The Optimization Of Injection-production In Offshore Oilfields With The Constraint Of Fluid Capacity

At present,the development mode of most of Chinese offshore oilfields is regionally joint development,in which each monoblock oilfield is a single object to be developed and all of them are connected by a unified production and transportation system.In this kind of development mode,all of monoblock oilfields are developed cooperatively.It is noteworthy that the giant monoblock oilfield is taken as the main body while the other small monoblock oilfields are taken as appurtenances.With the development of offshore oilfields,there are more and more serious problems appearing in their regionally cooperative development.First,most of offshore oilfields have been in moderate or high water cut stage and suffered from significant production decline.Second,the expenditure of constructing new production and transportation facilities for offshore oilfield is extremely high due to the effects of its special geographical conditions.Nevertheless,the capacity of existing production and transportation system is not enough for enhancing total daily liquid production rate of each monoblock oilfield.In addition,each monoblock oilfield’s oil production potential changes and their demand of total daily liquid production rate becomes different compared with the initial design condition,because of the influences of taking specific measures for offshore oilfields production.Therefore,how to optimize the injection and production schedule of offshore oilfields with the limitation of production and transportation system’s capacity is a significant problem to be solved.However,due to the large-scale trait of offshore oilfields and the high computational cost of reservoir numerical simulation,the conventional numerical-simulation-based oilfield production optimization methods are no longer appropriate for handling the optimization of injection and production in offshore oilfields(OIPOO).With this in mind,two novel techniques are proposed in this paper,to effectively solve injection and production optimization problems in offshore oilfields.First of all,the production and transportation modes of offshore oilfields were summarized,and a four-tier structure of offshore oilfields’ production and transportation system was proposed in this paper.On the basis of that,a mathematical model of OIPOO was established.Then,two kinds of methods for the optimization of injection and production in offshore oilfields were presented.In the first method,the relation curve of the change magnitude of field water cut versus the change magnitude of liquid withdrawal rate is plotted for each monoblock oilfield according to the results of some runs of reservoir numerical simulation,in order to estimate the production performance of each oilfield with the allocated daily liquid production rate.And the optimization of injection and production in offshore oilfields is divided into two stages in this method,that is the optimization of splitting total daily liquid production rate among all offshore oilfields and the production optimization of each monoblock oilfield.In the second method,in order to realize OIPOO,the Long Short-Term Memory(LSTM)network is employed as the surrogate model of reservoir numerical simulation,forecasting each monoblock oilfield’s production performance under different production schedules.Moreover,the covariance matrix adaptation evolutionary strategies(CMA-ES)and Gaussian-distribution-based simultaneous perturbation stochastic approximation algorithm(GSPSA)are used to solve different subproblems in the optimization.Finally,the efficacy of the two methods proposed above were validated in the case of offshore oilfield CY.The results showed that the proposed methods possessed their unique advantages in solving the optimization of injection and production in offshore oilfields.All wells’ daily water injection rates and daily liquid production rates in the production schedule,which is obtained by the curve-based optimization method,change smoothly during the whole production optimization period.By using this schedule,the development efficiency of oilfield in moderate water cut is enhanced significantly at the negligible cost of the development efficiency of oilfield in high water cut,and the schedule shows superior performance on slowing down the increase of oilfield water cut.By contrast,the production schedule generated by the LSTMbased method could adjust the allocation of total daily liquid production rate at each time step,meanwhile,all wells’ daily water injection rates and daily liquid production rates in the schedule appeared as staircase curves.The development efficiency of all oilfields was widely boosted through using the schedule.Besides,the proposed technique performs well on reducing the computational cost and enhancing the oil production.In a word,with the limitation of production and transportation system’s capacity,the significant improvement of development efficiency and economic benefit on offshore oilfield CY demonstrated the effectiveness of the proposed methods.