Effects of Confined Space on Production from Tight Reservoirs
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AbstractA rigorous method is presented for incorporating confined space into thermodynamic modeling for use in compositional reservoir simulation. Capillary pressure is incorporated as a function of pore size, wettability, and fluid phase properties. Simulated reservoir production is compared for cases with and without including capillary pressure effects due to small pores. During thermodynamic modeling, oil and gas phases are assumed to exist at different pressures, which difference is the capillary pressure. This becomes significant for small pores and is described by the Young-Laplace equation. Including capillary pressure using this method effectively and rigorously couples pore size with the thermodynamic calculations involved in compositional reservoir simulation. The interfacial tension (IFT) required for capillary pressure calculations is determined using the Weinaug and Katz parachor method. These methods are incorporated into an in-house developed compositional reservoir simulator to predict changes in production. Results from modeling show changes in fluid behavior due to confinement. The phase envelope calculated including capillary pressure exhibits a decrease in the cricondenbar and increase in cricondentherm compared to the bulk space phase envelope. Therefore, the bubble point pressure decreases and two-phase flow is reached later in production from an oil reservoir. When the reservoir contains both oil and gas phases, the oil saturation is higher in confined space compared to bulk space. Simulation results show that in a tight oil reservoir, oil production rate including confinement is larger than when assuming no capillary pressure due to this higher liquid saturation. Therefore, produced gas-oil ratio is smaller. Implementation of capillary pressure into compositional reservoir simulation until present has been achieved by use of the Leverett j-function with and without IFT correction. The method proposed here is more rigorous in that capillary pressure is calculated based on pore size and fluid compositions. Thus, as fluid compositions in the reservoir change due to production, capillary pressure successfully reflects those changes.
