Abstract
Abstract. We present a method for accelerated optimisation of CO2 injection into petroleum reservoirs. The optimisation assumes maximisation of the net present value by coupling reservoir models with the calculation of cash flows. The proposed method is based on the construction of a hierarchy of compositional reservoir models of increasing complexity. We show that in dimensionless volumes, the optimal water and gas slugs are very close for the 1-D and 2-D areal reservoir models of the water-alternating-gas (WAG) process. Therefore, the solution to the 1-D optimisation problem gives a good approximation of the solution to the 2-D problem. The proposed method is designed by using this observation. It employs a larger number of less computationally expensive 1-D compositional simulations to obtain a good initial guess for the injection volumes in much more expensive 2-D simulations. We suggest using the non-gradient optimisation algorithms for the coarse models on low levels of the hierarchy to guarantee convergence to the global maximum of the net present value. Then, we switch to the gradient methods only on the upper levels. We give examples of the algorithm application for optimisation of different WAG strategies and discuss its performance. We propose that 1-D compositional simulations can be efficient for optimising areal CO2 flooding patterns.
Highlights
Gas flooding is a well-established method of enhanced oil recovery (EOR)
Our key findings related to the optimisation of enhanced oil recovery are
– 1-D compositional simulations are efficient for optimising areal CO2 flooding patterns
Summary
Gas flooding is a well-established method of enhanced oil recovery (EOR). The injection of gas, CO2 or the associated petroleum gas, into oil reservoirs allows for a significant increase in the microscopic displacement efficiency, ED, caused by the compositional exchanges between oil and gas, oil swelling and viscosity reduction (Lake, 1989; Blunt et al, 1993; Christensen et al, 2001; Thomas, 2008). Optimisation of gas flooding often requires determining the well patterns, the distances between injection and producing wells, and the volumes of water and gas slugs in the WAG injection (Sanchez, 1999; Kovscek and Cakici, 2005; Chen and Pawar, 2018). Thereby, the optimisation assumes finding an injection strategy, e.g. the volumes of gas and water slugs, that maximises NPV (Fig. 1). Low for detailed estimation of compositional exchanges between the gas and oil phases (Coats, 1980; Orr et al, 1995) This complicates the optimisation, because compositional modelling is computationally expensive. Since the refined model requires lager computational resources, we aim to reduce the number of simulation runs for this model This is achieved by ensuring that the coarser model from a previous level provides a good estimate for the solution to the optimisation study on the level.
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