Salt Precipitation in Gas Reservoirs

Abstract

Abstract This paper reports a study of halite precipitation in gas reservoirs and the ensuing formation damage. The examination of productivity decline in several wells revealed that halite precipitation is most likely due to water evaporation with pressure drop in the vicinity of the well-bore. Hence the model was developed in an attempt to capture the main features of this phenomenon. In addition to the classical mass and momentum conservation equations the model includes equations describing: (a) the evaporation and salt precipitation kinetics and (b) the modification of porosity and permeability due to restriction of the pores by the halite precipitate. The equations were solved numerically for the one dimensional case, using standard reservoir simulation methods. Laboratory experiments were performed using a sand pack containing highly saline water at ‘irreducible’ water saturation. The sand pack was conical in shape to simulate radial inflow effects. CT scans of the sand pack were used during the experiments, in conjunction with pressure drops monitored over several sand pack segments, to reveal the details of salt precipitation. This enabled us also to quantify the water saturation and to check the integrity of the sand pack, as pressured gas flow might cause viscous fingering. Simulator results clearly show increased pressure drop over the length of the sand pack; especially so at the narrow outlet section of the sand pack, indicating that salt precipitation occurs predominantly in what would be the near wellbore region. CT scans and conductivity measurements of individual sand pack segments have confirmed these results in the lab.