Changes in relative permeability curves for natural gas hydrate decomposition due to particle migration

Abstract

It is very important to study the fluid flow in a natural gas hydrate (NGH) reservoir for hydrate exploitation. During the decomposition of NGH, some particles fall off and flow with the fluid as a result of poor cementation, which usually leads to sand production in the hydrate mining process. In this study, a three-dimensional pore network model (PNM) was established to simulate the three-phase flow of the gas phase (CH4), water phase, and solid phase (particles). Relevant parameters were set considering the deposition, blockage, and flow in the process of particle migration. Then, the gas–water relative permeability curves under different hydrate saturations, particle diameters, and particle numbers were obtained, considering the influence of the change in the pore-throat space during the hydrate decomposition process. The results showed that the gas–water relative permeability equal points (isotonic points) first turned toward the left and then to the right. The analysis results showed that particle deposition and blockage mainly influenced the relative permeability curves during the early period of the hydrate decomposition process. An increase in the pore-throat space was the main factor affecting the relative permeability curves in the late period of the hydrate decomposition process. Moreover, increasing the particle diameter and number of particles caused the isosmotic points of the relative permeability curves moving to the left as a whole because the pore-throat space decreased. Increasing the number of particles first affected the fluid flow in the hydrate reservoir when the temperature was 280 K, pressure was 5.76 MPa, and hydrate saturation was 0.48. In the actual hydrate mining process, this could cause sudden changes in the relative permeability and production. Thus, this condition needs to be avoided. It has important significance in reference to the process of hydrate mining with sand production.