Tight oil reservoirs show less than 10% recovery in the US, creating a strong incentive for development and application of corresponding techniques that enhance recovery from such reservoirs. Enhanced oil recovery (EOR) techniques applied to tight oil include huff-n-puff (a cyclic injection-soaking-production approach) and flooding (injection of an oil displacing fluid from injection wells into the reservoir for production through production wells). Both approaches have seen limited success because of fundamental reasons. Huff-n-puff requires substantial mass transfer into and out of a tight oil formation during each injection-soaking-production cycle, a task that is inherently challenging because of low formation permeability. Similarly, flooding using fractured horizontal wells for injection and production faces the challenge of short-circuits created by communicating fractures from the injectors and producers. An alternative EOR approach for tight oil is to rely on a single well that is recompleted, so that both injection and production can be accomplished by that well, through corresponding injection and production fracture sections in an alternating pattern, separated from each other by packers. A dual completion can be used, which enables continuous injection and production for fracture sections connected to the corresponding (injection or production) part of the completion. A challenge of this approach is complexity, cost, and size limitations, all resulting from the dual completion configuration. An alternative is to use a single completion featuring multiple remotely activated valves, which can be used to asynchronously inject into and produce from corresponding fracture sections in an alternating pattern along the well, separated from each other with packers. When injection valves are open, production valves are closed, and vice versa, in a repeated cycle. In the first part of this cycle (production valves closed), injection increases pressure in the formation, and in the second part of the cycle (injection valves closed) pressure declines as production proceeds.
While the above idea has been proposed qualitatively, quantitative effects of various reservoir, well completion, and operational variables have not been studied. Such variables include reservoir permeability, secondary fractures, cement leaks, injection rate, and cycling pattern. The purpose of this presentation is to study the effect of such variables in a study of a single-well alternating production (SWAP), and compare the results to alternatives such as huff-n-puff.
Simulation using compositional reservoir modeling was performed. The computational domain consists of one horizontal well with 10 hydraulic fractures. The base model uses typical fluid and rock properties from the Bakken formation. We use CO2 for miscible injection. For the model studied, the results show more than 45% recovery factor for ten years, indicating that SWAP has the potential for manifold improvement of oil recovery from tight oil formation. Corresponding optimization using huff-n-puff resulted in recovery factor of about 20%, well below the recovery delivered by SWAP. We ascribe the superior performance of SWAP to the fact that, unlike huff-n-puff, in each cycle of SWAP only the pressure front has to traverse the area between injection and production fractures, as the injection fluid sweeps that area.
The proposed methodology demonstrates a workable single-well EOR strategy that could guide the design and operation of a completion capable of cost effectively increasing oil recovery from tight oil formations.
