Effect of Hydration of Montmorillonite on the Permeability to Gas of Water-Sensitive Reservoir Rocks

Abstract

Abstract Laboratory research has been conducted to evaluate the effect of clay hydration on the permeability to gas of water-sensitive reservoir sands. samples of a sandstone containing trace amounts of montmorillonite and a sample of montmorillonite were studied in the laboratory to determine whether swelling or dispersion was the cause of permeability reduction in these samples. Helium, containing various amounts of water vapor, was used to hydrate the clay minerals and to determine the gas permeability at various stages of clay hydration. The amount of water adsorbed by the samples using this method is small. The nonwetting-phase permeability at higher water saturations was investigated by saturating the samples with water and measuring the permeability to humid helium while decreasing the water saturation. Relative-permeability curves obtained from results of these procedures were used to estimate the effect of the swelling of trace amounts of montmorillonite on the permeability of the samples. Most of the damage to the permeability when reservoir sands containing trace amounts of montmorillonite are exposed to fresh water is due to dispersion and movement of clays. Blockage of pores by the increased volume of expanded montmorillonite is believed to result in permeability damage that is small in comparison to the observed damage to the samples tested. Introduction Studies have shown that permeability is severely damaged when sands containing only small amounts of montmorillonite are contacted by fresh water. When samples of sands containing large amounts of montmorillonite are placed in fresh water in the laboratory, these samples may completely disintegrate, forming an unconsolidated mass of larger volume than that occupied by the dry sample. In this case, it is apparent that the swelling of montmorillonite has destroyed the pore structure of the sand. If only a trace of montmorillonite is present in a sand, samples may remain intact when saturated with water, although the permeability to water is a small fraction of the gas permeability of the dry sample. Many workers in the field of water sensitivity have attributed this reduction in permeability to the blocking of pores and reduction of pore size by the increased volume occupied by expanded montmorillonite, if the sand contains a detectable amount of montmorillonite or mixed-layer clay containing montmorillonite. Logically, the smaller amount of montmorillonite present in a sand, the smaller should be the effect of montmorillonite swelling on permeability; however, the quantity of montmorillonite sufficient to cause severe damage by swelling is not known. Although hundreds of samples have been tested in our laboratory, no correlation has been established between the amount of montmorillonite in samples and the permeability reduction caused by fresh water. To many petroleum engineers, the phrase "clay swelling" is synonymous with "water sensitivity", or "permeability reduction" implying that any formation damage due to the hydration of clays is caused by swelling. Although all clays adsorb water on their surfaces, montmorillonite is the only clay mineral commonly found in reservoir rocks which adsorbs water between intercrystalline layers, resulting in expansion of the clay particle. As montmorillonite swells, the first few layers of water adsorbed between platelets are strongly held and well oriented, and the montmorillonite retains its crystalline structure, although expanded. As swelling of sodium montmorillonite continues, the platelets become farther apart and the forces orienting the platelets in the crystalline structure become weaker, resulting in a less orderly orientation of platelets. In an abundance of water, small groups of platelets may become detached from the original montmorillonite particle and may be dispersed throughout the water phase. Because of its swelling properties, sodium montmorillonite is very easily dispersed in water. Particles of other clay minerals, such as illite and kaolinite may also be dispersed in water, causing water sensitivity of sands not containing montmorillonite. The presence of an immobile layer of water adsorbed on the surface of clays has been considered a possible cause of the low permeability to water of dirty sands. Grim states that the thickness of the layer of immobile water held by sodium montmorillonite is three molecular layers or 7.5 A (angstroms), with some orientation of water extending to 100 A. Assuming a very thick, immobile water layer adsorbed on the surface of a pore represented by a capillary tube, the maximum effect of the water layer on permeability can be calculated. Using a pore radius of 10(-4) cm and an immobile water layer of 50 A, the calculation shows the permeability to be reduced only 2 per cent. Similar calculations can be used to show that the effect of electro-osmotic counterflow is of the same order of magnitude as that of bound water. JPT P. 1213ˆ