Permeability reduction is one of the significant problems in the water injection process.Several mechanisms may be involved in permeability reduction including scale formation, suspended particle invasion and fines migration.On the other hand, rock dissolution may also lead to permeability improvement of carbonate rocks.In this research, effect of rock lithol. along with incompatibility of injection and formation brines, scale formation and plugging of pores by particles in the injection water for a layered reservoir were examinedDiluted formation brine (produced from desalination unit) was considered as injection water.Simulation of scale formation due to thermodn. conditions and mixing of formation and injection brines were performed to investigate amount and types of scales.Simulation and jar tests showed that Scale precipitation decreased with increasing the mixing ratio of injection water to 75%.For core flooding tests, both carbonate and sandstone core samples from a layered candidate reservoir were selected for monitoring of formation damage and permeability variation due to incompatibility of fluids.SEM and EDX anal. were used for static and dynamic tests.Carbonate core sample showed sever damage and competition between scale precipitation and rock dissolutionHigher flow rates contribute to permeability enhancement of carbonate rocks by prevention of scale sedimentation (less scale formation) and acceleration of carbonate rock dissolutionAs a pressure difference increasing of 60% was shown in the first stage of water injection process into the carbonate rock.Eventually, after increasing and decreasing the injection rate, the pressure difference decreased to 30 presences.Scale formation mechanism is dominant in sandstone rock samples and can be removed by backflow.As pressure difference decreased 27% in sandstone rock sample, which resulted in permeability decrease from 365 to 265 mD.