1887

Abstract

Summary

Alternating injection of water and gas (WAG) has been widely applied as an oil recovery strategy since the late 1950s. The expected benefits are improved macroscopic sweep, with the water and gas sweeping lower and upper zones of the reservoir respectively, and improved microscopic sweep due to various effects leading to lowering of the residual oil saturation. Benefits on the microscopic scale are expected especially if the injected gas is miscible with the oil. WAG has been applied to various North Sea assets such as Snorre, Statfjord and Gullfaks.

WAG strategies are typically designed using trial simulations of different scenarios. For fields with many wells it is not generally possible to design an optimal strategy without the use of approaches to systematically explore alternative strategies. Mathematical optimization theory provides such methods. Previously, we have applied such methods to determine optimal drilling sequences for new field developments for a number of Norwegian assets under uncertainty. Here we apply similar concepts to additionally optimize the optimal injection and production strategies for drilled wells.

The development period may take a number of years if many wells are to be drilled, leading to time-varying capacity to (re-)inject gas that is difficult to take into account when the order in which injectors become available is not a priori fixed. Therefore we investigate alternative approaches to characterize WAG strategies during the field development stage, namely switching time controls and injection type controls, also in combination with injection rate controls.

We present a number of examples of numerical experiments for a representative test model. Multiple geological realizations of the model are used to represent the uncertainty. Results indicate that significant improvements in economic returns can be obtained through optimization relative to reasonable base strategies, also if the WAG strategy is optimized for a fixed drilling sequence. We show that not only the expected value can be increased, but that also the value for the worst performing realizations can be improved, thereby reducing risk. Finally, we provide physical interpretations of the optimal strategies in support of decision maturation.

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/content/papers/10.3997/2214-4609.201700326
2017-04-24
2020-09-28
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References

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