The significant formation damage with the use of conventional workover fluids due to filterate invasion of water sensitive zones in the wells having subhydrostatic reservoir pressures can be avoided by the use of nondamaging subhydrostatic workover fluids. New formulations of oil-base workover fluid with minimum specific gravity (0.86) and requisite Theological properties were developed through extensive laboratory studies by using LSHF HSD or a mixture of HSD and stabilised connate crude oil (1:1) and OBM chemicals. The formulations were subjected to field trial in subhydrostatic wells of the Kalol oil field in Western India. The bottomhole studies and / or production performance of the wells worked over with this system, have indicated revival of the wells to their optimum production, which would have been impossible otherwise. The success of the system has found its preferential use for workover operations at all subhydrostatic wells in the Kalol oil field.

The oil-base subhydrostatic fluid system has been introduced for the first time in ONGCL, India. Economically, it is very much viable due to its reusability at different wells. The use of stabilised /treated connate crude oil in equal parts with HSD further reduces the cost of the system with additional benefit of its wax dissolving capacity at perforations.

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