64 PESGB May 2016

Enhanced Oil Recovery (EOR) has been implemented since 1975 to help address growing domestic con-sumption in India. However, less than 4% of India’s oil is currently being produced by EOR techniques. Further resources could be tapped in the future to help reduce the country’s dependence on imported oil which now ac-counts for 79% of total consumption. This is particularly important as India became the 3rd largest oil consumer in mid-2015, behind China and the United States, with demand set to rise in line with increases in population and development.

Two of the basins where EOR has been applied are the Cambay and Barmer Basins, located in the states of Gujarat and Rajasthan respectively. The basins were formed due to continental rifting during the Late Cretaceous associated with the break-up of the super continent Gondwana. The rifting created two elongated NNW-SSE depressions which have been filled with fluvial and marine deposits resulting in the source, seal and res-ervoir rocks which are present today. Important trapping mechanisms include rift related structural traps as well as stratigraphic pinch outs.

Despite similar evolutions for both basins, there has been a marked difference in the exploration of the two. The Cambay Basin has entered into a mature stage after extensive exploration and production over the past 70 years and still accounts for ~13% of India’s yearly oil pro-duction. Conversely, the Barmer Basin was unattractive for exploration until 1999, when new seismic surveying and processing techniques were applied. A total of 37 discoveries have been made since then and the basin now accounts for ~23% of India’s yearly oil production.

This paper looks into the impact of two EOR tech-niques: In-Situ Combustion (ISC) and Chemical Flooding, in the Cambay and Barmer Basins respectively. The pro-duction gain from these techniques, combined with cost reduction through improved efficiencies, could help to promote the techniques to other fields in the country.

IN-SITU COMBUSTION IN THE CAMBAY BASINISC is a process used in targeting heavy oil reservoirs (<20.0° API) and can be viewed as having a dual effect of heating the oil in place reducing its viscosity as well as gas injection increasing reservoir pressure. Oil is ignited

in the reservoir by the injection of hot air and/or lowering heaters down injection wells forming a fire front (Figure-1). With continued injection of air, combustion is maintained at around 350-450°C utilising only a small percentage of oil in place as fuel. As the fire front sweeps through the reservoir, the remaining oil ahead of the fire front in-creases in temperature and reduces in viscosity, mobilis-ing it towards producer wells. The combustion effect also generates flue gasses which have the additional effect of charging the reservoir with a secondary gas cap, increas-ing the reservoir pressure, resulting in additional mobi-lisation of oil. As with other gas injection schemes, ISC also requires the process to begin up-structure, forcing oil down dip through gravity drainage towards producer wells. Combinations of pattern or line drive well configu-rations have been applied in the past.Balol and Santhal fields, operated by Oil and Natural Gas Corporation Ltd. (ONGC), are located in the north of the Cambay Basin and are the sites of two out of only four commercial ISC EOR schemes currently operational worldwide. The fields are part of a NNW-SSE striking narrow heavy oil belt within the Eocene Kalol Formation, which is host to thick fluvial channel sandstone bodies with good quality reservoir characteristics. The Kalol Formation can be subdivided into the KS-I, II and III pay zones with the KS-I being the most important contain-ing ~80% of OIIP. At both fields, the KS-I pay zone has a porosity of ~28%, permeability between 3,000-8,000 mD and a thickness ranging from 2-10 m. The oil water contact ranges between 988-1,018 m with the oil having an API gravity of 13-18° and a viscosity of 150-1,500 cp making it very immobile within the reservoir.

Santhal, containing 351 MMbo OIIP, is located in the southern end of the heavy oil belt and is separated by an E-W fault to Balol, lying immediately to the north, con-taining 168 MMbo. Balol was discovered in 1970 a year before Santhal and they were both put on conventional production in a 300 m x 300 m well pattern configuration in 1985 and 1974 respectively. There is a strong active edge water drive present at both fields and due to an ad-verse mobility contrast between the heavy oil and water, primary recovery was fairly low: <13% at Balol and <17% at Santhal. Peak oil production at Balol was achieved in 1991 with a rate of ~6,600 bo/d, but had declined to

News Feature

Tristan Reilly – Field Researcher, IHS

Advantages of Enhanced Oil Recovery Techniques in the Cambay and Barmer Basins, Western India

PESGB May 2016 65

~2,500 bo/d with 75% water-cut in early 1997. Santhal also experienced declines in production from a peak of ~11,500 bo/d in 1987 declining to ~4,700 bo/d also with a water-cut of 75% in early 1997.

It became apparent that thermal stimulation of the oil was required to target the heavier oil accumulations within the reservoir. Cyclic Steam Injection and ISC were both considered but ISC was adopted as the thin res-ervoir with its moderate depth meant that steam injec-tion would condense too quickly downhole for it to be effective. Accordingly, in 1990 ONGC conducted a pilot ISC scheme in a pattern configuration at the Balol field which yielded encouraging results. The similar charac-teristics of the Santhal field meant that full implementa-tion of ISC techniques were applied there in April 1997 without a pilot study, followed by full implementation at Balol in October 1997. Air was injected at pressures of 1,200-1,600 psi with peak injection rates of 17.6 MMcf/d at Balol and 53.0 MMcf/d at Santhal both achieved in mid-2004.

Initially, pre-existing infrastructure was utilised whereby former producer wells were converted into air injection wells. However, it soon became apparent that re-com-pletion and the drilling of purpose designed injection wells was required with optimal cement and metallurgy for ISC as the original wellbores became heavily cor-roded. Additionally, newly designed air injectors were required as a number of complications and explosions in

the early stage injectors hampered production. Important safety considerations were also developed to deal with produced flue gasses including H2S, CO2 and SO2 which necessitated careful handling at the surface.

The implementation of ISC technology resulted in a reduction in water-cut and an increased production rate (Figure-2). Critically, an increase in the recoverable volume at Balol to 65 MMbo (39% recovery) and 153 MMbo (43% recovery) at Santhal was achieved.

Figure 1: Schematic diagram indicating the temperature pro-file in the reservoir during ISC

Figure 2: Graph indicating the arrest in production decline due to the implementation of ISC

66 PESGB May 2016

CHEMICAL FLOODING IN THE BARMER BASINChemical flooding techniques have been developed since the late 1960s to alter the physicochemical properties of the injected fluid and those of the oil in place, to aid in the mobilisation of oil which would be by-passed during normal waterflood operations. Adding high molecular weight polyacrylamide and other long chained polymers to the injected fluid increases the viscosity to that of the oil in place. This reduces the mobility contrast between the two fluids and increases the sweep efficiency as a more uniform displacement front is formed throughout the reservoir, combatting the effects of reservoir hetero-geneities which can cause water-fingering (Figure-3). The addition of alkali and surfactant chemicals to the polymer flood (ASP Flood) can help to emulsify residual oil, lower-ing the interfacial tension between the oil and water and in doing so, promote oil mobility within the reservoir.

Polymer and ASP schemes are being implemented in the Lower Paleogene Fategarh Formation in the Mangala, Bhagyam and Aishwariya (MBA) fields in the Barmer Basin, operated by Cairn India Ltd. (CIL). All three fields were discovered in 2004; Mangala, the largest, holds 1,283 MMbo OIIP followed by Bhagyam and Aishwariya at 557 MMbo and 540 MMbo respectively. The Fategarh Formation consists of fluvial sandstones, siltstones and conglomerates. The reservoir is thicker and of more op-timal reservoir quality in the north of the basin in the area of the MBA fields, with porosities between 21-30% and a permeability of ~5,000 mD. The reservoir is also relatively shallow at ~700 m and as such the reservoir pressure is moderate-low at 1,625 psi. Additionally, the oil in place has a high paraffin wax content (~25%), moderate vis-cosity (10-20 cp) and medium gravity (~27° API), making it fairly immobile within the reservoir.

Mangala was placed on water injection from the begin-ning of production in 2009 with an expected recovery of 35%. The IOR operation saw a large rise in water cut as water fingering was exacerbated thus requiring a large amount of water treatment. To combat water fingering and high water cut, a pilot EOR scheme was implemented in Q4 2010 in which 4 injector wells were drilled in a 100 m x 100 m square with a producer well at the centre. By August 2011 the pilot polymer injection phase was initiated; water-cut declined to ~30% for 4-5 months and resulted in an 11% increase in production rate over wa-terflood. By October 2012, ASP injection began and dem-onstrated the mobilisation of un-swept oil post polymer flooding, resulting in a decrease in water cut by 20-30% lasting for 2.5 months after initial injection. Lab testing in-dicated that 10-15% incremental recovery would also be achieved over polymer flooding. Further polymer injection and final water injection stages were then conducted.

In October 2014 CIL began full field implementation of

polymer flooding at Mangala and by the end of 2015 pro-duction reached 19,000 boe/d. By January 2016 ~330,000 bbl/d of polymer was being injected and this is scheduled to increase to a maximum injection rate of 400,000 bbl/d in Q4 FY17. When full implementation is achieved across all the MBA fields, CIL anticipates producing >300 MMbo incremental over 636 MMbo expected from water flooding alone which is reported, by the company, to be the world’s largest chemical flood operation.

IS EOR THE KEY TO A BRIGHTER PRODUCTION FUTURE IN INDIA?The application of EOR to existing fields could be a more attractive proposition to operators as a means of achiev-ing more rapid increases in production, compared with the longer turn-around time of new exploration and de-velopment projects. Cost is still the main issue for EOR application, but this could be improved with greater ef-ficiencies and technical capabilities. Both ONGC and CIL have demonstrated this in becoming world experts with respect to some of the technological innovations applied in the Cambay and Barmer Basins.

ISC has been proven to be effective in producing heavy oil and IHS has estimated the operational costs to be ~$16/bbl and ~$20/bbl at Balol and Santhal respectively. This moderate cost level means that the process is an economi-cally viable technique even in the current low oil price envi-ronment (<$40/bbl). ONGC has now begun implementing the ISC technique at the Lanwa field lying to the immediate north of Balol. Initially, Lanwa was developed for primary production in 1986 on the same well pattern configuration as Santhal and Balol. In 1992 an ISC pilot study was un-dertaken at Lanwa but full field implementation was not ap-plied due to the presence of problematic coal layers caus-ing safety and operational concerns. However, geocellular modelling in 2004 highlighted safer areas for ISC operation, leading to the field being included in ONGCs redevelop-ment plans for the area in 2012 for full ISC implementation.

Chemical flooding, meanwhile, can be applied to a large range of field types and it is this versatility, coupled with further cost reduction, which makes the technique increas-ingly more attractive. IHS estimates the polymer injection process at Mangala to cost ~$11/bbl extracted and CIL aims to see production costs reduce to <$10/bbl after full implementation is achieved in FY17. With the cost ef-ficiencies in mind, the technique has also been applied by ONGC at the Bechraji field, lying <10 km to the west of Santhal. Bechraji originally underwent an unsuccessful ISC pilot in 2002 but a chemical flood EOR pilot scheme was conducted there in FY15. The chemical flood operation showed positive results and it is now believed that a full implementation development plan is under consideration.

For further information please contact:

Anthony Jaep, Field Researcher – NW Europe, IHSAnthony.Jaep@ihs.com

News Feature

Advantages of Enhanced Oil Recovery Techniques in the Cambay and Barmer Basins, Western India (cont.)