Hydrate-bearing sediment of the South China Sea: Microstructure and mechanical characteristics

Self Cite

The effective stress and types of particles significantly affect the permeability of hydrate sediments. In this study, effects of hydrate saturation, hydrate decomposition, effective stress, and particle type on gas permeability and sample deformation of montmorillonite samples and quartz-montmorillonite mixtures were investigated. The results showed that the gas phase permeability increased with hydrate formation. In addition, the decline of sample porosity is the fundamental factor determining the permeability failure rate under effective stress. During methane hydrate decomposition by depressurization, the compression coefficient of samples increases with decreasing pore pressure, which means that decomposing natural gas hydrate makes experimental samples more sensitive to stress. Furthermore, the addition of sand particles significantly affects the compressibility of sediments. In addition, because of the decline of sample porosity and the swelling of montmorillonite, the results of gas permeability show a downward trend when the pore pressure decreases during the decomposition of methane hydrate. Gas permeability of the sample ranges from a few millidarcies to several hundred millidarcies. Furthermore, the content of clay and the particle size in the sample are the key factors in determining the permeability damage rate when the stress increases during decomposition of methane hydrate. Therefore, the addition of quartz sand can reduce damage of gas permeability of sediments caused by effective stress and hydrate decomposition.

Section: Methodsmentioning

confidence: 99%

Experimental Study on the Evolution of Compressibility and Gas Permeability of Sediments after Hydrate Decomposition under Effective Stress

Zhang

Wang

et al. 2022

Self Cite

“…Natural gas hydrates (cage compounds) are transparent or translucent solid compounds formed by hydrogen bonding of guest molecules such as methane gas or carbon dioxide with water. − In the natural environment, gas hydrate exists stably in the form of methane and water crystallization under low-temperature and high-pressure conditions, which is also widely distributed in offshore areas and permafrost zones. − Under the background of the exhaustion of traditional fossil resources and continuously increasing energy demand, − as a clean and efficient unconventional fossil energy with wide distribution and large reserves, natural gas hydrate can provide critical support for solving the energy shortage problem. , However, the geological characteristics of hydrate-bearing sediments are complex and the sedimentary environment is sensitive, , and the exploitation of hydrate is accompanied by the decomposition of hydrate. , The decomposition of hydrate will reduce the sediment strength, which will lead to landslide, deposition, and another marine geological disaster. − Therefore, the study of the mechanical characteristics of methane hydrate-bearing sediment (MHBS) under triaxial stress is of great importance to the safety and stability exploitation and commercialization of hydrate engineering …”

Section: Introductionmentioning

confidence: 99%

Mechanical Characteristics of Underconsolidated Methane Hydrate-Bearing Clayed-Silty Sediments

Liu

Chen

et al. 2023

Self Cite

Natural gas hydrate reserves in the South China Sea are huge, and their commercial prospects have been proved by repeated trial production. It is essential to systematically clarify the mechanical properties of hydrate-bearing sediments for the safe drilling of hydrate reservoirs. To date, most research studies focus on the undrained mechanical behavior of hydrate-bearing sandy or clayed sediments under normal consolidation conditions, but there are few reports on underconsolidated-undrained triaxial shear experiments. In this study, the present research conducts undrained triaxial shear tests on hydrate-bearing clayed-silty sediments. The results show that underconsolidated hydrate-bearing sediments appear to show a strain-hardening phenomenon, which are different from hydratebearing sands in Nankai Trough. The excess pore water pressure of hydrate-bearing sediments remains positive during shear. The increasing degree of consolidation will increase the failure strength and the Secant Young's modulus E 50 . According to the Mohr− Coulomb criterion, the corresponding predicting formulas are established, and the cohesion increases with the increase in the degree of consolidation, while the internal friction angle is not sensitive to the degree of consolidation. In addition, the failure strength and the Secant Young's modulus E 50 increase with the increase in initial effective confining stress before shearing.

“…Previous studies have shown that important factors influence hydrate decomposition such as permeability (k), porosity (Φ), and hydrate saturation (S H ). k is the measurement index of the hydrate reservoir seepage capacity and largely controls the production potential of the hydrate reservoir; reservoir Φ and S H would obviously influence methane gas and water production in the depressurization process. − So, accurately describing the 3D heterogeneity characteristics of k, Φ, and S H in the geological model is the necessary prerequisite to exactly study the hydrate reservoir production, spatial evolution of temperature and pressure field, and productivity effect of reservoir reconstruction.…”

Section: Introductionmentioning

confidence: 99%

Study on Gas Hydrate Reservoir Reconstruction for Enhanced Gas Production in the Shenhu Area of the South China Sea Based on a 3D Heterogeneous Geological Model

Xin

et al. 2022

Natural gas hydrate (NGH) has the huge exploitation potential for clean energy in the current world energy pattern. Its total organic carbon storage is about twice the sum of carbon content of oil, gas, and coal and hence attracts the attention of the world. China has carried out some gas hydrate exploitation works and delineation of three gas hydrate deposits whose reserve scale is over 100 billion cubic meters, and the NGH resource is abundant. The NGH reservoir in the South China Sea is of low porosity and permeability, is unconsolidated, has poor cementation, and is easy to fragment in hydrate production. On the one hand, the condition of low porosity and permeability around the well would severely reduce the production effect; on the other hand, the hydrate decomposition would more easily bring wellbore instability caused by stratum stress change. Therefore, to achieve safe and efficient exploitation of NGH, proper reservoir reconstruct technology needs to be chosen for enhancing the reservoir’s permeability and stability. This paper chose the Shenhu area of South China Sea as the research area, constructed the three-dimensional heterogeneous geological model of hydrate-bearing sediments which can be approximated realistically to depict, then used high-pressure jet grouting (HPJG) technology to reconstruct the hydrate reservoir, optimized the grouting hole direction, location, and spacing, and assessed the productivity increasing effect. The research results show the following: (a) Because of the higher hydrate saturation (SH) and lower permeability (k) around the well, the HPJG in the vertical direction was profitable for reservoir productivity. (b) Considering the heterogeneous SH and k in the horizontal direction, the HPJG in the front of the horizontal well was profitable for the reservoir productivity. (c) The HPJG spacing needs to be properly designed in combination with the absolute standard (V P), the relative standard (R GM), and the specific production index (J). (d) Under the conditions of this paper setting, the optimal HPJG reconstruction scheme is as follows: the HPJG in the vertical direction was arranged at the front of the horizontal well, and the spacing between the holes was 10 m. The productivity after reconstruction ranged up to 55.69% compared with before.