The Precipitation of Salt in Gas Producing Wells

Abstract

Abstract The entrainment of reservoir water from gas reservoirs and the associated precipitation of dissolved salts in the late production stage can impair the productivity and may even result in the abandonment of wells, even in reservoirs without active edge water drive. The causes of precipitation of dissolved salts from the entrained water, the chemical composition and plugging behaviour of the precipitates, as well as stimulation concepts with field results are discussed. On the basis of the thermodynamic equilibrium between reservoir water and gas, on the one hand, and between the dissolved salts and the reservoir water, on the other hand, the effect of the reservoir pressure on the aqueous phase is investigated, and the mechanisms of the transport and precipitation of salts are explained. Special emphasis is placed on early identification of down-hole salt precipitation on the basis of the chemical composition and derived ionic parameters of wellhead water samples from different gas wells. The results obtained with water from Rotliegende, Dolomite and Bunter reservoirs are discussed. Experience gained in operations and stimulation with the measures thus derived for controlling the salt problems is described. Introduction In the course of gas production from reservoirs in Northern Germany, salt precipitation from the reservoir water is observed to an increasing extent as recovery progresses. This phenomenon likewise occurs during the injection of dry gas into porous aquifers. Salt deposits can form in the production string as well as in the perforation zone. It may be assumed that the surrounding formation is also affected by salt precipitation in the proximity of the well. In the area of an aquifer employed for gas storage, NaCl precipitates have been detected during well testing by pressure draw-down [Kleinitz, 1982]; these deposits had resulted from filling of the pores with saturated brine. During production, the precipitation of salt results in a significant decline in productivity, which may culminate in total plugging and ultimately in the abandonment of wells. For the elimination of deposits as well as the prevention of salt precipitation in the vicinity of the wells, fresh water treatments are applied in production operations, in addition to the mechanical removal of halite scale from the tubing and perforation zone with the use of scrapers. The object of all such measures is to maintain or restore the original permeability conditions. The cause of halite precipitation in depleted gas wells is illustrated in figure 1. In this figure, the maximal water content in methane is plotted as a function of the pressure. The recovery factor for a Northern German gas well is also indicated in the diagram. The decline in reservoir pressure can be derived from the p/Z behaviour. As known from experience, the reservoir pressure declines as recovery progresses. However, the solubility of water in the gas increases with decreasing reservoir pressure. As shown in figure 1, the water content in the methane increases decidedly beyond 200 bar in the reservoir under consideration here. At this point, the recovery factor is about 60 per cent. In the course of production, the pressure declines, and the salt concentration in the existing reservoir water thus increases to a value at which halite scale is deposited. A scanning electron micrograph [Kleinitz, 1982] of formation rock with precipitated salt in a pore channel is presented in figure 2. The coverage of the internal surface of the pores with halite, the associated decrease in available cross-sectional area, and thus the impairment of permeability are clearly evident. With a pore diameter of about 30 µm, the thickness of the NaCl deposit is between 3 and 4.5 µm. At the centre of the micrograph, the inward growth of an NaCl crystal into the volume of the pore is clearly visible.