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Investigation of Gas Recycling for Development of Carbonate Oil Reservoir

A. Khormali, H. Jafarpour, A.V. Petukhov, D.G. Petrakov

National Mineral Resources University (Mining)

Summary

The effectiveness of gas recirculation and simplicity of its implementation in carbonate reservoirs can determine that increasing in production rate can be carried out at the stage of drafting development and operation of oil fields. In this work, a new model was developed to consider the effect of gas recycling on production indicators depending on the amount of gas injection by Eclipse (Schlumberger) software. This developed model has been used for considering the reservoir pressure, gas oil ratio, recovery factor and water cut of the carbonate reservoir during its long period of production. The observations showed that model has optimum time period at high amount of recycling into the synthetic carbonate reservoir.

Recovery factor of the reservoir was increased twenty times more than the case without injection after the defined production time. The optimum reservoir pressure is determined by this simulation method with minimum water cut. Also, in case of gas recycling, the reservoir pressure is maintained constantly over the production time that it leads to increase the efficiency for production and keep it in a desirable level during the time that In fact it's one of the noticeable points for keeping and preserving the reservoir during its production time.

Introduction

In recent years, the structure of proven hydrocarbon reserves is undergoing noticeable qualitative change. Recycle-gas injection is a promising recovery process to produce oil and gas. Great prospects of gas recycling tendency are associated with the region, where there are fields with high potential in the gas content of the condensate and volatile oils (Aghabozorgi and Khormali, 2014). These fields are characterized by very hard thermobaric conditions. In connection with that, interest, which is based on injection of the gaseous agent, is renewed to technology of field development (Stuart et al. 2013).

The combination of the cycling process with the injection of high pressure gas into the oil-saturated interval is represented by the presence of the attractive oil rim. Reservoir pressure maintenance in the gas-condensate zone and the implementation of an effective mechanism of oil displacement (in the region of condensation or evaporation) allows count on to fold increase of recovery factor of hydrocarbons in comparison with the development via depletion (Romanov and Zolnikova, 2008). Deformation of contacts phases (oil and oil-water-gas) is considered an important problem in the equilibrium gas recycling, i.e. in the absence of phase transitions.

The most significant hydrocarbon reserves are confined to sandy and carbonate reservoirs. Identification of oil reservoirs is conducted complex of well logging and analysis of laboratory data, taking into account the entire of geological information for the field (Petukhov, 2002). In the study of carbonate reservoirs, in addition to traditional lithologic and geophysical methods, ultrasonic method, capillary saturation of rocks phosphors and other methods are used (Shawket et al. 2006).

Optimization of pressure on the separation stage gas recirculation allows the reduction in the compressor capacity, energy costs for gas compression and loss of hydrocarbons from condensing (Eva and Nestor, 1998). Models of carbonate reservoirs play a major role for optimization of injection pressure during oil field development.

The objective of this study is the investigation of gas recycling process during the production time in a carbonate reservoir by simulating the process. The pressure and gas oil ratio (GOR) and production rate changes in the reservoir are considered. The model of this work is simulated by Eclipse (Schlumberger) software. Consequently, the possibility of simulation applying exists for solving complex problems associated with the development of gas condensate fields with oil banks, avoiding the cumbersome modeling.

Material and Simulator Development

In this study, a simulator has been developed based on the properties of a carbonate oil reservoir, which are shown in Table 1. The following procedure exists during the modeling of the carbonate reservoir:

1. Production with any gas injection for 9000 days from the field.

2. Production from the reservoir with injection of 1/4 of average gas production rate (0.25 Qgp).

3. Production from the reservoir with injection of 1/2 of average gas production rate (0.5 Qgp).

4. Production from the reservoir with injection of 3/4 of average gas production rate (0.75 Qgp).

The following parameters are considered for evaluation of the simulation model: oil field efficiency, oil field production rate, produced total oil from field, gas field production rate, produced total gas from the field, recovery factor and water cut.

Each injection group is assigned a certain fraction of the available gas from the source. These fractions are initially calculated in proportion to the groups' gas injection potentials. After each pass through the injection calculation loop, these fractions are recalculated using the actual gas injection rates. Thus, if a particular injection group injects significantly less than its allocated share of available gas, its unused share of available gas is re-allocated to the other groups. It may take several loops around the injection calculation before the available gas fractions converge. With the composition of formation fluid, which is formed at each mixing, have been calculated the contact of condensation, a result of which, the compositions of the liquid and gas phases are obtained at different pressures.

Table1 The properties of the synthetic reservoir model used in simulations for carbonate oil reservoir.

gas recycling oil reservoir

Results and Discussion

The simulated reservoir model is based on the properties stated in Table 1.

Figure 1 Oil production rate of the reservoir over the production time.

Figure 1 illustrates that in the position of natural production (without gas recycling) the production rate decreases during the production time and lead to zero. In the position of injecting gas with three different rates, it could be seen that oil production is increased to a limit point (pick) and then decreased to a constant value and continued constantly during the time. Also, it could be seen that higher gas injection rate is leaded to higher oil production rate until the pick point.

a b

Figure 2 Gas oil ratio (GOR) (a) and Gas production rate (b) during production

As in figure 2a, in the position of natural production (without gas recycling), over the time, GOR is constant. Also in the position of injecting gas in the rate of 0.25 Qgp there is no effect on GOR over the time. But, in the position of injecting gas in higher rate (0.5, 0.75 Qgp), it could be seen the increasing of GOR over the time.

As could be seen in Figure 2b, in the position of natural production (without gas recycling) the gas production rate decreases during the production time and lead to almost zero. In the position of injecting gas with three different rates, it could be seen that gas production rate is increased. Also, we have seen that higher gas injection rate is leaded to higher gas production rate over the time.

Figure 3 Recovery factor of reservoir.

Figure 3 shows that in natural production (without gas recycling) the recovery factor is increased with the very low slope during the production time. In case of injecting gas with three different rates, it could be seen that recovery factor is increased. Also, it illustrates that higher gas injection rate is leaded to higher recovery factor over the time.

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a b

Figure 4 The dependency of the reservoir pressure vs. field efficiency (a) and total water cut during production (b).

As shown in Figure 4a, in case of natural production (without gas recycling) the reservoir pressure is decreased with the sharp slope. In the position of injecting gas with three different rates, it could be seen that reservoir pressure is increased to a limit point (pick) and then decreased. Also, we have seen that higher gas injection rate is leaded to higher reservoir pressure until the pick point.

Figure 4b depicts that in natural production (without gas recycling) the total water cut has been increased over the production time. In the position of injecting gas with three different rates, it could be seen that total water cut is decreased with the sharp slope. Also, it shows that the higher gas injection rate is leaded to lower water cut over the time.

Conclusion

The following conclusions can be drawn from this research for the simulated carbonate oil reservoir:

· The oil production rate is increased during 3000 days by increasing gas injection rate, after that goes to a constant value for all gas injection rates.

· GOR is constant during 3000 days, but after this time, for high gas recycling injection rates GOR is increased sharply. GOR remains constant at low gas injection rates.

· Gas production rate and reservoir recovery are increased at high gas recycling rates at any time of production. The long pressure maintenance occurs at high gas injection rates until the first 3000 days of production; water cut is declined rapidly by gas recycling.

References

1. Aghabozorgi-Nafchi, S. and Khormali, A. [2014] Rapid Determination of Optimum Wettability Alteration Radius for Gas-condensate Reservoirs. 6th EAGE Saint Petersburg International Conference and Exhibition, Russia, Extended Abstracts, Th CH 05.

2. Eva, S.O. and Nestor M.R. [1998] A New Methodology to Determine the Optimal Pressures of the Stages in the Separation Oil-Gas. SPE 39859, International Petroleum Conference and Exhibition of Mexico, Villahermosa, Mexico.

3. Petukhov, A.V. [2002] Theory and methodology of structural and spatial zoning study in fractured oil and gas reservoirs. Ukhta State Technical University, Ukhta, (in Russian).

4. Romanov, A.S. and Zolnikova, E.F. [2008] Maintenance Of Reservoir Pressure In Gas And Oil Deposit By Gas Injection. SPE 117426, Russian Oil and Gas Technical Conference and Exhibition, Moscow, Russia.

5. Shawket, G.G., Bertrand, M.T. and Douglas, A.B. [2006] Modeling Original Water Saturation in the Transition Zone of a Carbonate Oil Reservoir. SPE 88756, Reservoir Evaluation & Engineering, 9(6), 681-687.

6. Stuart, R.Y., Ken, J.F. and Gerard, F.C.N. [2013] Recycling redundant coreholes. SPE 167050,

7. Unconventional Resources Conference and Exhibition-Asia Pacific, Brisbane, Australia.

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