Permeability Evolution at Various Pressure Gradients in Natural Gas Hydrate Reservoir at the Shenhu Area in the South China Sea

Abstract

The sediment of the hydrate reservoir in the Shenhu Area is mainly clayey silt. Its characteristic small particles and poor cementation challenge the quantification of the reservoir permeability during gas production. An accurate description of the seepage mechanism of the clayey-silt reservoir is the basis, and also a difficulty, of effective development of gas in the South China Sea. In this study, four sets of water seepage experiments under different pressure gradients are carried out using the clayey-silt reservoir sediments, in which the fourth sample was subjected to computed tomographic (CT) scans. The experimental results shows that the clayey-silt reservoir has a compression of the pore structure and decreasing permeability with the increasing pressure gradient. CT images are used to show the reduction of pore spaces for fluid flow. When the pressure gradient is less than 3 MPa per meter, the pore structure of the reservoir has minor changes. When the pressure gradient is greater than this value, the pore structure of the reservoir will be quickly compressed. This leads to a rapid decrease in permeability, and the process of permeability reduction is irreversible. The decrease of permeability can be predicted directly by establishing a power law model with the change of porosity. Our experimental results preliminarily reveal the dynamic evolution law of pore structure and permeability of clayey-silt reservoir in the process of gas hydrate exploitation via depressurization. The permeability evolution law at various pressure gradients provides a scientific and reasonable basis of a productivity control system for clayey-silt gas hydrate in depressurized gas production.

Pore Structure Fractal Characterization and Permeability Simulation of Natural Gas Hydrate Reservoir Based on CT Images

“…Yu and Cheng [22] established the fractal permeability model for the bidispersed porous media and introduced the algorithm of pore space fractal dimension and tortuosity fractal dimension based on the box-counting method. Cai et al [23] established a fractal permeability model for creeping microstructures under different pressure conditions based on the data of water flooding experiment combined with CT scanning [24] for clayey silt sediments. Liu et al [25] used fractal dimensions to establish a fractal theory-based relative permeability model of hydrate-bearing sediments.…”

Experimental Study on the Propagation Characteristics of Hydraulic Fracture in Clayey-Silt Sediments

“…Clay and quartz are dominant, which account for 47.2% and 36.4%, respectively. The clay minerals are mainly montmorillonite and illite [19,37].…”

“…The formation energy depletes at the later stage of hydrate gas production, causing the decline in thermal and pressure conductivity of the reservoir, which is difficult to spread to the far end of the wellbore [33][34][35]. Meanwhile, lack of gas channel to flow 3 Geofluids from the dissociation front to the wellbore, the gas production is seriously affected [36][37][38][39]. Therefore, the fracture propagation characteristics of clayey-silt sediments in hydrate dissociation are an important issue.…”

The Characteristics of Gas-Water Two-Phase Radial Flow in Clay-Silt Sediment and Effects on Hydrate Production

“…In the early experiment of water flow in the clay-silt sediment core from the South China Sea, it is found that the permeability decreased gradually with the increase in differential pressure and this process is irreversible [18]. CT scanning was used to observe the phenomenon of pore volume creep in the sample during the flow process, and there was a clear power law relationship between the effective permeability and porosity of sediments under different pressure differences.…”