Study on the relations between controlling mechanisms and dissociation front of gas hydrate reservoirs

Abstract

It is significant to understand the dissociation front of gas hydrate reservoirs not only for the prediction of production performance but also for the control of geological hazards. However, great differences can be found among dissociation modes and features of dissociation front advance reported in the literature. In this study, controlling mechanisms, i.e., fluid flow, heat transfer, and hydrate dissociation, are introduced to analyze the dissociation front of a one-dimensional (1D) hydrate reservoir conditioned to depressurization. The relations between the controlling mechanisms and hydrate dissociation modes as well as the features of dissociation front advance have been identified by use of 12 well-designed scenarios considering different thermal boundary conditions, intrinsic permeability, and hydrate dissociation models. Note that the exponential function has been used to quantify the features of dissociation front advance curves that are in a convex shape. It has been revealed that a piston-like dissociation mode is formed while the dissociation of hydrate reservoirs is controlled by either the fluid flow mechanism or the heat transfer mechanism. Regarding the fitting exponent of dissociation front advance curve, it ranges from 0.5 to 1.0 for the fluid flow controlling mechanism and floats around 0.5 for the heat transfer controlling mechanism. Given that hydrate dissociation is the controlling mechanism, an extending non-piston-like dissociation mode emerges and the advance of dissociation front accelerates at the late stage. Furthermore, the dynamic variations of dissociation modes and features of dissociation front advance curves have been investigated when the controlling mechanism is changed. The dissociation mode tends to transfer from a piston-like one to a non-piston-like one and the convex shape of dissociation front advance curve is inclined to change to a concave one when the dominance of fluid flow mechanism is alleviated. Consequently, a pragmatic criterion has been developed to estimate the controlling mechanisms based on dissociation modes and advance features of dissociation front, or vice versa.