, logging problem, bad cementation job, time overrun and more importantly formation damage arising out of long exposure time. OIL adopted many measures in steps and lessons learned from previous wells are implemented on the next well to overcome and improve upon the previous problems. OIL so far has introduced change in casing policy, mud system and mud weight to overcome few of its drilling problem.

Introduction

Drilling of deep Eocene wells in Moran started in July 1998 with the drilling of a well in Moran's south block and presence of hydrocarbon in Eocene was established from this very well. The next well drilled in Moran's North block also produced oil. Since then six more wells were drilled in Moran area. Though presence of hydrocarbon in both North and South block of Moran were established from the first two wells, these wells could not produce for a longer period due to high water cuts.

Hole instability and consequent failure to drill a gauge hole in the pay zone was prevalent in almost all deep Eocene wells drilled so far by OIL. Though problems in drilling of shallow Eocene wells (depth less than 4000m) could be solved by adopting improved drilling fluid technology with appropriate mud weight, problem in drilling deep Eocene wells could not be solved fully yet in spite of taking several measures. OIL so far has introduced change in casing policy, mud system and mud weight to overcome few of its drilling problem to drill deep Eocene wells with some success.

This note discusses in brief the continuous improvements made so far to overcome the serious drilling problems and the road ahead for further improvement to achieve successful drilling with optimising productivity.

Geology

Assam - Arakan basin is bounded to North-West by Himalayan Thrust sheet and South- East by Naga Thrust. The basement rock between two thrusts is relatively narrow (50 -100 km) and is entirely overlain by shallow dipping tertiary sediments up to about 4800m thick. These tertiary sediments which are both source rock and reservoir of Assam oil fields contain intersecting faults of relatively small displacement. These faults divide the sediments into separate blocks that greatly influence the hydrocarbon distribution. Tectonic activity manifested in the thrust belts and faulting immediately introduces the possibility of high and unequal ground stresses that may contribute to drilling problem in this region.

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