In-situ Combustion Technique to enhance heavy oil recovery at Mehsana, ONGC - A success story

Abstract

Abstract The northern part of the Cambay Basin located in Mehsana, Gujarat (India), is fenced by a number of heavy oil fields reserving about 140 MMT OOIP. Balol and Santhal fields form a part of this heavy oil belt with API gravity 15–18. The oil viscosity ranging from 50 to 450 cps at reservoir pressure of 100 kg/cm2 and 70°C temperature. High mobility contrast between viscous oil and water has resulted into low primary recovery. Subsequent Artificial lift methods resulted into high water production rather than oil. Thus, it necessitated for in-situ combustion technique. In-situ combustion is a thermal enhanced oil recovery process. The technique is based on the principle of reducing viscosity by heating of oil in the reservoir itself. It involves ignition in the well and injection of air to sustain flame front. In this process a small quantity of in-place oil burns, producing CO2, water vapours and heat. It helps in lowering the viscosity of oil, which increases the oil mobility and displace it towards the surrounding producers. It is a matter of pride that ONGC, Mehsana has successfully implemented this technology. The project was initiated at Balol field on pilot scale in 1991. Based on encouraging results, in-situ combustion technique was implemented on commercial scale in 1997 at Balol and Santhal fields. It has not only given a new lease of life to these fields but has also increased the oil recovery factor from 6–12% to 39–45%. Production engineering for in situ combustion involves careful planning and monitoring for successful implementation. Based on field experience, some of the major issues which came up during implementation are summarized in this paper. It can be helpful for successful planning and implementation of new projects Introduction Oil and Natural Gas Corporation Ltd. (ONGC) is India's biggest integrated oil company having major interest in E&P activities. Mehsana asset, located in Northern part of Gujrat State, is the highest oil producing onshore asset of ONGC. The annual crude oil production of the Asset is in the tune of 2.35 MMT. Mehsana Asset is having oil fields producing both heaviest crude and one of the lightest crude in India with API gravity ranging from 13–42°. Heavy oil fields at Mehsana were found during 1970–71. These heavy oil fields belong to northern part of Cambay basin are Santhal, Balol, Lanwa and Becharaji which. These fields cover an area of about 70 Sq Km and contain around 150 MMT oil in place. Heavy oil exhibits gravity less than 20° API. They are characterized by high viscosity that increases as API gravity decreases, low hydrogen/carbon ratios, low gas/oil ratio, as well as significant asphaltenes, sulfur and heavy metals. In short, heavy oil reservoirs generally present low-energy and low productivity wells. These characteristics make recovery challenging. Balol and Santhal are two major fields of the heavy oil belt bearing oil with API gravity between 15°−18°. These fields encompass 22.17 MMt and 53.56 MMt of OOIP respectively. The crude is asphaltic in nature containing 6–8% asphaltene and has oil viscosity in the range of 50 to 450 cps in reservoir conditions. Reservoir pressure and temperature are 100 kg/cm2 and 70°C respectively at 990 m (MSL). Reservoirs have permeability of the order of 3–8 darcies and are operating under active water drive. High mobility contrast between oil and water has resulted into primary recovery below 13%. Moreover on the application of artificial lift water production was on rise. In many wells it became 95–100% and some wells had to be closed due to high water cut. The poor primary recovery and less effective artificial lift method necessitated search for suitable enhanced oil recovery method for improving ultimate recovery from these fields. One of the methods was In-situ combustion technique.