Multiphase flow is commonly exhibited for wells producing from low-permeability liquid-rich gas or oil reservoirs. In such cases, rate-transient analysis (RTA) methods that account for multiphase flow may be applied to derive critical hydraulic fracture, reservoir and fluid-in-place estimates. One RTA approach is to use a numerical reservoir simulator to history-match production rates and/or flowing pressures to derive this information. For this application, relative permeability curves must be entered into the simulator, and variables such as fracture half-length, permeability and fluid-in-place are adjusted until the production data is matched. In many situations, laboratory-derived relative permeability curves are not available, and the relative permeability curves used in the model are arbitrarily-defined; the only criterion often used is that the curves are "reasonable".
This paper investigates the effect that different relative permeability curves have on history-match results, and specifically the impact on the linear flow parameter (LFP = total fracture area×square-root of permeability) and drainage area. This study demonstrates that different curves can result in equally acceptable history-matches of the data (for the oil cases studied) but yield different values of LFP and drainage area. Further, it is demonstrated that the impact of relative permeability on the gas-oil-ratio (GOR) depends on the flow regime: a) during transient flow, only the shape of the relative permeability curves affects GOR, and b) during boundary-dominated flow, the length of the drainage area and the permeability also affect the GOR.
Finally, it is suggested that, rather than focusing on history-matching oil rates, it is much more efficient to first history-match the GOR by adjusting the relative permeability curves. Only after the GOR has been history-matched should the individual rates be history-matched by varying fracture half-length.
The results of this study provide important guidance to reservoir engineers performing RTA or model history-matching of wells completed in reservoirs in which multiphase flow is expected.