Feasibility of Development of Marginal Fields Through Horizontal Well Technology
Authors: Srinivasan S. , Joshi J.M. , Narasimham J.L. , Rao Mohan , Barua Shyamal , Jha Krishna Nand 
Published: December 2007

Bombay Offshore basin is the largest oil accumulation area in India. Oil was discovered in reservoirs varying from a few metres to about 100 m (328 ft) thickness in small and large pools scattered across the exclusive economic zone. Besides exploitation through a large number of conventional wells, as on date about 32 horizontal wells and 10 drain holes were completed and recompleted in the conventional wells respectively in major oil pools. L-II carbonate reservoir of Bombay High field, developed with a mix of horizontal and conventional wells, by far has the longest production history of horizontal wells. Horizontal well completion in this field under gas and water coning situations is considered as an excellent example of techno-economic success. However, much needs to be done in the areas of workover jobs, well testing and stimulation of these horizontal wells.

There are several marginal pools discovered around major offshore fields having similar gas and water coning problems and/or low productivity. With rich experience gained through horizontal well technology, it is now possible to exploit the marginal fields with horizontal wells for improving oil recovery in Bombay offshore basin.

Introduction

Since 1974 a large number of big and small oil/gas fields were discovered in offshore waters of Western India (Fig. 1). Some of these fields, including the giant oil field Bombay High. are on commercial production for several years. By and large these fields were developed through conventional vertical or inclined wells. Two reservoirs of the offshore oil fields viz. L-II reservoir of Bombay High which is overlain by gas cap and B zone reservoir of Panna field, wherein the thin oil column is sandwiched between active gas and water column were initially considered for drilling horizontal wells for controlling gas/water coning and for increasing well productivity. Subsequently several number of horizontal wells were drilled in these two reservoirs as well as in other medium size fields, Neelam and Heera.

The first two successful horizontal wells in L-II reservoir of Bombay High field were drilled in 1987 under the expert guidance of M/s. HORWELL. France. The very first well NR-H of this field was completed with casing (5" liner) and cemented. Out of the 309 m (1013 ft) horizontal section drilled, 259 in (850 ft) were perforated using TCP gun. Rest of horizontal wells drilled till date were either left bare foot or completed with slotted liner. NR-H completed with cemented casing and perforated with TCP gun, by far has given the best production performance in all respects. Even the horizontal wells drilled and completed with slotted liners in other fields like Panna have outperformed conventional wells in terms of production and controlled gas/water coning. Many horizontal wells or drainholes are being planned in Heera field for maximising the oil production in a cost effective manner.

While big and medium size fields received the necessary impetus for exploitation. the marginal fields discovered in offshore waters did not receive much attention all these years in view of anticipated poor economic returns. if developed under conventional well technology. 3D seismic data being acquired is also expected to lead to the discovery of many such marginal finds as well as peripheral extension of existing fields in the mature Bombay Offshore Basin. Exploitation of these fields in the most economic way calls for introduction of innovative drilling, production and processing technologies.

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