Same-Well Enhanced Oil Recovery (SWEOR) - Cyclic Hydrocarbon Production/Hydrocarbon Gas Injection in a Hydraulic Multi-Fractured Horizontal Well
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Unconventional reservoirs have gained big attention in the oil industry as these reservoirs constitutes huge amounts of hydrocarbon reserves. The required advanced production techniques as well as costs related to implementation and improvement of these techniques makes the field development of unconventional reservoirs challenging. The result is a high break-even price, which is a significant challenge for field development, especially because of the low and unstable oil price the last years. Today, the most commonly used production technique is to drill extensive lateral wells combined with a multi-stage hydraulic fracture stimulation process. Increasing the conductivity between the wellbore and the formation allows for economically high production rates by pressure depletion. However, even with the use of these advanced methods, the production rate is typically declining within a short period of time, causing an oil recovery factor in the range of a few percentages. Therefore, using enhanced oil recovery techniques to improve oil recovery is necessary. This study will analyze same-well EOR methods (SWEOR) that uses hydrocarbon gas injection to increase oil recovery. The main EOR strategy considered is called Same-Well Cyclic Hydrocarbon Production/Hydrocarbon Gas Injection (SWEOR) or in shorter terms cyclic EOR. This strategy aims to increase oil recovery by placing a single tubing string into a well that is already producing by pressure depletion. The idea is to increase the oil recovery with a low capital cost related to the implementation of the EOR strategy by exploiting what s already in place. The other SWEOR strategy is referred to as Same-Well Continuous Hydrocarbon Production/Hydrocarbon Gas Injection (SWEOR) or in simpler terms continuous EOR. This strategy uses a dual tubing string configuration, and is included during this work for comparison with the cyclic EOR strategy the latter as the main strategy. SENSOR reservoir simulator developed by Coats Engineering, Inc., was used to perform numerical simulation on reservoir models reflecting properties of unconventional reservoirs. The results obtained during this study shows that the cyclic EOR has a very good potential to increase oil recovery in such reservoirs. By assigning porosity- and permeability values to cement grid blocks, the effect of introducing hydraulic fracture communication through cement was considered. Taking the actual inflow area of the cement layer into account, the results showed that the cyclic EOR strategy is not sensitive to hydraulic fracture communication at all. Sensitivity on various surface controlled parameters was considered. Increasing the gas injection pressure can be done to achieve a faster gas displacement process for a given matrix permeability km. The performance of the cyclic EOR strategy is not sensitive to the duration of the pressure depletion period prior to the initiation time. Optimization of production-injection cycle durations ∆t showed that the cycle durations can be optimized to give a higher oil recovery factor at a given point in time prior to gas breakthrough.