Reservoir Simulation for Reinjected Water Return in Produced Fluid of Wells at Ogiri Geothermal Field, Japan

Numerical simulations of the Ogiri geothermal field were conducted using a fractured model for evaluating return of reinjected water to production wells. In this study, a MINC method was applied to grid blocks both in production and reinjection zones to describe the return of reinjected water in the reservoir. A mixing ratio of reinjected water to the produced fluid at respective production wells were quantitatively evaluated by using a function of two kinds of waters built in the TOUGH2 numerical simulator. 1. INTRODUCTION Reinjecting water back into a formation is a useful technique of recharging fluid into a reservoir. Reinjected water is heated as it flows in the formation towards the production zone. However, excessive and quick return of reinjected water to the production zone causes cooling, and consequently decreases in steam production of wells. It is then necessary to properly manage the reinjection scheme for a sustainable development of the field. Reservoir simulations provide visualization of the condition of the reservoir systems based on measured field data. Itoi et al. (2011) conducted a reservoir simulation of reinjected-water return at the Ogiri geothermal field using a three-dimensional reservoir model. The model included a grid system applied with a MINC method (Pruess et al., 1999), which was only for the shallow production zone, and the grids in other domain in the model were of porous type. In this study, we assigned grids with the MINC method to the other domains of fault zone and reinjection zone and carried out reservoir simulations using TOUGH2 (Pruess et al., 1999) to quantify mass ratios of reinjected water to produced fluid of wells. The temporal change of mass ratio of reinjected water from respective reinjection wells was analyzed.