Fluid movement in the carbonate reservoirs is much more complex than in sandstone reservoirs, where the porosity and permeability development are quite predictable. In carbonate reservoirs, the fluid movement is controlled to a greater extent by the secondary porosity. The secondary porosity is generated through the process of dissolution of carbonate rock by meteoric water. The rainwater dissolves atmospheric carbon-dioxide forming carbonic acid that reacts with the limestone country rock to form various types of solution-induced features. It invades the rock through the cracks, fissures and joints present, widening and enlarging them by dissolving along the walls. Some of the solution-induced features are solution channels, vugs, sinkholes, roof collapses and breccia.
Neelam, a carbonate reservoir in Western Offshore India, is an oil field with a daily oil production of 30000 bpd. It is producing from the Bassein Formation (Upper) which is Mid to Late Eocene in age and constitutes the Main Pay in the field. It is also known as B-zone; A-zone being the Mukta Formation (gas bearing) of Late Oligocene age, the two being separated by a major unconformity. In 1994, the field was put on full-fledged production as well as on water injection (peripheral). However, soon the front row of producers facing the injectors started producing with high water-cut due to suspected water breakthrough. Efforts to combat this situation met with only partial success.
The situation warranted a thorough review of the geo-scientific data and fluid dynamics of the reservoir. Detailed core studies were carried out, which revealed that being associated with an unconformity, the reservoir had undergone extensive diagenetic changes and the invading meteoric waters had generated a lot of secondary porosity in the form of vugs and solution channels. An integrated study of cores and logs has helped in identifying eight major facies in the reservoir. The relationship of these facies is seen better with Density and Neutron logs than with Gamma and Resistivity logs, because the primary petrofabric has been altered by the percolating meteoric waters. Further, geo-statistical technique has been used for identifying the distribution of facies all over the field to understand the porosity pattern and fluid dynamics inside the reservoir.
The shift in study from genetic to diagenetic aspects has helped to understand the reservoir in a better way and in identifying layers of varying permeability for addressing carbonate reservoir management problems more effectively.
