Degradation of permeability due to the application of hydraulic fracturing fluid has been postulated to be one of the reasons why massive hydraulic fractures fail (Davis, 1975; Clark, 1977). The selection of fracturing fluid for a fracture job is not only dependent on its effectiveness of creating the fracture, in propagating proppants, but also its damaging nature. Several previous studies (Simon, et aL. 1977; Davis, 1975; Haiwka, 1972) have indicated that certain fracturing fluids are less damaging and behave differently to specific formation lithologies. The following factors need to be considered in the se1~ction of the fluid that will cause the least amount of damage in relation to the time of damage recovery, 1. Initial reservoir permeability 2. Relationship of back flow pressure to the reservoir pressure. In a previous work of fracturing fluid/rock interaction by Simonson (1976) it was concluded that tests should be performed at in situ conditions. He attained a wide contrast of results between tests at atmospheric conditions of temperature, and saturation levels and conditions at simulated in situ conditi.ons. This necessitates the requirement of studying such fracturing fluid/rock interaction for samples at their in situ conditions.