CHAPTER-16 ENHANCED OIL RECOVERY (EOR) EOR and.lOR stand for Enhanced Oil Recovery and Improved Oil Recovery. As the names suggest, these recoveries relate to oil that come at some advanced stage of exploitation as they either enhance or improve upon the existing recoveries. Not many techniques are used for enhancing gas recoveries and thus the present discussion will pertain to oil reservoirs only. The general mechanism of oil recovery is movement of the hydrocarbons to production wells, due to a pressure difference between the reservoir and the production wells. Enhanced Oil recovery processes may be sub-divided into three major categories: • • •
Primary Secondary Tertiary or EOR / IOR
In the primary process, the oil is forced out of the petroleum reservoir by existing natural pressure of the trapped fluids in the reservoir. When the reservoir pressure is reduced to a certain point where it is no longer effective, producing wells may be put on suitable artificial lifts, namely gas' lift, SRP, etc. Water is also injected to maintain the existing pressure in the reservoir. This is termed as primary recovery stage.(Fig:16-01) At this stage, pressure may further be augmented through water flooding using some existing wells or newly drilled injectors. The recoveries achieved at this stage are referred to as secondary stage. The oil recovered by both primary & secondary processes ranges from 20 to 50 % depending on oil and reservoir properties. (Fig:16-01 This shows that still at this stage, a huge quantum of oil remains to be exploited in the reservoir and thus clearly establishes the need and justification of designing and implementing new techniques to harvest some additional oil. The recovery thus achieved will be termed as enhanced oil recovery and the corresponding techniques termed as EOR processes. Since all the reservoirs are unique in nature, EOR process has to be case specific. Further, EOR processes being cost intensive, proper selection/ execution of the process becomes very crucial. (Fig:16-01) The enhanced oil recovery processes can be divided into three major categories: • • •
Chemical Miscible/ Immiscible Thermal
Thermal processes have been used extensively for the displacement of heavy oils, whereas chemical and miscible displacement processes have been employed for the recovery of light oils. Among the various processes for oil recovery, thermal processes have the least uncertainty, and offer a promising approach for about 70% of the world's EOR production. At present, surfactant flooding is the most complex and, therefore, has the highest degree of uncertainty. However, if the surfactant formulation for oil recovery is properly designed and controlled in the reservoir, it has a high potential for achieving maximum oil recovery. 287
Chemical Processes: There are broadly two categories: • •
Polymer Flooding Surfactant Flooding
Polymer Flooding: Polymer flooding is very useful to control the mobility of displacing phase i.e water. The fingering process is controlled and good amount of water free oil can be produced. In ONGC, polymer flooding has been successfully implemented in Sanand field of Ahmedabad Asset.(Fig:16-02) Surfactant Flooding: The mechanism of oil displacement by surfactant is based on reducing the interfacial tension and is useful in bringing into main stream the irreducible oil component which as such is not possible with water injection alone. Addition of caustic injected into the reservoir reacts chemically with the fatty acids and forms in-situ sodium salts of fatty acids. The formation of these surfactants results in ultra-low interfacial tension which brings globules of oil back in continuous phase and pushes towards producers to enhance recoveries. In ONGC, ASP (Alkaline Surfactant Polymer) flooding is successfully implemented in Viraj field of Ahmedabad Asset.(Fig16-02) Miscible/ Immiscible Processes: The miscible displacement process involves the injection of a solvent such as alcohol, refined hydrocarbons, condensed hydrocarbon gases, LPG, hydrocarbon gases and carbon dioxide gas etc. which can dissolve in the reservoir oil. In ONGC, miscible gas injection has been used for GS-12 sand of Gandhar field of Ankleshwar Asset. The results are highly encouraging and recoveries are expected to be around 60%. Even immiscible gas processes also give good amount of additional recoveries.(Fig:17-03) Thermal Processes: When petroleum reservoirs contain a low gravity (less than 20o API), highviscosity oil and have a high porosity, secondary recovery methods are not effective for displacement of oil. For such reservoirs, thermal processes have received the most attention. All the processes used basically reduce the viscosity of the crude and thus lead to increased mobility of reservoir oil. Depending on the way in which heat is generated in the reservoir, the thermal processes can be divided into three categories. (Fig: 16-04) • • •
In-Situ Combustion Steam Injection Wet Combustion
In-situ Combustion: For in-situ combustion, the crude oil near the wellbore is ignited using chemicals, downhole electric heaters or downhole gas burners. After completing ignition in the vicinity of the wellbore, continuous air injection promotes movement in the burning zone toward the producing wells. Propagation of a continuous burning zone results in almost complete 288
removal of all reservoir liquids and leaves behind hot, clean rock, which heats the injected air before it reaches the burning zone ahead. In ONGC, in-situ combustion is successfully implemented in Santhal & Balol fields of Mehsana Asset with highly encouraging results. (Fig:16-04) Steam Injection: In steam injection process, steam is injected into the reservoir either continuously or in cycles. Continuous steam injection involves both injection and production wells, whereas cyclic injection involves one well only which serves as both injection and production well. For the same pattern size, the response time is 25-50% lower than the response time for additional production by in-situ combustion. Duri field of Indonesia is a classic case of steam injection which is monitored rigorously with the help of 4-D seismics.(Fig:16-05) Wet Combustion: In the in-situ combustion process, a large amount of heat is left behind in the swept formation as waste heat. The heat utilization and efficiency of the process can be improved by water injection. Water is injected with the air. Superheated steam forms in an evaporation front and travels behind the combustion front. The important advantage of this process is that the amount of residual oil left to be burned as fuel by the burning front is considerably decreased, which in turn displaces more oil and less air is required to burn a unit volume of oil in the reservoir. MEOR, an Unconventional EOR Process: Microbial Enhanced Oil recovery process is known to the industry for long. However, some recent advancement has been attempted for reservoirs having quite high temperatures. Micro-organisms and their metabolic products stimulate oil production. The technique involves injection of selected micro-organisms into the reservoir and the subsequent stimulation and transportation of their in-situ growth products. Their presence aids in further reduction of residual oil left in the reservoir after secondary recovery. MEOR process in unlikely to replace conventional EOR processes because MEOR itself has certain constraints. In ONGC, MEOR has been attempted in Kosamba (Western Onshore), Badarpur (Silchar) and very recently in Charada field of Mehsana Asset. Points to ponder over EOR Processes: AS observed, EOR processes are very case specific and require large initial investments and careful monitoring during application. The selection of appropriate process itself asks for a rigorous study. The process is studied in laboratory and then taken to field on pilot scale. If found successful and commercial, it is then implemented in the field. It is quite often observed that even encouraging results at both laboratory and pilot scales do not yield good results at field level. Handling of environment is also a big issue when using EOR processes. Although these constraints to EOR processes cause hesitancy in their applications, yet the lure of getting additional recoveries from established results remains a major point of attraction. This gets still more importance keeping in minds the uncertainties in international market and those days of discovering easy oil are gone.
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IOR Methods: As such all EOR processes are IOR processes. Whereas, EOR methods use one process or the other being implemented on reservoir scale, IOR also covers augmentation of recoveries through drilling of high tech wells. This as well makes use of infill wells to produce by-pass oil which as such is not possible through existing wells. Mumbai High, a giant ONGC field is making use of IOR process through drilling of high tech and infill wells to gain additional recovery.
Fig: 16-01
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Fig:16-02
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Fig:16-04
Fig: 16-03
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