Experiments on Water Vapourization in Porous Media
Authors: Zuluaga E. , Grondona J.C. Monsalve 
Published: July 2010

Introduction
Vapourization of water in gas production can result in precipitation of salts in reservoirs. Water vapourization in oil fields occurs basically under two different scenarios:as a result of the increase of the molar water content in the gaseous phase as the pressure declines at a constant temperature around the wellbore; and,when dry gas is flowing in the reservoir, water is vapourized in order to fulfill the thermodynamic requirements at a given pressure, temperature, and salinity of the brine.
Water vapourization has been 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 reservoirs (HP/HT) which are characterized by very high salinity brines. The reduction in permeability can be aggravated over time due to capillary imbibition in water wet reservoirs into the zones where the water saturation has been reduced due to vapourization. Knowing the water content of the gas being produced is an important parameter to monitor and to prevent potential scaling problems due to water vapourization. In this study, rates of water vapourization were measured at reservoir conditions for dry gas flow. The authors are not aware of previous experimental studies performed to determine rates of water vapourization in reservoirs. The effect of reservoir pressure on the rates of water vapourization in rocks at initial water saturation was also determined.

Experiments
Pristine Berea cores at initial water saturation of approximately 18% were employed for the experiments. The initial water saturation in the cores was established by the porous plate technique since it is the only method that guarantees the uniformity of saturation distribution along the core sample. Brine at a concentration of 100 kg/m3 of sodium chloride (NaCl) and methane technical grade with very low water content were used in the experiments. The vapourization experiments were performed by flowing dry methane through the core samples. Two silica gel packs were located at the outlet end of the core to measure the humidity of the flowing gas and one silica gel pack was placed at the inlet to retain any water content of the gas injected. The core holder containing the core sample is located inside the oven to provide a constant temperature of 90 °C throughout the test.

Results and Discussion
Figure 1 presents the curves obtained for water vapourization at 90 °C and at pressures of 6,895 kPa, 10,342 kPa, and 13,445 kPa. The flow rate was maintained at approximately 8.5x10–5 m3/s measured at standard conditions (0.18 SCF/min). In these curves, the x-axis is the drying time and the y-axis is the amount of water recovered from the flowing gas. Two vapourization periods are identified in the curves: a constant rate period and a falling rate period. During the constant rate period, the water is vapourized at constant velocity. The rates of water vapourization are obtained from the slope of the curves during the constant rate period and will be referred to as CRV (Constant Rate Vapourization).