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

Lu Lu,Luo Luo,Xia Xia,Qiu Qiu,Sun Sun,Bian Bian,Cai Cai

doi:10.3390/en12193688

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.

Highlights

With the tightening of conventional oil and gas resources, vigorously developing unconventional oil and gas reservoirs becomes an important way to alleviate current resource shortages, adjust energy utilization structures, and ensure national energy security [1,2]
The natural gas hydrate reservoir at the Shenhu Area in the South China Sea is different from sandstone reservoirs found in other regions and countries [5]
The reduction of about half of the porosity can further explain the significant decrease of permeability (13.4–3.92 mD). All these analyses prove that the pore structure change is the primary cause for the decrease of permeability with the increase of pressure difference

Summary

Introduction

With the tightening of conventional oil and gas resources, vigorously developing unconventional oil and gas reservoirs becomes an important way to alleviate current resource shortages, adjust energy utilization structures, and ensure national energy security [1,2]. In 2017, the China Geological Survey conducted the test production of natural gas hydrate reservoir via depressurization [3,4] at the Shenhu. The natural gas hydrate reservoir at the Shenhu Area in the South China Sea is different from sandstone reservoirs found in other regions and countries [5]. When the bottom flow pressure of the hydrate reservoir production well starts to reduce, the hydrate phase equilibrium condition will be broken, and the decomposed methane gas and water will discharge from the decomposition front edge to the production wellbore through the clayey-silt porous media. If the control of the depressurization amplitude is unreasonable, the pore-throat structure of the clayey-silt porous media will be deformed, which will affect seepage capacity around the production well and reduce single-well production [14,15,16]

Methods

Results

Conclusion