In this study an attempt was made to understand the reduction in permeability caused by water vaporisation on reservoirs. For this purpose, experiments were performed to determine reduction in permeability caused by the simultaneous flow of gas and brine. In some cases, water is immobile but vaporisation can still take place. In the latter scenario, experiments were performed to determine the rate of water vaporisation at different pore pressures in the range of 1000 to 1950 psi, at constant temperature of 90 °C. This is probably the first attempt to measure water vaporisation in porous media at high pressure and temperature. Tests indicate that the simultaneous flow of dry gas and brine (50 g NaCl/l) at GWR of 1 could cause a reduction in permeability of approximately 35%. Also, it was found that, at constant temperature, the rate of water vaporisation in reservoirs at initial water saturation increases as the pressure decreases. The values of water vaporisation ranged from 0.027 to 0.013 gH2O/min for 1000 psi and 1950 psi, respectively.
Vaporisation of water due to gas production can result in precipitation of halite in reservoirs. Furthermore, the solubility of other scales such as CaCO3, CaSO4, SrSO4 and BaSO4 decreases when the amount of NaCl dissolved in the formation water decreases. Water vaporisation in oil fields can occur in different scenarios: a) As a result of the increase of the molar water content in the gaseous phase as the pressure declines at constant temperature around the wellbore, and b) when dry gas is flowing in the reservoir water is vaporised in order to fulfill the thermodynamic requirements at a given pressure, temperature and salinity of the brine. Vaporisation occurs even at conditions of initial water saturation, favoring the precipitation of salts especially around the wellbore. Usually, precipitation of halite has been associated to water vaporisation and reported as the cause of permeability reduction in some oil fields and as a potential problem in many others, especially in high pressure, high temperature oil fields (HP/HT) which are characterised by very high salinity brines1. Some measurements of water vaporisation have been performed in PVT cells2,3, but the authors of the present study are not aware of measurements done before in porous media at high pressure and temperature. Knowing the water content of the gas being produced is a key parameter to monitor possible dehydration around the wellbore and to prevent potential scaling problems due to water vaporization.
Experimental studies in PVT cells to determine water vaporisation have been reported by Dodson and Standing2. It was found that the amount of water in the gaseous phase increases with temperature and decreases with pressure and solids content. The authors did not consider the effect of the porous media or water saturation on the vaporisation rate.
Morin and Montel4 studied the dehydration process in order to predict the conditions for precipitation of NaCl in the tubing.
