Comparison Between Gas Injection and Water Flooding, in Aspect of Secondary Recovery in One of Iranian Oil Reservoirs

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1 Comparison Between Gas Injection and Water Flooding, in Aspect of Secondary Recovery in One of Iranian Oil Reservoirs BABAK AMINSHAHIDY 1, MEHDI FOROOZANFAR 2 1 Department of Petroleum Engineering,University of Tehran 2 Department of Petroleum Engineering, International Campus, University of Tehran, Kish Island IRAN aminshahidy@gmail.com 1, m.foroozanfar@ut.ac.ir 2 Abstract : - The second stage of hydrocarbon production during which an external fluid such as water or gas is injected into the reservoir through injection wells located in rock that has fluid communication with production wells.the purpose of secondary recovery is to maintain reservoir pressure and to displace hydrocarbons toward the wellbore. The most common secondary recovery techniques are gas injection and water flooding. Normally, gas is injected into the gas cap and water in injected into the aquifer. In this paper as case study in one of Iranian oil reservoir gas injection and water flooding were evaluated in two different pattern ; Five Spot and Peripheral. Eclipse 100 was employed to simulate the reservoir. It was found that the best result is obtained by gas injection in 5-Spot pattern. Key-words : - Secondary Recovery,Gas Injection,Water Flooding, Pressure Maintenance. 1. Introduction Secondary recovery involves artificial energy into the reservoir via gas injection or water flooding. Secondary oil recovery is employed when the pressure inside the well drops to the levels that make primary recovery no longer viable. Pressure is the key to collecting oil from the natural underground rock formations in which it forms. When a well is drilled, the pressure inside the formation pushes the oil deposits from the fissures and pores where it collects and into the well where it can be recovered. But this initial pressure is finite. In order to continue collecting the oil, the pressure must be maintained through other means. This tactics are referred to as secondary recovery techniques. 1.1 Water Flooding Water Flooding is implemented by injecting water into a set of wells while producing from the surrounding wells. Water flooding projects are generally implemented to accomplish any of the following objectives or a combination of them : Reservoir pressure Maintenance Dispose of brine water and/or produced formation water As a water drive to displace oil from the injector wells to the producer wells Over the years, water flooding has been the most widely used secondary recovery method worldwide. Some of the reasons for the general acceptance of water flooding are as follows ; Water is an efficient agent for displacing oil of light to medium gravity, water is relatively easy to inject into oil-bearing formations, water is generally available and inexpensive and water flooding involves relatively lower capital investment and operating costs that leads to favorable economics. 87

2 1.2 Gas Injection Immiscible gas is injected to maintain formation pressure, to slow the rate of decline of natural reservoir drives, and sometimes to enhanced gravity drainage. Immiscible gas is commonly injected in alternating steps with water to improve recovery. Immiscible gases include natural gas produced with the oil, nitrogen or flue gases. Immiscible gas injected into the well behaves in a manner similar to that in a gas-cap drive : the gas expands to force additional quantities of oil to the surface. Gas injection requires the use of compressors to raise the pressure of the gas so that it will enter the formation pores. 2. Introducing the Reservoir This reservoir is located in southwestern of Iran. Table 1, 2 and 3 illustrate general data, rock and fluid properties of the reservoir respectively, reservoir temperature is 220. Table 1 : General Data of the Reservoir Phases Gas, Oil, Water Start of Production 1 Oct 1999 Start of Injection 1 Oct 2009 Datum Depth(ft) 900 Gas Oil Contact(ft) 900 Water Oil Contact(ft) 1950 Initial Pressure (Psia) 4100 Number of Production 3 Wells Permeability in X Direction(md) Permeability in Z Direction(md) Table 2 : Rock Properties Max : 3.39 Min : 0.71 Max : 0.33 Min : 0.07 Porosity Max : 0.08 Min : 0.03 Table 3 : Fluid Properties in Pressure 4100 Psia Water Compressibility 2.1E-6 Water Viscosity(Cp) 0.9 Gas Viscosity(Cp) 0.02 Oil Viscosity(Cp) 0.4 Gas Density(lb/ft3) 16.5 Oil Density(lb/ft3) API 32 Initial Gas Saturation Initial Oil Saturation Initial Water Saturation Model Properties In this paper with regard to design the suitable well configuration and pressure maintenance method, the different injection scenarios with different well patterns were designed by using commercial simulator. Based on Geological data, reservoir static data including porosity, absolute permeability and NTG were calculated for all grid blocks using geological model of reservoir and up scaling techniques and were used as input data to simulator. Figure 1 shows the selected model. Fig.1 Three dimension view of selected model Economic limitation which is considered for oil production wells are listed in table 4. Table 4 : Economic Limitation for Production Wells Min Oil Production Rate(STB/Day) Max Water Cut Max GOR(Mscf/Day) This reservoir was simulated from 1 oct 1999 to 2020, as a first scenario it produced by it s own energy as a primary recovery for 20 years. This 88

3 scenario is as base study for other scenarios to analyze their influences on the reservoir. The injection operation(gas injection or water flooding) to maintain the reservoir pressure was started from Introducing the Scenarios 4.1 Natural Depletion In this scenario the selected model has 3 production oil wells with bottom hole pressure 1000 Psia, consider figure Gas Injection, Peripheral Pattern Specifications of this scenario are : 3 oil production wells with bottom hole pressure 1000 psia. 8 gas injection wells, total injection rate is 3200 Mscf/Day. Consider figure 4. Fig.4 Location of Wells in Gas Injection Peripheral Fig2. Location of Wells in Natural Depletion 4.2 Water Flooding, Peripheral Pattern Specifications of this scenario are : 3 oil Production wells with bottom hole pressure 1000 Psia. 8 water injection wells, total injection rate is 1600 STB/Day. 4.4 Water Flooding,5-Spot pattern Specifications of this scenario are : 3 oil production wells with bottom hole pressure 1000 Psia. 8 water injection wells, total injection rate is 1600 STB/Day. Consider figure 5. Consider figure 3. Fig5. Location of Wells in Water Flooding 5-Spot Pattern Fig.3 Location of Wells in Water Flooding Peripheral 4.4Gas Injection 5-Spot Pattern Specifications of this pattern are : 89

4 3 oil Production wells with bottom hole pressure 1000 Psia. 8 gas injection wells, total injection rate is 3200 Mscf/Day. Consider figure 6. Fig8. Amount of Oil Production Rate(STB/Day) at year 2020 Fig6. Location of Wells in Gas Injection 5-Spot Pattern 5. Comparison All the Scenarios 5.3 Oil Remaining in the Reservoir at 2020 Now we compare all the scenarios in different aspects which injection has influence on them. 5.1 Reservoir Pressure at 2020 Fig9. Amount of Oil Remaining (STB) in the reservoir at year Oil Production Total at 2020 Fig7. Amount of Reservoir Pressure (Psia) at year Oil Production Rate at

5 Fig11. Effect of Free Gas Saturation on Sor Fig10. Amount of Oil Production Total (STB) at year Conclusion According to Figure 9 we can recognize the reservoir rock is water wet, about the rocks which are water wet Cole(1969) presented two theory : 1. Cole postulates that since the interfacial tension of a gas-oil system is less than the interfacial tension of a gas-water system, in a three phase system containing gas, water and oil the reservoir fluids will tend to arrange themselves in a minimum energy relationship. In this case, this would dictate that the gas molecules enclose themselves in an oil blanket. This increase the effective size of any oil globules, which have enclosed some gas. When the oil displaced by water, the oil globules are reduced to some size dictated by the flow mechanics. If a gas bubble existed on the inside of the oil globule, the amount of residual oil left in the reservoir would be reduced by the size of the gas bubble within the oil globule [1]. Figure Cole(1969) points out that reports on other laboratory experiments have noted the increased recovery obtained by flooding cores with air after water flooding. These cores were classified as water wet at the time the laboratory experiments were conducted. On the basis of this experiments, it was postulated that the residual oil saturation was located in the larger pore spaces, since the water would be preferentially pulled into the smaller pore spaces by capillary action in the water wet rocks. At a later time, when air was flooded through the core, it moved preferentially through the larger pore spaces since it was nonwetting. However, in passing through this large pores spaces, the air displaced some of the residual oil left by water displacement [2]. Figure 12. Fig12. Effect of Free Gas Saturation on Sor Therefore, for this reservoir rock which is water wet and it s initial water saturation in compare of 91

6 initial gas saturation is much higher, gas injection in 5-Spot pattern has more positive influences for this oil reservoir. References [1]. Tarek Ahmed, Hydrocarbon Phase behavior,gulf Publishing Co,1989. [2]. Tarek Ahmed, Hydrocarbon Phase behavior,gulf Publishing Co,1989. [3]. Ma, T.D.,ARCO Alaska Inc., Youngren., G.K.,ARCO E&P Technology,Society of Petroleum Engineering. ISBN: ,1994. [4]. Maliheh Alsadat Mousavifar,Ashkan Parchizadeh,Sedigheh Mahdavi, Comparison Between EOR Method In One Of fractured Iranian Reservoir, International Journal of scientific & Engineering Research, Volume 3, Issue 4,April [5]. Sedigheh Mahdavi,Mahdi Bahraini, Simulation Study of WAG and SWAG Injection Scenarios in One of Iranian Oil Fields, International Journal of scientific & Engineering Research, Volume 4, Issue 4,May [6]. MARCO R.THIELE, ROD P.BATYCKY, L.KENT THOMAS, Miscible WAG Simulation Using Streamline,European Conference on the Mathematics of Oil Recovery _ Freiberg, Germany, 3-6 September [7]. S.Mohammadi, M.Khalili, M.Mehranfar, Optimal Conditions for immiscible Recycle Gas Injection Process :A Simulation Study for one of the Iranian Oil Reservoirs, Scientia Iranica, Chemistry and Chemical Engineering , [8]. B.Palsson, D. R. Davies, A.C.Todd and J. M.Somerville, The Water Injection Process A Technical and Integrated Approach, Heriot-Watt university, Edinburgh, UK, Vol 81,Part A, Mar

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