Study of effects of downhole conditions on the setting time and compressive strength of MOS settable system by orthogonal experimental design

Abstract

Lost circulations of drilling fluids during drilling and completion can be an expensive and time-consuming problem. Commonly used lost circulation materials (LCMs) like nutshells or cross-linked polymer gels can’t be easily removed after plugging with a risk of formation damage. Magnesium oxysulfate cement (MOS cement) provides a reliable solution for solving the severe loss that occurs in production and non-production zones. In this paper, the effects of harsh downhole conditions including density, bentonite content, salt content, and temperature on the setting time and compressive strength of the MOS settable system are studied using the orthogonal experimental analysis method. At the same time, laboratory plugging experiments and field application are used to evaluate the ability of the MOS settable system to seal the severe loss. The results show that the downhole conditions have a significant effect on the setting time and compressive strength of the system. By adding calcium carbonate particles, the permeability of the MOS settable system can be reduced and the pressure-bearing capacity of the MOS solidified plug can be improved. Field application shows that this settable system can solve severe loss that conventional LCMs cannot solve and has a good application prospect.