An Integrated WAG Characterization Study for an Offshore Oilfield

Abstract

Water-Alternating-Gas (WAG) injection is an enhanced recovery method that is being applied in some brazilian offshore oilfields as an alternative to combine effective pressure maintenance policies, flexible produced gas management strategies and increased recoveries. In this technology, gas plays the role of reducing residual oil saturation while water controls, by multiphase flow-in-porous-medium effects, gas adverse mobility. In addition to the extra engineering tasks needed to design development plans and production facilities, an intense characterization programme should be put in practice in order to reduce risks, increase predictability and optimize WAG floods. Much more complex laboratory tests than the ones usually performed for continuous water/gas floods should be done, particularly to tackle multiphase flow in porous medium phenomena. The proper characterization, modeling and simulation of these effects are vital for a representative WAG design and evaluation. This paper presents the results of a pragmatic and integrated laboratory characterization, modeling and simulation study for an offshore oilfield that was focused on relative permeability hysteresis effects. Firstly, it summarizes the best relative permeability hysteresis model (Larsen & Skauge) available in most commercial reservoir simulators. Then, it describes a special laboratory WAG multiphase flow-in-porous-media characterization program, designed and executed to obtain the parameters of this hysteresis model. Experiments were done at reservoir conditions and with rock and fluids from an actual offshore field. Following, results of these tests were evaluated in light of Larsen & Skauge model, in order to confirm its validity. Finally, WAG simulations of a sector of the target field were performed to access the impacts of including or neglecting relative permeability hysteresis phenomena. Conclusions of this work contribute to increase predictability and reduce uncertainties related to field-scale implementation of WAG technology. Knowledge acquired from this study promoted a better estimation of oil recovery, gas production and overriding. It also supports the design of subsea and topside equipment, which is critical in the offshore scenario.