Microseismic events along preexisting zones of weakness occur in a reservoir due to pore pressure buildup and fracturing during fluid injection. We have used a multiphase fluid-flow numerical simulator to model water injection in a gas reservoir. Previous studies generally consider a single fluid, in which the relative permeabilities and capillary pressure play no role. We analyze the effects of partial saturation on the injection process. On the basis of a spatial distribution of weak stress zones and a threshold pore pressure, the simulator models fluid transport in the formation and allows us to obtain the spatiotemporal distribution of the microseismic events. We consider uniform and fractal distributions of the pore pressure at which microearthquakes are triggered. We analyze the influence of the initial water saturation and the presence of preexisting natural fractures, as well as the effect of updating the rock properties after the microseismic events occur. Moreover, we perform simulations in a low-permeability reservoir in which the borehole pressure increment generates a system of fractures that propagate into the reservoir. The importance of considering two-phase fluid flow resides in the fact that partial saturation greatly affects the trigger time evolution. This is mainly due to the difference in compressibility of the two phases.