Abstract
Three-phase relative permeability is one of the main parameters in flow predictions of tertiary recovery processes in petroleum reservoirs. In particular, an understanding of the hysteresis in three phase relative permeability is essential to describe cyclic processes such as water alternating gas (WAG) injection. Several studies measured three-phase relative permeabilities through steady-state and unsteady-state indirect methods, with their own advantage and disadvantages. Among these, unsteady-state direct measurements are preferred since they mimic the natural reservoir processes while no assumptions are made for relative permeability calculations. But, previous unsteady-state direct measurements of three-phase relative permeability obtained relative permeabilities only during drainage processes. In this work, we extend this method to directly obtain oil and water relative permeabilities during three-phase flow through different flow cycles. The method involves in-situ measurements of phase saturation, and a combination of applied gas and water flow to overcome the limitations of previous direct relative permeability measurements. This method is used to directly measure the three-phase hysteresis effects in relative permeability. Specifically, we perform flow experiments in a 3ft long, vertically oriented sand pack; the sand pack is initialized at a specific oil saturation using two-phase flow. As opposed to previous unsteady-state direct measurements of three-phase permeabilities, gas and water are injected separately as different cycles and, the saturation along the sand pack is measured at different times using computerized tomography scanning. The relative permeability of each phase is then obtained directly from the measured in-situ saturations using Darcy's law. The results show that, we should pick only the middle 40cm of the saturation data to avoid the effect of capillary entry and capillary end effects on saturation data. Experimentally, we find that, the oil relative permeability rises at the beginning of each WAG cycle and then decreases almost as abruptly, with very little dependence on the phase saturations. On the other hand, the water relative permeability remains almost constant. The results also show that, the residual oil saturation decreases during the WAG process at different cycles. The results are discussed in terms of local flow processes, and potential implications for reservoir flooding are given.
