Gravel Pack and Formation Sandstone Dissolution During Steam Injection

Abstract

Laboratory and field flow studies show gravel-pack sands and formation sandstones may be dissolved rapidly in steam generator effluents. Dissolution rates are affected markedly by changes in solution pH and temperature. Photographs and scanning electron micrographs show changes in rock morphology caused by silica dissolution. Introduction Steam stimulation and steamflooding of oil reservoirs have been used for many years to help produce viscous oil more effectively. Recently it was shown that steam drive also was applied successfully in a relatively low-viscosity oil reservoir. This success combined with the need to increase domestic oil production may lead to expanded use of steam injection in the oil field.In cyclic steam stimulation it is common to inject perhaps 3000 m3 (18,870 bbl) of 80% quality steam at about 260 degrees C (500 degrees F) on each of several stimulation cycles. Thus, a few thousand cubic meters of liquid-phase water (20% of total) may be injected into a well during several steam stimulation cycles. In the case of a steam-drive injection well, tens of thousands of cubic meters of liquid are injected into the formation over the life of a project. If the formation rock and any gravel pack that may be in the well have any significant solubility in the flowing fluid, a rather large amount of rock could be dissolved. For example, if rock solubility is only 100 ppm (100 mg/L), 10 000 m3 of injected fluid will dissolve 1000 kg (2,200 lbm) of rock. If this dissolution occurs rapidly, a gravel pack would be short-lived. In the case of a large loss of formation rock from the immediate well area, formation caving and perhaps even casing or liner collapse could result.Quartz and other siliceous minerals have very low solubilities at room temperature, but at elevated temperatures solubilities increase rather sharply (Fig. 1) as reported by Krauskopf. Thus, there is ample Si solubility at normal steam injection temperatures to cause concern, although the actual rate of dissolution is not known. However, the rate is as important as solubility because it determines if severe dissolution occurs in or immediately adjacent to the injection wellbore or if it occurs less intensively over a larger radius.Solution pH also is an important factor influencing the extent and rate of Si dissolution. When HCO3 ions in steam generator feedwater decompose to CO2 and OH ions, the CO2 partitions to the vapor phase and the OH ions partition to the liquid phase. This causes the pH of the generator liquid effluent to increase sharply, the extent of which is dependent on the HCO3 content of the feedwater. Some of the OH ions react with remaining HCO3 ions to form CO3 ions, which also are quite alkaline. Measurements on samples of liquid effluent collected from field generators show that about one-third to one-half of the alkalinity is from CO3 ions and the rest is from OH ions. JPT P. 941＾