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One of the key uncertainties associated with miscible water-alternating-gas (WAG) projects is the well injectivity. The underestimation of productivity indexes of the injectors always results in the loss of additional oil production. This paper describes the current solutions and approaches to the design and implementation of nitrogen injection pilot into the injection well on the remote oil field in order to adjust the technical and economic indicators of miscible WAG process. First of all, according to the probability distribution of permeability on the field, the well selection process was carried out. At the second stage, due to the high capital costs of pipeline construction, a gaseous agent(nitrogen) that may be transported in the liquid form was selected. Then, for each well, based on the simulation models, the required nitrogen resources were estimated to ensure that the steady-state flow was achieved. The final stage of pilot works design includes the selection of the measuring instruments to record injection rates, pressures (BHP, THP), temperatures in the well and production logging. The main result of the pilot injection of the nitrogen into the reservoir was the confirmation of the key success criteria: productivity indexes and gas injectivities, tightness of the cement in the wells and the possibility of displacing the water from the wellbore to the reservoir at the current tubing head pressure. During the analysis of the results, the simulation model which were designed to calculate the predicted effects of the implementation of the miscible WAG on the field were adjusted to match pilot tests results. Application of the results of pilot injection allowed to increase a duration of the water injection cycles without loss of the gas injection volumes and gas utilization levels. The duration change in the operating cycles of the WAG, in turn, makes it possible to increase the displacement efficiency, which leads to an increase of the cumulative oil production by 10%. Based on the key uncertainty management studies: laboratory studies to study the processes of oil and gas interaction (slim-tube test, asphaltenes precipitation test), special core analysis, pilot gas injection and economical evaluation, a decision was made to launch a miscible WAG project in the oil field. In this paper, it is shown that the implementation of a nitrogen injection pilot allows us to reduce the uncertainties in injectivity for wells originally constructed not for gas injection. Correct analysis of the results gives a deeper understanding of the processes occurring in the injection wells during the transition period from water to gas injection. This field test of nitrogen injectivity gives a good opportunity for the full filed WAG project implementation.

The paper describes the development of an EOR project for a supergiant oil rim field, and how the optimal injected gas composition was determined. This includes the laboratory tests to develop a miscible gas equation of state (EOS), cryogenic plant design considerations, and the evaluation of gas EOR development strategies for the different Novoportovskoye reservoirs. Facilities models were built to evaluate options to increase extraction of C2-C4 components from produced gas. This rich gas can then be mixed with lean gas to create miscible injectant (MI) for EOR. Because the gas caps are so lean (90+% methane), the best options were to chill the produced gas to either −55°C or −80°C with a turbo-expander unit. The lower temperature option requires special metallurgy, but it boosted extraction by 35%. The first EOR PVT model was designed from the limited compositional data available. MI compositions for each reservoir were developed. A laboratory experimental program was undertaken to calibrate the EOS for Novoportovskoye EOR. The new EOS was used to determine the optimal composition of the MI. According to slimtube tests and simulation studies, the optimal composition of miscible injectant is 62-67% methane, with the rest being C2-C4. The currently planned waterflood patterns and gravity drainage/gas cap expansion areas were studied to determine the best areal and cross-sectional location of the MI injectors. An integrated model was built to account for injected and produced gas volumes and compositions. This allowed the analysis of bottle-necks in the production network, cryogenic plant, and gas facilities. WAG in most areas performed better in terms of gas utilization, which leads to increased incremental oil given the same amount of MI available. By choosing the most efficient patterns in terms of gas utilization it is possible to increase oil recovery in these areas by 10-15% OOIP.

ВThe application of miscible displacement technology is associated with significant risks and uncertainties. Therefore, it is important to plan a lab programme carefully with the aim of mitigation or eradication of those risks at the design stage. This stage conventionally includes lab analysis, pilot projects, simulations, an analysis of and interactions with potential technology suppliers, etc. This article presents an overview of an adequately detailed laboratory research programme required to apply miscible displacement, and also highlights the important aspects of each experiment that deserve consideration.

Currently, for the most oil companies it is possible to identify a general tendency that the reservoir permeability for the new prospects is decline. Reserves quality deterioration increases the risk of the future oil production failure. This paper describes the organization of an integrated approach to source data analysis, simulation model constructon and miscible displacement design in a low-permeable reservoir. At the first stage, a reservoir fluid model is constructed. Based on the lack of a qualitative oil sample from the well (original sample was significantly depressurized during testing) and on the analysis of the fluid properties of the analog fields, the prediction of the main oil parameters was made. At the second stage, using static parameters of a low-permeable reservoir, probabilistic simulation models of the field was constructed, providing a coverage of the main spectrum of reservoir uncertainties. Further, import gas resources were estimated for the injection implementation. The main result is an integrated assessment of the impact of uncertainties for oil properties and gas import resources on key economic indicators for a miscible displacement at the field with a low-permeable reservoir using the reservoir simulation model. The analysis verified the key parameters that have the greatest impact on the economy of the project, determined the threshold values for the volume of gas imports, and based on the feasibility study made recommendations on the choice of the pilot area for injection. A program for conducting additional laboratory experiments was also prepared, this study will allow the narrowing of the oil composition uncertainties and its interaction with injection gas. In addition, a program is developed to carry out special core experiments on gas injection, which will make it possible to correct the behavior of phase permeabilities for a miscible displacement. This paper shows that on the basis of reservoir simulation an integrated approach organization for the verification, accounting and analysis of uncertainties on reservoir characteristics, fluid properties and volume of imported third-party gas allows to search for and select an economically viable and sustainable option to develop an oil field using miscible gas injection technology. An implementation of this approach provides a recovery factor increase from 7% to 25%.