The technology of horizontal, multilateral and ERD wells has come from outer edge to mainstay in redevelopment of Mumbai High. The paper focuses on the technological improvements in engineering, placement and drilling of horizontal and multilateral wells, both infill and sidetrack, to exploit the bypassed, un-swept / less depleted reserves in the heterogeneous multi-layered carbonate
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Mumbai High field, located in western continental shelf of India, is on production since 1976 and is the largest contributor to indigenous oil production. The field comprises of several pay zones of which "P" reservoir of early middle Miocene age is the major multi-layered limestone reservoir characterized with widely varying petrophysical properties. After attaining a production of about 400,000 bopd during 1985–90 the field had entered in declining phase. The problems of differential depletion and preferential water movement in the sub-layers, gas and water channeling /cusping occurred in the reservoir effecting the sweep efficiency.
Based on the re-assessment of the reservoir with updated geological model and reservoir simulation an extensive footage campaign was launched in 2001 as a "redevelopment" IOR project. During implementation, various test experiments have been conducted on on-going basis and better strategies are being evolved. With successful application of enhanced drilling technology of horizontal, multilateral and ERD wells, and particular emphasis on targeting individual layers within the "P" reservoir, the declining production trend has already been reversed in the initial implementation phase. Oil production is showing an encouraging upward trend and an additional upside of the projected reserves is becoming visible.
The paper highlights the approach followed for managing the reservoir with emphasis on absorption of emerging technologies. Analysis of gain in productivity due to new well engineering associated with layer thickness, heterogeneity and drainage pattern have also been discussed in the paper with the future scope for tapping the more difficult reserves and improving oil recovery in cost effective way through high tech wells.
Introduction
The Mumbai High field, the largest offshore oil field of India, was discovered in 1974. The field is situated in Western continental shelf towards west north west of Mumbai city at an average water depth of about 75 m. The field consists of heterogeneous, multilayered carbonate reservoirs. Being on continuous production for last 27 years, the field is in mature stage of its producing life. The level rate of oil production from the field was of the order of 0.4 million bopd during 1985 to 1990. During mid and late nineties the field had entered a crucial phase of "mid-life crisis" and production level dropped to about 0.21 million-bopd levels with rise in gas oil ratio and water cut. The oil recovery was about 20 %.
To offset the declining trend and improve recovery, a "redevelopment" program was initiated during 1998–99. Reservoir re-evaluation was carried out through integration of G&G data and comprehensive review and analysis by the multi-disciplinary teams. Pilots were conducted to assess the potential of by-passed oil areas and to examine the feasibility of oil drainage from the area below the gas-cap through successful placement of the drainhole in thin zones using the new generation direction drilling tools and redesigned mud system.
The refined reservoir characterization, use of state of the art technology for visualization of sub-surface complexities in immerse environment and changes in development pattern have facilitated to revitalize the field. The overall impact, even with limited implementation of the redevelopment program, has been very encouraging.
Geological Description
Mumbai High is a doubly plunging anticline structure with large areal extent. The hydrocarbon bearing carbonate formations are developed from Oligocene to Miocene with few regression and transgression cycles of deposition. The reservoir is bounded on the east by a major NNW- SSE trending fault. The western limb of the structure is gentle and the limit is defined by OWC. There are two highs separated by an east west trending low permeability barrier. The northern part is called Mumbai High North (MHN) while the southern part is Mumbai High South (MHS) (Figure-1).