1.INTRODUCTION
EOR is a generic term for techniques for increasing the amount of crude of crude oil that can be extracted from an oil field. field. Using EOR, 30 to 60 percent or more of the reservoir's original oil can be extracted, compared with 20 to 40 percent using primary using primary and secondary recovery. recovery. Enhanced oil recovery is also called improved oil recovery or tertiary recovery (as opposed to primary and secondary recovery).[1] Enhanced oil recovery is achieved by 1.Gas injection 2.Chemical injection 3.Microbial injection 4.Thermal recovery (which includes cyclic or continuous steam, steam flooding, flooding, and fire flooding)
2.TYPES OF EOR STRATEGIES
2.1. WATER INJECTION AND WATER FLOODING Conventionally, based on the type of production and nature of reservoir the following patters are followed an injection well. Once the primary energy of the reservoir tends to deplete it becomes necessary to maintain the pressure inside the reservoir to achieve optimum production and maximise ultimate recovery. In such condition the pressure maintenance can be done by injecting water into the reservoir which is compatible to the formation water present in the reservoir through several water injection wells. Such kind of operation is known as water flooding/Water injection. In this process, the primary objective is to fill the voidage created by the produced oil fractions thus avoiding the reservoir pressure to decrease with the increased production. When the water is injected in the reservoir, it tends to push the oil towards upwards thus increasing the life and the ultimate recovery of the reservoir. Water injection and water flooding are quite similar terms the only difference being the level at which injection water is being discharged and the displacement phenomena. Normally only 30% of the oil in a reservoir can be extracted, but water injection increases that percentage (known as the recovery factor) and maintains the production rate of a reservoir over a longer period.
Water injection:
In water injection operation, the injected water is discharged in the aquifer through several injection wells surrounding the production well. The injected water creates a bottom water drive on the oil zone pushing the oil upwards. In earlier practices, water injection was done in the later phase of the reservoir life but now it is carried out in the earlier phase so that voidage and the secondary gas cap in the reservoir are not created. Using water injection in earlier phase helps in improving the production as once secondary gas cap is formed the injected water initially tends to compress free gas cap and later on pushes the oil thus the amount of injection water required is much more. The water injection is generally carried out when solution gas drive is present or
water drive is weak. Thus for better economy the water injection is carried out when the reservoir pressure is more than saturation pressure. The selection of injection water {displacing fluid} depends upon the mobility rate between the displacing fluid {injection water} and the displaced fluid {oil}
DISADVANTAGE OF WATER INJECTION
• Reaction of injected water with the formation water can cause formation damage. • Corrosion of surface and sub-surface equipment.
Water flooding:
In this operation, displacing fluid is injected in the oil zone through the surrounding water injection wells creating an edge water drive flooding oil towards the production well. For better efficiency, the pressure of the reservoir should be such that no secondary gas cap is formed. Water flooding is generally more effective than water injection when no voidage is being
created. Water flooding in other regards is similar to water injection including selection parameters of the displacing fluid, the only difference being the displacing phenomenon. However, in practice the above patterns are not necessarily followed It is to be noted that water injection and water flooding are essentially the same process, however, while in the case of former we inject the water in the water zone of the reservoir, in the latter we inject it directly into the hydrocarbon zone 2.2.GAS INJECTION
Gas injection is presently the most-commonly used approach in enhanced oil recovery. In addition to the beneficial effect of the pressure, this method sometimes aids recovery by reducing the viscosity of the crude oil as the gas mixes with it. Gases used include CO2, natural gas or nitrogen. Oil displacement by carbon dioxide injection relies on the phase behaviour of the mixtures of that gas and the crude, which are strongly dependent on reservoir temperature, pressure and crude oil composition.
In high pressure applications with lighter oils, CO2 is miscible with the oil, with resultant swelling of the oil, and reduction in viscosity, and possibly also with a reduction in the surface tension with the reservoir rock. In the case of low pressure reservoirs or heavy oils, CO2 will
form an immiscible fluid, or will only partially mix with the oil. Some oil swelling may occur, and oil viscosity can still be significantly reduced. In these applications, between one-half and two-thirds of the injected CO2 returns with the produced oil and is usually re-injected into the reservoir to minimize operating costs. The remainder is trapped in the oil reservoir by various means. CO2 is pumped into the reservoir through an injection well. It mixes with the remaining oil, forming a “miscible zone.” The pressure from the CO2 and expanding oil pushes an oil bank
toward the production well, where it rises to the surface. Then the CO2 is separated from the oil and may be added to the stream of CO2 going into the injection well. A side effect of enhanced oil recovery is that the CO2 that was used to force oil out of the formation is now sequestered.
2.3.CHEMICAL INJECTION
The injection of various chemicals, usually as dilute solutions, have been used to improve oil recovery. Injection of alkaline or caustic solutions into reservoirs with oil that has organic acids naturally occurring in the oil will result in the production of soap that may lower the interfacial tension enough to increase production. Injection of a dilute solution of a water soluble polymer to increase the viscosity of the injected water can increase the amount of oil recovered in some formations. Dilute solutions of surfactants such as petroleum sulfonates or bio surfactants such as rhamnolipids may be injected to lower the interfacial tension or capillary pressure that impedes oil droplets from moving through a reservoir. Special formulations of oil, water and surfactant, microemulsions, can be particularly effective in this. Application of these methods is usually limited by the cost of the chemicals and their adsorption and loss onto the rock of the oil containing formation. In all of these methods the chemicals are injected into several wells and the production occurs in other nearby wells. Chemical EOR is the addition of chemical agents to the injected water to aid mobility and the reduction in surface tension. Polymer flooding is a means of injecting long chain polymer molecules in an effort to increase the injected water viscosity. The addition of these chemicals means that the fluid would behave like a non-Newtonian fluid; at low velocities it is resistant to flow. This method not only
improves the mobility ratio (by lowering it) but also the vertical and areal sweep efficiency. The polymer causes a reduction in the permeability and allows the preferential filling of the high permeable zones in the reservoir. This lowers flow velocity and increases the sweep area. Surfactant Polymer flooding these are surface active agents that help to break down the surface tension between the oil and water. This allows for the oil and water to separate. The effect of the surfactant depends on the concentration. In low concentrations the rate is gradual but in higher concentrations the rate is increased until such time that the surfactant is diluted by the formation fluids. It also improves the mobility of the fluids and reverses the rock wettablity. Primary surfactants usually have Co-surfactants, activity boosters, Co-solvents added to them to improve stability of the formulation.
2.4.MICROBIAL INJECTION
Microbial injection is part of microbial enhanced oil recovery and is presently rarely used, both because of its higher cost and because the developments in this field are more recent than other techniques. Strains of microbes have been both discovered and developed (using gene mutation) which function either by generating biosurfactants, or by emitting carbon dioxide. Three approaches have been used to achieve microbial injection. In the first approach, bacterial cultures mixed with a food source (a carbohydrate such as molasses is commonly used) are injected into the oil field. In the second approach, used since 1985, nutrients are injected into the ground to nurture existing microbial bodies; these nutrients cause the bacteria to increase production
of
the
natural
surfactants they
normally use
to
metabolize
crude
oil
underground. After the injected nutrients are consumed, the microbes go into near-shutdown mode, their exteriors become hydrophilic, and they migrate to the oil-water interface area, where they cause oil droplets to form from the larger oil mass, making the droplets more likely to migrate to the wellhead. The third approach is used to address the problem of paraffin components of the crude oil, which tend to separate from the crude as it flows to the surface. Since the Earth's surface is considerably cooler than the petroleum deposits (a temperature drop of 13-14 degree F per thousand feet of depth is usual), the paraffin's higher melting point causes it to solidify as it is cooled during the
upward flow. Bacteria capable of breaking these paraffin chains into smaller chains (which would then flow more easily) are injected into the wellhead, either near the point of first congealment or in the rock stratum itself.
2.5.THERMAL ENHANCED OIL RECOVERY METHOD
Thermal enhanced oil recovery techniques are generally applied to relatively shallow (less than 3,000 feet) very viscous heavy oil (generally defined as oil with API gravity between 10 and 20 degrees). These techniques include conventional steam floods with injectors and producers drilled in tight spacing patterns; cyclic production where the steam is injected and allowed to “soak”, then produced out of the same well; steam assisted gravity drainage where the steam is
injected in one horizontal well and produced from another lower horizontal well; and rarely in an in-situ combustion project. Heavy oil typically has a viscosity between 100 and 10,000 cp and does not flow unless diluted with a solvent or heated.
Thermal EOR has its own set of opportunities and obstacles. Although the daily production from thermal methods, mainly in California has been in decline for the last few years, there are still sizeable reserves, not only California, but also Alaska, North and South Dakota, Wyoming, and Texas. Several successful in-situ combustion projects have been established in South Dakota. In a study performed for the DOE, ARI has identified 100 billion barrels of heavy oil in place in the U.S. with 42 billion in California and 25 billion barrels in Alaska and that technically possible production could be increased from the current level of 225 thousand barrels/day to as high as 500 thousand barrels/day. The current production is based on current best practices and economics. New technology will be required to address resources deeper than 3,000 feet and the more shallow, but environmentally-sensitive Arctic resources. The challenges of thermal EOR are economics and environmental. The conventional steam flood is usually found in large fields, where economics of scale apply. The upfront capital costs are considerable, with wells drilled on 2 acre spacing, expensive steam generation and production facilities and insulated flow lines. For environmental reasons, most steam generators are fired with natural gas, making the operating expenses considerably higher than convention production. The other economic issue relates to the price of oil. The thermal EOR projects not only suffer the same effects of volatile oil prices over time as all oil producers, but the heavy oil, which results in less higher end products when refined, historically have been priced at a $10 to $15 discount to West Texas Intermediate.
Environmental concerns present a considerable obstacle to current producing fields and because of tighter regulatory requirements of new development, even more difficult for new projects. The first issue is the surface use. With the tight spacing and considerable steam generator, producing facility and flow lines, there is little else that the surface can be used for. Obviously, a development of this sort would not be allowed where the surface is residential or developed for other high end purposes. Also, for the steam projects, some or all of the water will be supplied by the municipalities, competing with human use for the quantities. In California in particular where much of the production is from unconsolidated sand reservoirs, subsidence of the surface may have major effect on surface infrastructure. Clean air can be an issue with the combustion products of the steam generators, particularly if a fuel other than natural gas is used.
Other specialized technologies present other challenges. The in-situ projects require special equipment to deal with the corrosion of sub-surface and surface equipment and also require careful process of the produced combustion gas. And in Alaska, while the production techniques are technicall capable of developing the thermal projects, new approaches are needed to allow production of the shallow Alaska North Slope resources while protecting the permafrost.
3.SELECTION BETWEEN ALTERNATIVE STRATEGIES The criteria for selection of a particular EOR process depends upon the various factors like reservoir conditions, crude properties and economics feasibility.
CO2 injection and surfactant flooding are considered best technically and economically but CO2 injection is preferred because of the following reasons: a) The amount of surfactant required for flooding is high therefore the cost increases b) CO2 injection gives higher oil recovery and cost is less as compared to surfactant flooding.
ENVIRONMENTAL IMPACTS OF EOR
Enhanced oil recovery wells typically produce large quantities of brine at the surface. The brine may contain toxic metals and radioactive substances, as well as being very salty. This can be very damaging to drinking water sources and the environment generally if not properly controlled.
CO2 is a major environmental pollutant therefore extra care to be taken while CO2 reinjection.
BIBLIOGRAPHY 1. www.wikipedia.org/wiki/enhanced_oil_recovery 2. www.ijastnet.com 3. Enhanced oil recovery - Wikiversity.htm 4. Petroleum and gas field processing by H.K. Abdel Aal, A.Mohamed, M.A. Fahim
