Quantitative Analysis of Mechanisms for Water-Related Sand Production

Abstract It is well known that saturation changes of the rock-wetting phase or alterations in wetness can lead to sand instability, therefore sand production. Several possible mechanisms are discussed in the paper, among which chemical reactions between water and sand and the changes of capillary force as a part of cohesive strength are focused upon quantitatively. Based on the time frame of sand production record, four major possible chemical reactions between reservoir sand and formation water are identified and analyzed, including quartz hydrolysis, carbonate dissolution, ferruginous deposits and clay swelling. Those reactions increase sand instability either through altering the surface energy of the sand and physically changing the shape and size of cement, (known as Rebinder effect), or locally increasing the pressure gradient. The possibility to quantify the effects of those reactions is discussed and the main difficulties are pointed out. Resorting to an analytical model at the grain scale, which accounts for meniscus behavior and properties in two-phase liquid systems and expresses the results in terms of capillary bond force and a tensile strength in a Mohr-Coulomb plot, the manner and magnitude of the capillary force influence on sand stability after water breakthrough are quantitatively described. It is found that, at grain scale level, the capillary cohesive force is one to three orders higher than the destabilizing seepage force from fluid pressure gradient, depending on particle sizes. Therefore it should not be neglected in the analysis of sand instability. Furthermore surface tension of fluids is found to be linearly related to capillary bond force and therefore to rock strength (tensile strength or UCS), the magnitude of which highly depends on the sand particle size, especially when particles are small. The model is able to successfully explain many reported phenomena about both capillary force and water-related sand production. As a conclusion, changes of capillary force with water saturation may have more influence on sand failure than chemical reactions in weakly consolidated sandstone at the beginning of water breakthrough. After some critical water saturation, the capillary strength diminishes and chemical reactions may become dominant over time. Introduction Sand production with water is a common problem in oil fields, especially for weakly consolidated sand1,2,3 and chalk4. Since most reservoir rock is water-wet, water breakthrough generally tends to destabilize the rock. Hall and Harrisberger1 showed that a sand arch can be stablized with respect to outward flow of the non-wetting liquid phase at a limited flow rate at residual saturation of the wetting phase, wheras outflow of the wetting phase destroyed the arch. In sand cavity experiments2, the critical global pressure gradient that activates sanding drops to half when water saturation was increased to 27%, comparing with the irreducible water saturation (23%). Furthermore, sanding appears to occur in an episodic manner: at a given flow rate and saturation condition, sand cavity starts to grow and then stabilize, but may again start to grow. Additional cavity growth requires either an increase of pressure gradient or a change in water saturation. Macroscopically, the difference in rock behavior before and after water breakthrough mainly results from the changes of rock properties5,6, including rock strength3,7,8,9,10. The reduction of Young's modulus, and bulk modulus results in an increase of rock deformability, which is usually neglected in current geomechanical models used to predict rock stability in multi-phase fluid flowing system.