Migration laws of natural gas hydrate solid particles with different abundance in horizontal wells

When a horizontal well is drilled through the natural gas hydrate (NGH) strata with different abundance, the NGH-bearing solid debris tends to lead to the deposit and adhesion of solid particles in the lateral sections and consequently cuttings carrying is not smooth. In this paper, the critical return rate model of drilling fluid in the case of cuttings rolling (normal drilling) and saltation (pump off for sand settling) under NGH adhesion in the laterals was established according to the force and migration laws of cuttings under the condition of multiphase flow in a horizontal well and the particle migration theory. Then, numerical simulation was conducted and the influential factors and migration laws of cuttings starting in the case of normal drilling and pump off were analyzed. And the following results were obtained. First, the critical starting flow rate decreases with the rise of NGH abundance and it is higher when the NGH adhesion is taken into account. Besides, the higher the NGH abundance, the greater the effect of NGH adhesion. Second, the critical starting flow rate increases with the rise of drilling cuttings particle size when the NGH abundance is less than 85%; and it decreases with the rise of drilling cuttings particle size when the NGH abundance is more than 85%. Third, the critical starting flow rate decreases with the rise of drilling fluid density and viscosity. And fourth, under the same conditions, the critical return rate for saltation is about 1.28 times that for rolling. It is suggested that the rolling model should be adopted for the normal drilling while the saltation model for the recycle after pump-off sand settling. The research results are of great significance to the optimization of NGH drilling parameters and the reduction of drilling safety risks.