Development of Universal Proxy Models for Screening and Optimization of Cyclic Pressure Pulsing in Naturally Fractured Reservoirs

Abstract

Abstract Economic success of an IOR project primarily depends on the effectiveness of the selected method in the reservoir of interest and the way it is deployed. Due to the heavy computational work required for a large number of simulations, it is rather an arduous task to determine optimum design schemes for a given project. Proxy models that mimic reservoir models can potentially offer an opportunity to reduce the number of simulation runs and allow timely reservoir-management decisions to be made. In this study, a high-performance screening/optimization work flow is presented to narrow the ranges of possible scenarios to be modeled using conventional simulation. The work flow proposed consists of three fundamental steps:Creating a knowledge base with simulations for a wide range reservoir characteristics and design scenariosTraining neural-network based proxy models with the knowledge base generated in Step 1Using the genetic algorithm to search for the optimum design scenario by evaluating the objective function via proxy models This work flow is applied to the cyclic pressure pulsing process with CO2 and N2 in naturally-fractured reservoirs. Developed proxies are universal such that the range of their applicabilities is extended to a wide spectrum of reservoir characteristics, including initial conditions, well spacing and reservoir fluid types (heavy, black, and volatile). Proxy models proved to be able to predict critical performance indicators such as cumulative oil production, and oil flow rates within high levels of accuracy. The genetic algorithm is coupled with proxy models to maximize the discounted oil volume produced per injected volume for a specified period of operation. After utilizing the aforementioned work flow, it is possible to significantly reduce the computational time and manpower expended with an initial screening of the cyclic pressure pulsing process. In this way, more effective design strategies can be structured for the reservoir under consideration. Introduction In low-permeability reservoirs that are dissected by a network of interconnected fractures, solution channels, and vugs, water-flooding and gas flooding are not fully effective, as the injected fluid tends to channel through the high-conductivity network and bypass the low-permeability, oil-bearing matrix.1;2 In this type of reservoirs, cyclic pressure pulsing with gas has been found to be more effective. Fractures provide a large contact area for the injected gas to penetrate and diffuse through the low-permeability matrix. Because it is a single-well process, well-to-well connectivity is not required. The initial capital investment is low and the payback period is rather short as compared to that of field-scale flooding projects. This makes the cyclic pressure pulsing a low-risk process. The process is characterized by three stages: During the injection period, the gas is injected into the reservoir. After the injection period, the well is shut-in to wait for the injected gas to interact with reservoir fluids by diffusing from fractures into the matrix. This period is called the soaking period and its duration is typically 2–4 weeks. After soaking is completed, the well is brought back to production. Typically, a large amount of gas is produced at the beginning, while the oil production rate starts to rise and reaches a peak rate. After this point, production may continue until the economic limits are reached, and if necessary, another cycle can be initiated. In Figure 1 these stages are illustrated with their impact on the oil flow rate with time.