Selection of well opening schedules in pre-salt reservoirs using WAG-CO2

Abstract

In field development projects involving many wells, the limitation of available rigs can increase the need for investments. In our literature research about the definition of well opening schedules, we found some proposals but could not find studies that deep dive into this topic, nor any that propose an approach that could be applied to the Brazilian pre-salt fields. Therefore, this research focuses on evaluating the influence in the NPV of well opening schedule of a project using WAG-CO2, inspired in pre-salt fields, and we propose a simple procedure to specify the well opening schedule without having to perform many simulations or complex analysis. We used the UNISIM-II-D-BO benchmark (synthetic simulation model with characteristics of a Brazilian pre-salt field) and a well configuration proposed in a previous study, with 11 producers and 10 injectors. First, we calculated the distance between the producer and injector wells and evaluated the well economic indicator (WEI) of each well by opening all at the same time at the beginning of the production schedule. We then evaluated 252 combinations of those injector wells, where five of them were injecting water and the other five were injecting gas, from where we defined three reference alternatives to check the repeatability of the results. With all these data, we tested predefined schedules: (1) with the 1st injector well injecting only gas or participating in the WAG cycle; (2) the producers' opening sequence based on their WEI; (3) the injectors’ opening based on their WEI or distance to producers; and (4) testing the relationship between producers and injectors (number of producers to open before the injectors), making sure that all the gas could be reinjected. For comparison purposes, we then run an optimization algorithm to select the schedule, using 2,660 simulations. At the end, we verified that the difference between the optimization with the algorithm (demanding thousands of simulations runs) and the proposed schedules (demanding few simulations) in the NPV was about 2%. Thus, to speed-up decision making, it is possible to have an opening schedule with good performance considering a simple procedure, as follows: 1st injector well collaborating with the WAG; producer wells opening from the highest to the lowest WEI; injector wells opening from the closest to the furthest distance to the producers; and the relationship between producers and injectors has to be investigated, not only aiming to find the best economic return, but also because of the environmental restriction (the need to reinject all the produced gas). This procedure reduces the number of simulations that must be performed if an engineer decides to use an optimization technique.