Enhanced Oil Recovery and its Relevance in Indian Oil Fields
Dr. Partha P. Mitra
Oil & Gas Star tup Mentor and
E & P Domain Specialist, India

Enhanced oil recovery, also called tertiary recovery, is the implementation of various techniques for increasing the amount of crude oil that can be extracted from an oil field. This article discusses the various EOR methods, their applicability, and the screening criteria for various reservoir types in Indian contexts.

Why Enhanced Oil Recovery?
India's domestic consumption of petroleum product was 194 Million Metric Tonnes (MMT) in the fiscal year 2016-17. The Honble Prime Minister in his vision at "Urja Sangam 2015" had set a target to reduce the domestic crude import by 10% by 2022. In reality, our crude impor t is increasing every year, 82% during fiscal 2016-17 from 78.5% during fiscal 2014-15. On the other hand, there has been steady decline in crude oil production from 2012-13. Globally, the rate of replacement of the produced reser ves by new discoveries has been declining steadily over decades and of late, exploration activity was badly hit by low oil price. India is not an exception. There has not been any big discovery in Indian Basins after Mangala field in Barmer in 2004. Considerable por tion of current world's oil production comes from the matured old fields. The majority of Indian producing oil and gas fields are in decline or in matured phase. The average recoveries from Indian matured fields are less than world average. To achieve reduction of crude impor t by 10% by 2022 looks ex tremely difficult unless we make some real big discoveries and bring them to early production. However, it is possible to increase the domestic production by focusing on improved recovery from our Indian matured fields and other initiative such as bio fuel which is rightly under the radar of NITI Ayog at present. While discovery of new fields is not entirely in our hands, the fast implementation of Enhanced Oil Recovery (EOR) mechanism is something doable and achievable.

Normally oil is recovered from the subsurface reservoir under three phases. Primary recovery is the mechanism where the hydrocarbon rises to the surface of its own or through some kind of artificial lifting mechanism using pumps. Secondary recovery is the process where water or gas is injected from injection well sand that displaces the oil to the producing well bores. Majority of our Indian fields are under these two categories. In a situation where the oil mobility is an issue either due to its fluid characteristics or depleted reservoir pressure, significant amount of already mapped oil remains undrained despite having production infrastructure. EOR is a tertiary mechanism which helps to recover this by passed oil and improve the overall recovery from the field. In India, it is estimated that only 4 % of our production comes from EOR campaign which is well below the world average of 20-40%. A mere 10-20% increase in oil production through EOR from Indian fields would generate an additional 4-8 MMT of oil a year.

Different Types of EOR
The EOR can be broadly classified under three headings namely thermal recovery, gas injection and chemical flooding. The success of EOR method largely depends on the type of reservoir rock, its physical properties and type of crude. It is estimated that about 75-80% present EOR projects are froms and stone reservoir while restare from carbonate ~18-20% and ~ 4 % from other rock types. As the name suggests, in thermal recovery, the reservoir is heated which reduces the viscosity of the oil and increases the mobility of the oil to flow to the producers. It works very well for viscous crude type and first success fully tried in Venezuela and subsequently in USA. In Canada, this still remains one of the major EOR drivers. The application of solar energy for thermal EOR has shown some very interesting results in fields of Oman. This could be very effective where adequate gas is not available or even if available; the same is used for commercial sale. Thermal EOR has been effective in Indian fields such as Santhal, Balol of Cambay. But it has its a dverse impact on environment and as a result despite its huge popularity and acceptance, it is being gradually replaced by other EOR methods.

In case of Gas injection, natural gas, nitrogen or carbon dioxide is injected into the reservoir and gases then expand and push gases through the reservoir, or mix with or dissolve within the oil. This gives rise to decreasing viscosity and increasing flow of the oil. This is also known as miscible injec tion. The two major fac tors that affect the per formance of a miscible flood are oil- displacement efficiency at the pore level and sweep efficiency on the field scale. EOR using Carbon dioxide (CO2-EOR) has gained significant popularity in Industry where technologies have been developed to injec t CO2 created as byproduct in place of naturally occurring CO2. This has been very effective in fields of US, Venezuela and some fields in Canada. Gas injec tion technique has been successfully applied in Gandhar of Cambay and Borholla of Assam by ONGC.

The EOR through chemical flooding is other EOR method and is quite popular globally. In Chemical EOR, Polymer is added to water to increase its viscosity and reduce water permeability, thus decreasing its mobility and when this mixture is pumped through the injector, the mixture acts as a piston to push the oil to the producers. The process involves pumping water containing surfactants to reduce the interfacial tension between the oil and water phases thereby altering the wettability of the reservoir rock resulting in enhanced oil recovery from the reservoir. Thereafter the Polymer is mixed with water and injected continuously for an extended period of time. After a significant reservoir pore volume is subjected to injection, the drive water is pumped into the injection well to push the polymer slug and the oil bank in front of it moves towards the nearby production wellbores. Surfactants are also used in conjunction with polymers which helps in reducing the surface tension between the oil and water for better sweep.

Screening and Trial
The execution of any EOR project normally involves the screening mechanism, pilot scale implementation and full field implementation after a successful pilot. During screening, it is extremely important to evaluate, model and simulate various EOR scenarios through feasibility study. It is important to note that type of rocks and quality of oil may vary drastically within the same basin and therefore the understanding of the basin and generation of oil are important. Trial on pilot provides important field data which helps in refining the screening and subsequent full field implementation covering the entire field. It is a normal practice to acquire various reservoird at a during the actual execution which is used to continuously update the performance and make necessary course correction if needed.

Polymers & EOR
Po lymers generally used for EOR can be broadly classified under two categories i) Synthetic polymers and ii) Bio -polymers. The most commonly used synthetic polymers are Polyacrylamide known as PAM and Hydrolyzed Polyacrylamide (HPAM). The high salinity drastically reduces the viscosity proper ties of the mix ture and PAM is found to be stable to 62C at seawater salinity and 90C at normal salinity. Therefore the application of PAM is generally restric ted to onshore operation. HPAM is derived by par tial hydrolysis of PAM and it has high tolerance to high mechanical forces, low cost and resistance to bac terial attack. Some modified HPAMs are found to be stable at 120C. However HPAM is found ineffective in reservoir with high salt or salinity. It is also sensitive to hardness and presence of surfactants or other chemicals.

The application of Bio-polymer as a replacement of synthetic polymer is possibly nex t big thing to happen in EOR application. Such bio-polymers can be extracted from forest fungus. Xanthan gum and Schizophyllan, a kind of polysaccharide, are being studied which are produced by different bacteria through fermentation of glucose or fructose. The molecules of such bio-polymers generally have high molecular weight and very rigid polymer chains. It has been established through laboratory studies that these bio-polymer exhibit better chemical, mechanical stability, improved injectivity and above all, are more environment friendly since they are biodegradable. However, the application of bio-polymer is at a nearly stage of development and therefore is quite expensive.

Application of polymer and its successful outcome will depend upon the compatibility of the polymers with the types of reservoir rock, the reservoir conditions, the type of fluids etc. Globally major EOR projects are from clastics/sandstones rock types. Carbonate reservoirs are always challenging in terms of mineral composition, quite often naturally fractured and characterized by heterogeneous porosity and permeability distributions. In the presence of fractures, there is a high possibility that the injected fluid will flow through the fractures network by passing oil in the rock. Therefore, the low porosity, presence of fractures and oil to mixed wet rock properties are likely to result in low recovery in Carbonate reservoirs. In a typical offshore environment, salinity, hardness of the water may adversely affect the effectiveness of any chemical EOR methods. EOR implementation in offshore fields is always more challenging than the on land fields. Designing an integrated facility and reengineering the old with new facilities becomes more complex and cost intensive. With the stricter environmental laws, the waste management remains another challenge but these are all manageable with adequate planning, adopting the right technology with sound execution plan.

EOR in India
India has large number of matured fields from big to small and some are p roducing over the decades. However the EOR is the area which probably didn't get the due attention it deserved. The Indian Upstream regulator Directorate General of Hydrocarbon has initiated a study on EOR best practices with stakeholders input for Indian fields. History of EOR in Indian dates back to early 80s with fields of Gujarat. Recovery through thermal processes were commercially tried in fields such as Santhal, Balol & Lanwa of Gujarat. Gas injection and chemical EOR process in other Gujarat field such as Gandhar and gas injection in Borholla of Assam are some of the other ongoing commercial EOR schemes of ONGC. However the success story of chemical EOR in the fields of Barmer basin of Rajas than by Cairn India with its Joint Venture partner ONGC has put Indiain global map of EOR campaign. The operator rightly recognized the importance of EOR in early stage of field lifecycle. Mangala, the largest producer of Cairn India was put on production in 2009 and the pilot study was initiated as early as 2011. The operator carried out a detailed EOR pilot for Mangala field with water flood followed by a polymer flood and subsequently Alkaline Surfactant Polymer(ASP) flood were successfully run to quantify the incremental recovery. Based on the positive outcome of the EOR pliot in Mangala field, the full field implantation is being carried out which has helped in maintaining the field plateau production better than what was envisaged during early development plan. The implementation of Chemical EOR for other Rajasthan fields such as Bhagyam and Aiswariya have already been initiated and under different phases of activities. The operator has leveraged the learning from Managla to other Rajasthan nearby fields. Today it is one of the largest such campaign in the world. The current assessment of the operator is that there exists an EOR resource base of 300 million barrel of incremental recoverable oil from these three fields put together. All our Indian present EOR fields are on land Assets and are sandstone/clastic reservoirs where as sizeable matured fields are in offshore and out of them many are carbonates. This makes the EOR implementation in Indian offshore more challenging. Media report suggests that ONGC is working on 28 EOR projects and Government of India is also planning to auction nominated fields for EOR which is encouraging. All these put together makes India as an ideal destination for EOR campaign.

1. Ann Muggeridge, Andrew Cockin, Kevin Webb, Harry Frampton, Ian Collins, Timoulds, Peter Salino - Recovery rates, enhanced oil recovery and technological limits, December 2013 - The Royal Society Publishing
2. Public domain Report on Solar Enhanced Oil Recovery - An incountr y value a ssessment on Oman, Jan 2014
3. Public domain Report on Enhanced Oil Recovery Chemical Flooding by CAIRN
4. Public domain Presentation on EOR 2017 in Indian Context by ONGC