Water injection or reinjection in geothermal reservoirs has been proven as a successful engineering technique with many benefits such as maintaining reservoir pressure and sustaining well productivity However, many questions related to water injection into geothermal reservoirs still remain unclear, for example, how the in-situ water saturation changes with reservoir pressure and temperature, and how the reservoir pressure influences well productivity In order to answer these questions, we studied the effects of temperature and pressure on the in-situ water saturation in a core sample using an experimental apparatus developed in this study It was found that the in-situ water saturation decreases very sharply near the saturation pressure but not to the residual water saturation When the mean pressure in the core sample decreases further, the in-situ water saturation decreases sharply again to the residual water saturation Also investigated were the effects of pressure on well productivity Index We found that well productivity Increased with an Increase of mean reservoir pressure within a certain range and then decreased The well productivity reached the maximum value at a pressure close to the saturation pressure The results of this study may be useful to evaluate projects such as the waste water injection scheme at The Geysers