Mumbai High, a highly heterogeneous multi-layered carbonate sequence with shale intercalation, played a critical role to quench the energy thirst of a big developing country like India. Mumbai High field is divided into two blocks i.e., Mumbai High North (MHN) and Mumbai High South (MHS) by a major WNW-ESE trending graben containing impermeable shale. After 34 years of production, the field is in its brown stage and radical decline in production has been observed. Horizontal drain holes were introduced to counter this problem by increasing the reservoir exposure. However the biggest challenge faced so far is the placement of the horizontal drain hole within 2 - 4m thick layer with significant structural variations. Exit into shale due to presence of sub-seismic fault and minor dip change is a common observation while drilling.

Geosteering technique has developed as the industry has shifted its focus on horizontal well drilling. Initially wells were placed using log correlation, through understanding of the reservoir which often helped to drill a better horizontal well. The simple log-correlation technique was improved with the introduction of azimuthal measurements. However the approach was still reactive and a need was felt to have a proactive approach. Present sophistication has met the requirement by providing early indication of the approaching boundary.

This paper will illustrate various techniques for successful placement of drain holes in such complex reservoir. Also, it will capture the chronological evaluation of Geosteering technology and how they are used to develop this field. The drilled wells in all the cases have proven to be the successful drain holes and prolific producers compared to the surrounding wells.


Mumbai Offshore Basin, a pericratonic rift basin in the western continental shelf of India, covers about 148,000 km2 from coast to 200 m isobaths and the sedimentary fill ranges from 1100–5000 m (Fig.-1). Several large oil and gas fields have been discovered in this Basin, and the presence of hydrocarbons has been established in the multiple pay zones belonging to L-III limestone reservoir of Miocene age in Mumbai high. The L-III limestone reservoir is a multi-layered reservoir and has been divided conventionally from top to bottom into major layers A, B, C, D, E, and F separated by intervening shale and argillaceous tight limestone. 'A' layer is further subdivided into Al, A2-I, A2-II, A2-III, A2-IV, A2-V, A2-VI and A2-VII and along with layers B, C constitutes the main producing horizon (Fig.-2).

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