Gas is initially in equilibrium with water at reservoir conditions. However, once a well is placed on production, the pressure drop that occurs around the wellbore causes an increase in the molar content of water in the gas phase. Water will be vaporized starting from the near wellbore zone where the pressure is small, and form a vaporization front that moves into the formation opposing flow. As water is vaporized, dissolved salts become more concentrated, and eventually minerals will precipitate into the formation. This might cause productivity impairment if water continuously supplies minerals to the wellbore area to be precipitated.
This study has developed a semi-analytical model to predict water vaporization for gas producers. The pseudo-steady state well model solves analytically the pseudo-pressure equation and uses the results to explicitly evaluate water saturation at each time step.The equilibrium mole fraction of water in the gas phase has been corrected for capillary pressure and salinity effects. A modified equilibrium constant has been derived that takes into account capillary pressure.
Water vaporization has been reported as a cause of permeability reduction in some oil fields and is a potential problem in others, especially in high pressure, high temperature reservoirs (HP/HT) that are characterized by very high salinity brines (Graham et al., 1997).
Morin and Montel (1995) studied the conditions for water vaporization as function of salinity and pressure. The authors mentioned three situations that contribute to water vaporization: the flow of gas from layers at different water saturation, significant pressure drop, and increase in water vaporization when the gas flows from the near wellbore area to the well. Precipitation of minerals in the near wellbore zone was also observed by Place and Smith (1984).
Graham et al. (1997) performed batch calculations to predict the precipitates expected in a North Sea formation. The results indicated the loss of about 40% of water because of evaporation at 195°C, which would deposit significant amounts of halite. However, the limitation of the model in terms of high salinity brines introduced uncertainty to the predictions.
Jasinski et al. (1997) studied a scale control strategy for the Heron Cluster, Abeerdeen. Analysis indicated extremely high salinity of the formation water. The scaling predictions indicated that NaCl precipitation was the main threat to production. This precipitation would be associated with the decrease of solubility of NaCl with pressure and as water evaporated at and below the bubble point pressure.
There has been little modeling of water vaporization for flow through permeable media. Most approaches have been based on modifications of existing compositional simulators by incorporating water as a component in the equation of state (Bette and Heinemman, 1989; Kurihara et al., 2000). The effect of salinity has been included either with salinity-dependent solubility tables (Morin and Montel, 1995) or by adding salt as a component in the equation of state (Lee and Lin, 1999). Some have modified material balance equations to account for water vaporization (Humpreys, 1991). All the approaches have neglected capillary pressure effects.
Zuluaga and Lake (2004) studied water vaporization in gas injectors as a traveling wave in which capillary diffusion occurs. The approach included capillary pressure and salinity effects. The reduction in mole fraction of water in the gas phase as saturation decreased was included by using Kelvin's equation. The effect of salinity was taken into account by using the equilibrium mole fraction of water in the gas phase measured experimentally.The solution obtained allowed predictions of remaining water saturation with distance and time.
However, mechanisms for water vaporization during gas injection and gas production are different. The traveling wave solution cannot be applied to predict water vaporization for gas production. For gas production, the only driving force for water vaporization is the pressure gradient that occurs in the near wellbore zone. Therefore, the effect of pressure variations on thermodynamic properties must be considered.