New Methods and Technologies in Petroleum Geology, Drilling and reservoir Engineering. June Krynica, Poland

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1 15 th International Scientific and Technical Conference New Methods and Technologies in Petroleum Geology, Drilling and reservoir Engineering June Krynica, Poland IMPROVED RECOVERY a GOOD OPTION or NOT? UNIVERSITY of ZAGREB Faculty of Mining, Geology, and Petroleum Engineering Department of Reservoir engineering Pierottijeva 6, Zagreb, Croatia Ivanka Jüttner Tel: Fax: o ijutt@rgn.hr Boris Kavedžija Tel: bkave@rgn.hr Abstract It is known that the days of cheap and easy exploited oil has been passed. When production of oil is declining by natural depletion and by water flooding, and new discoveries are smaller in size the implementation of new technology processes would be the contribution to oil production. Substantial amounts of hydrocarbons have been uncovered on a macroscopic and microscopic scale in reservoirs. Remaining reserves of oil in the Republic Croatia are to be found in old reservoirs in their final development phase. Article gives an overview about selective introduction of new technologies to improved recovery on a macroscopic and microscopic scale in known reservoirs. Key words: improved oil recovery, enhanced oil recovery, gas injection, and immiscible or miscible condition. Introduction New crude oil deposits when found are produced in the most part by primary mechanisms such as solution gas, gas cap, gravity and water drive. However, as these reservoirs became depleted of their natural energy, it is fact that great quantities of residual hydrocarbons are still in place. At the end of this depletion period, the recovery factor is very low and for improving oil recovery, water or gas can be injected in order to maintain or to restore the

2 reservoir pressure. Most of secondary recovery methods are displacement methods and depend upon injecting fluid that displaces oil. The degree of success of any secondary recovery process will depend on the reservoir rock and fluid properties and of the properties of the injected fluid. But, by these conventional techniques, a very significant amount of the oil in place remains in the reservoir, and it has been the impulse for further investigation more efficient recovery methods. Tertiary processes, or enhanced oil recovery (EOR) consists of methods aimed at increasing ultimate oil recovery by injecting appropriate agents to the reservoir. Enhanced oil recovery includes thermal, chemical, and gas miscible processes. One of miscible process is injection of carbon dioxide to enhance oil recovery and it is applicable to both secondary (mobile oil) and tertiary (residual oil) floods, and to either continue injection of carbon dioxide or water alternating gas (WAG) processes. The recovery increasing Compensation of decreasing domestic recoverable reserves can be supported by further development of existing producing fields. in addition to successful exploration. Many simulations of enhance oil recovery (EOR) displacement processes have been performed for some oil field, which are at the end of secondary production. Three oil fields were selected as basis runs Ivanić, Žutica and Križ. After simulations results show that miscible carbon dioxide flooding would be optimal of predictive models, the obtained enhance oil recovery method for Ivanić oil field. Geological Description The Ivanić oil field is located 35 km SE of Zagreb. The Ivanić structure lies within Sava depression, and is relatively deeply positioned. It is low develop anticline striking from northwest to southeast and southwest to northeast. The oil and gas saturated sediments are of miocene age and the productive reservoir of this field are named Gamma series. The reservoir rock of the stratigrafic formation is sandstone, fine-grained quartz to medium and micaceous, with clayish binding material. The productive layers of Gamma series consist of two layers: Gama 5, and Gamma 4-1, and they are production object of oil field Ivanić. In a deeper layer Gamma 4-1 are seven intervals Gama 4, Gama 3, Gama 2/1, Gama 2/2, Gama 2/3, Gama 2/4, and Gama 1. The average reservoir depth was on 1570 m, depth of initial water-oil contact is 1600 m, and the average reservoir thickness is 48,5 m, The initial formation pressure was 187,5 bar, and the formation temperature 98 0 C. The oil reserve Gamma series of oil field Ivanić was m 3, and the reserve of Gama 2/4 interval m 3. The petrophisical characteristic of reservoir rock: porosity 22,8 %, permeability µm 2, and the average water saturation 32 %. The deepest interval Gama 2/4 is the object for pilot test of tertiary method, injection of carbon dioxide in Ivanić oil field.

3 OIL WATER Figure 1. Cross section of oil field Ivanić The production History In the last forty years through primary and secondary phases in oil field Ivanić were produced m 3 of oil, and 1,246 MM m 3 of gas. The oil recovery has been about 40, 4 % of proven reserves. Remaining oil in the reservoir is about m 3. In primary phase was dominant solution gas drive, and maximum depletion was achieved in the year First time in Croatia have been applying injection of water in entire oil reservoir, started in the year The injection well system is staggered line drive, with four production wells between two edges lines and one line in the middle. The reservoir pressure declined from initial value of 187,5 to 107 bars. Maximum production in secondary phase oil field Ivanić was in the year The matching between predicted and real production is very good. (Fig. 2.) As is above mentioned, the deepest layer Gama 2/4 of all seven oil layers is the object for pilot test of carbon dioxide injection in Ivanić oil field. The initial reserves in Gama 2/4 layer are m 3 of oil, and the recovery in primary phase was only 8-9 %. To moderate the natural reservoir energy decrease the controlled injection of water began in the middle of the year The measured reservoir pressure was above 160 bars. When invasion of water started in well Iva-11, and Iva 19, and production was ceased. It is observed that in period from the year 1972 to 1979 in Gama 2/4 were injected layer significantly larger quantities of water than it was produced fluid.

4 REAL CALCULATED OIL PRODUCTION WATER PRODUCTION VOLUME OF INJECTED WATER Fig. 2. Comparison of actual produced and calculated quantities of oil and gas vs. amount of injected water Due to technical reason in the Gamma 2/4 layer, the well Iva-11 produced fluid m 3 /day with fraction of 75 % water, and Iva-19 produced 20 m 3 /day with fraction of 85 % water. Before injection of carbon dioxide the reservoir pressure was 140 bars. Most principles applied in enhanced oil recovery (EOR) methods have been known for a long time, and the last three decades numerous researches, and laboratory studies have been intensives. According to screening criteria for particular method a series of EOR simulations were performed and the obtained results show that miscible carbon dioxide flooding would be optimal enhanced oil recovery for Ivanić oil field. During laboratory experiments the PVT analysis of oil in place and carbon dioxide have been conducted to define the influence on oil properties. To achieve miscibility between carbon dioxide (CO 2 ) and reservoir oil, leading to oil swelling, viscosity reduction and better oil recovery. Changes in viscosity and swelling factor versus injection pressure are shown in figure 3. VISCOSITY SWELLING FACTOR SWELLING FACTOR, m 3 Injection pressure CO 2, bar Fig. 3. Changes in properties of reservoir oil, swelling factor, viscosity vs. injection pressure

5 A practical method of defining a characteristic multiple contact miscibility, between injected fluid and oil is the slim-tube test, from which the minimum miscibility pressure (MMP) was determined. It has been performed on four samples of reservoir oil. The obtained results are shown in fig.4. OIL RECOVERY % MINIMUM MISCIBILITY PRESSURE, bar Figure 4. Minimum miscibility pressure vs. oil recovery The simulation of depletion oil by injection of carbon dioxide has performed on Reservoir simulator COMP3 Scientific Software Intercomp. The process have been consist of two phase: The increasing of reservoir pressure from 140 bars level of MMP by injection of water without any production, Depletion of oil by alternating injection slug of carbon dioxide and water in five cycles and than injection of water to the end of process. Increasing of reservoir pressure would long 1.3 year, through two wells with daily capacity of 400 m 3, and after that injection the same calculated volumes of carbon dioxide and water in reservoir condition (WAG 1:1). Calculated volumes of carbon dioxide in every cycle are 8 millions, and water m 3, about 5 % of pore volume saturated with hydrocarbons. After last cycle continue with injection of water till the end of process (WOR= 10), respectively to water flooding of 90 %. Initial simulated reserve of oil, are m 3 Initial volumetric reserve of oil, are m 3 Reservoir pressure, bars Minimum dynamic pressure on the bottom of injection well, 350 bars Initial daily volume of CO m 3. Initial daily volume of water 160 m 3.

6 The basic characteristics of simulated process are: The production increasing would be achieved in 4 th cycle. In 4 th cycle, the GOR is increasing, respectively breakthrough of CO 2 in production wells Additional recovery till criterion of flooding WOR=10, is 8.6 % of initial simulated reserves. Total volumes of injected CO 2 through five cycles are 40 millions m 3, Total volumes of injected water through five cycles are m 3. On the characteristic of depletion the process continues without early breakthrough of CO 2 in production wells. But, geological model of reservoir with four intervals in vertical cross section doesn t show real reservoir heterogeneity of reservoir. Intervals with high permeability (33 md) but small height (10-20 cm) exist, would be influence to earlier breakthrough of CO 2. Conclusions: Pilot test of injection of carbon dioxide to Gamma 2/4 layer is a tertiary phase of production, which indicate new reservoir engineering in Ivanić oil field. Even though the process was performed on a one part of oil field Ivanić, it gives a new hope for additional recovery of oil. The results of the pilot test can be extended to the whole Ivanić oil field. In the case of the large scale recovery further experiments are suggested. References: 1. Latil, M., Enhanced Oil Recovery, Gulf Publishing, Houston, (1980.), pp Glaso, O., Generalized minimum miscibility pressure correlation. Soc. Petroleum Engineers J. 25(6) (1985) Benham, A.L., Dowden, W.E- & Kuzman, W.J., Miscible fluid displacementprediction of miscibility. Petroleum Trans AIME, 219, (1960.) Simon, R., Rosman, A. & Zana, E., Phase Behavior properties of CO 2 - reservoir oil systems. Soc. Petroleum Engineers J. 18(1) 1978.) Mungan, N., Carbon Dioxide flooding-fundamentals. Canada Petroleum Technol., 20(1) (1981.), Jüttner, I., Dry Gas Injection for Miscible Displacement on Žutica Oil Field. (1996.) Geologia Croatica vol. 49/2,