(371ah) A Control Methodology for CO2 Injection to Optimize Tight Oil Production from a Fractured Horizontal Well

A common approach to enhancing the recovery of hydrocarbons from conventional reservoirs is to inject a fluid through injection wells, so that this fluid can displace hydrocarbons out of the reservoir through production wells to the surface. The technique is known as secondary oil recovery. Recent efforts to apply this technique for enhanced oil recovery (EOR) in tight oil fractured horizontal wells have shown limited enhancement in oil recovery. An alternative to using injection and production wells for enhanced recovery of tight oil has been to combine injection and production in the same well, using some sort of dual completion. A variety of dual-completion configurations have been proposed, all of which accomplish injection and production through the same well using means of separating injection fractures from production fractures in an alternating pattern (e.g. through packers) and connecting each group of fractures to the surface by a separate conduit (e.g. tubing). Remotely activated valves can be used to adjust the flow rate in each individual injection and production section. While the feasibility of this approach to EOR for tight oil has been established, it turns out the adjusting the fluid flow rate into each individual injection section makes a significant difference in EOR economics. The main reason is that the time needed for fluid injected through injection fracture sections to break through out of adjacent production sections varies along the well. Therefore, there is an incentive to develop a methodology for optimizing injection rates over injection fracture sections over. The purpose of this presentation is to provide a relatively simple methodology developed for this purpose.

To demonstrate the proposed methodology, we use a numerical simulator as a virtual well. One horizontal well with five injection fractures and five production fractures is the base model with corresponding fluid and rock properties typical of the Bakken formation. The injection fluid is CO₂. The production pressure is above miscible pressure, so that there is only one phase in the reservoir.

The proposed methodology optimizes the oil production rate in real time by adjusting individual CO­2 injection rates into each injection fracture section­­. A simple criterion has been developed to detect injection fluid breakthrough. This criterion makes use of real-time data either through a detectable tracer or through measurement of individual section injection rate and density of total production. Manipulation consists of appropriately adjusting the flow rate from each injection or production fracture for which breakthrough of the injection fluid has been observed in an adjacent production section. Simulations indicate that using injection rather than production valves to manipulate corresponding flow rates gives better results in terms of recovered oil for an amount of CO2 injected. In addition, simulations also indicate that the proposed control strategy addresses well situations where short-circuits between injection and production fractures exists, e.g. through secondary fractures or through cement leaks

The developed real-time optimization methodology demonstrates a potential workable system for tight oil EOR through a single horizontal well.