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.
