Experimental Study on the Depressurization of Methane Hydrate in the Clayey Silt Sediments via Hydraulic Fracturing

Abstract

Poor seepage capacity always occurs in the clayey silt hydrate reservoir, making it more difficult to achieve depressurization. Hydraulic fracturing is an important means of enhancing reservoir seepage and increasing gas production rates. In this work, a series of depressurization experiments were performed in a triaxial hydraulic fracturing apparatus. The samples were prepared according to the physical parameters of samples obtained from the South China Sea. The fracturing behavior and gas production characteristics of methane hydrate-bearing sediments were investigated. The effects of hydrate saturation, fracturing fluid properties, and in-situ stress on fracturing behavior were analyzed. It was found that it was feasible to be fractured for sediments with low hydrate saturation (S h < 0.30), and the fracture initiation pressure was positively correlated with the hydrate saturation. The results showed that medium-viscosity fracturing fluid had excellent flow and filtration loss reduction, which were beneficial to the maintenance of fractures inside sediments. The in-situ stress difference determined the direction of fracture propagation, which was always along the vertical direction of the minimum principal stress. Besides, the gas production enhancement and permeability improvement by hydraulic fracturing were evaluated. The average gas production rate and peak rate in the fractured run were 3.01 times and 5.84 times of those in the unfractured run, respectively. To further enhance the seepage capacity, the inorganic salt-based clay stabilizer solution was added to the fracturing fluid. The results showed that the permeability of the sediments fracturing with NH 4 Cl was 3.07 times of that in the unfractured run due to the inhibition of the hydration swelling of clay particles. The results are valuable to provide a basic understanding and suggestions for the application of hydraulic fracturing in gas hydrate reservoirs.

Review and Outlook on Fracturing Technology and Mechanism of Hydrate-Bearing Sediments

“…Complementary studies have combined hydraulic fracturing technology with depressurization strategies. Researches indicate that higher permeability rates coupled with extended radial lengths within fractured domains substantially enhance pressure drop propagation efficiency across such formations. ,, Moreover, they demonstrate that integrated approaches combining both hydraulic fracturing and depressurization yield superior gas production performance compared to relying solely on single-pressure reduction tactics. , The following table demonstrates the simulation studies and main conclusions of reservoir fracturing under different fracturing methods in recent years. It should be noted that researchers can also develop other models, such as molecular dynamics models, according to the actual situation to describe the mechanical behavior of hydrates in a more refined way. , …”

“…44,47,108 Moreover, they demonstrate that integrated approaches combining both hydraulic fracturing and depressurization yield superior gas production performance compared to relying solely on single-pressure reduction tactics. 131,132 The following table demonstrates the simulation studies and main conclusions of reservoir fracturing under different fracturing methods in recent years. It should be noted that researchers can also develop other models, such as molecular dynamics models, according to the actual situation to describe the mechanical behavior of hydrates in a more refined way.…”

“…Moreover, they observed an escalation in the fracture initiation pressure with rising fluid viscosities and noted that sediments exhibiting heightened hydrate saturation levels can manifest a brittle fracture response akin to that observed in loose sandstones� characteristics that render them amenable for hydraulic fracturing technology. Building upon these insights, Wang et al 132 conducted a series of triaxial hydraulic fracturing experiments and found that fracturing fluids with high viscosities have lower filtration loss rates, which are favorable to the rapid transmission and diffusion of fracturing fluids in the rock, and they can better cut and break the rock, and increase the connectivity of the fracture network.…”

Gas Production Enhancement by Horizontal Wells with Hydraulic Fractures in a Natural Gas Hydrate Reservoir: A Thermo-Hydro-Chemical Study

“…Nie et al 39 synthesized clayey-silty sediments (45% montmorillonite, 30% illite, 15% chlorite, and 10% kaolinite) bearing with 40% tetrahydrofuran hydrate and conducted true triaxial hydraulic fracturing experiments in a laboratory. According to the physical parameters of hydrate obtained from the South China Sea, Wang et al 40 synthesized hydrate sediment samples with methane gas, deionized water, and 1000 mesh quartz in a triaxial hydraulic fracturing reactor (diameter × height = 100 mm × 200 mm), and the hydrate saturation is 9−30%. The axial pressure is 6.5−8.0 MPa, and the confining pressure is 6.0 MPa.…”

Class-III Hydrate Reservoir Depressurization Production Enhancement by Volume Fracturing and Cyclic N2 Stimulation Combination

“…Liu et al 24 proposed an evaluation model of hydraulic fracturing in HBLs based on representative samples and found that fractures could be created in HBL when the fracability index was higher than 0.72. Wang et al 25 investigated the fracturing behavior of MH-bearing sediments using a triaxial hydraulic fracturing apparatus and found the fracturing initiation pressure was positive with the hydrate saturation and the average gas production rate was 3 times higher after fracturing. As mentioned above, hydraulic fracturing has great potential to improve the depressurization effect of hydrate reservoirs through artificially created fractures in HBLs.…”