Rock strength degradation induced by salt precipitation: A new mechanical mechanism of sand production in ultra-deep fractured tight sandstone gas reservoirs

I take a typical ultra-deep tight sandstone gas reservoir located in Tarim Basin as an example to investigate the rare sand production problem. The situation of sand production was presented, and then conventional analyses were conducted. Six tight sandstone core plugs were selected to conduct rock mechanical tests before and after salt precipitation. Ultimately, a mathematical model was established to investigate the mechanism of rock strength degradation. The results of sand production prediction indicated that sand production from rock skeletons should never appear, but the rock skeletons was observed in the sand samples, and thus the impact of salt precipitation was taken into account. The experiments proved salt precipitation caused a degradation in rock strength, and the difference between actual- and predicted critical sand production pressure drop based on the weakened rock strength reduced significantly. Furthermore, the stress intensity factor on the fracture tip induced by salt precipitation reached up to 1.22 MPa·m^1/2, which was greater than the fracture toughness of tight sandstone, and it was used to explain the rock strength degradation. The results are helpful to the knowledge of the sand production problem in ultra-deep fractured tight sandstone gas reservoirs.