Experimental And Simulation Study On The Reservoir Stimulation Of Clayey Silt Hydrate Via Splitting Grouting

Since the 21st century,China has become the largest energy consumer in the world with rapid economic development and increasing energy consumption.Natural gas hydrate（NGH）is considered as the most promising energy in the 21st century,and is widely stored in the marine sediments of South China Sea（SCS）.The economic,safe and efficient development of NGH resources in the SCS is of great strategic importance to safeguard national energy security,change the energy consumption structure and achieve the goal of carbon peaking and carbon neutrality.However,the field trials in the SCS have encountered a series of problems such as insufficient single-well capacity,limited mining area and short stablized production.There is a long way to achieve the goal of commercial exploitation of NGH.Therefore,in this paper,to further improve the gas production efficiency of marine NGH,experimental and simulation study on the reservoir stimulation of clayey silt hydrate via splitting grouting was conducted.Clayey silt hydrate reservoir is characterized by fine sediment particles,high clay content,poor cementation and low permeability,and is recognized as the most difficult type to develop.To simulate the gas production of hydrate-bearing sediments in real marine environment,in this study,a one-dimensional long-tube reactor was used to carry out the depressurization decomposition experiments of methane hydrate-bearing sediments.The results showed that the water saturation was an important factor in determining the mass transfer efficiency and gas production rate.The slow movement of hydrate decomposition front and low gas production rate in water-rich environment were due to the large amount of pore water hindered the gas release and transport in the sediments and reduced the heat and mass transfer efficiency inside the reservoir.Then,the variation of permeability before and after hydrate decomposition was studied.The results showed that the sediments permeability decreased significantly after hydrate decomposition.The mechanism of hydration and swelling of clay minerals to reduce lpermeability was revealed.After the interaction between hydrate decomposed water and clay mineral particles,the clay particle blocked the internal seepage channels of sediments and caused the damage of seepage capacity.Hence,the damage effect increased with the increase of clay content.To solve the above problems,the NH₄Cl solution,an inorganic salt-based clay stabilizer,was pre-injected into the hydrate-bearing sediments in this study,and the results showed the permeability of the sediments pre-saturated with NH₄Cl was maintained at～75%of the initial values after gas hydrate decomposition as the exchange of Na⁺with NH₄⁺in sediments was proved to decrease the bound water in montmorillonite interlayers and inhibit the hydration swelling of clay particles in the Nuclear Magnetic Resonance analysis experiments.To further enhance the gas-water seepage capacity of clayey silt hydrate reservoir,splitting grouting is applied to the clayey silt hydrate reservoir stimulation,and the fracture initiation pressure and fracture expansion shape of the hydrate-bearing sediment samples were studied under different hydrate saturation,splitting fluid viscosity and in-situ ground stress,which revealed the fracture expansion pattern of clayey silt hydrate.It was found that the hydrate saturation is the key factor to determine the fracture initiation pressure and fracture extension pattern of hydrate-bearing sediments.When the hydrate saturation≤0.30,the fracture initiation pressure was positively correlated with the hydrate saturation,while the fracture area was negatively correlated with the hydrate saturation.Meanwhile,the fractures always preferentially expanded toward the area with low hydrate saturation.Regarding the viscosity of the splitting fluid,the fracture initiation pressure of the hydrate-bearing sediments was negatively correlated with the viscosity of the splitting fluid.The results showed that the splitting fluid with medium viscosity（434 m Pa·s）had good fluidity and excellent anti-filtration property,which was beneficial to achieve the fracture initiation and maintenance of internal fractures in the reservoir.In addition,the enhancement of permeability and gas production under different splitting conditions was clarified.The results showed that the permeability maintenance ratio after splitting was 3.07 times higher than that of the unsplit run,and the average gas production rate and peak gas production rate were 3.58 times and 5.84 times higher than that of the unsplit run,respectively.The mechanism of increasing seepage and production by splitting was revealed.The presence of NH₄Cl raised the salinity of the splitting zone and shifted the hydrate phase equilibrium in the direction of easy decomposition;it shortened the spacing of the clay particle crystal layers and weakened the hydration swelling effect of clay;the heat absorption reaction of hydrate decomposition induced NH₄Cl particle crystallization and supported and maintained the stability of the fractures.However,splitting grouting is a transient and fine-scale process,and macroscopic indoor experiments tend to be difficult to reflect the grout diffusion pattern and dynamic grouting process.In this paper,a 2D discrete element method was used to establish a fine-scale model for NGH reservoir and numerical tests were conducted on splitting grouting by using Particle Flow Code.The pattern,direction and range of the grouting expansion were analyzed under different conditions of hydrate saturation,injection pressure,grout viscosity,injection well size,in-situ stress,and the number of injection wells.The results showed that hydrate saturation was an important factor affecting grout diffusion morphologies.With the increase of hydrate saturation,the stronger the cementation of the reservoir,the smaller the diffusion range of grout.Meanwhile,the injection pressure was the decisive factor in determining the grout diffusion rate.When the injection pressure was less than the initial grouting diffusion pressure,the grouting pressure was stagnant at the wellbore,and the grouting stage was completed only at the wellbore,and it cannot continue to split and form grout veins inside the reservoir.When the injection pressure was greater than the grout initial diffusion pressure,with the increase of the injection pressure,the faster the diffusion speed of the grout veins,the wider the diffusion range of the grout veins,and the more the number of secondary grout veins formed.In addition,wellbore size affected the difficulty of grout spreading.Under the same conditions,the larger the diameter of the wellbore,the greater the spreading of the grout.The grout viscosity affected the diffusion resistance of the grout.The higher the viscosity,the worse the injectability of the grout.Therefore,it was better to control the viscosity within 1000 m Pa·s,which can have stronger injectability.A coordinated multi-well stimulation approach was not a simple superposition of single-well stimulations.Simultaneous operation of multiple wellbores maked main grout veins then extended in both directions along the wellbore arrangement,greatly increasing the diffusion rate and lowering the threshold for reservoir grouting stimulations.Based on the results of the above study,the formulation study of foamed superfine cement-based reservoir stimulation grout was carried out in this study,and the variation of the rheology and curing time of the grout under different composition and ratio conditions were studied.A three-dimensional permeability testing apparatus was used to test the permeability of the grout consolidation body,and the variation of porosity and permeability of the consolidation body under different formulation conditions were studied.The results showed that the consolidation body was loose and porous,with porosity in the range of 9.41%-46.59%and permeability in the range of 51.23 m D-789.07 m D,and its internal pore channels were evenly distributed with thin pore walls,forming a large number of pore strings,which can be used as multi-stage percolation channels for gas and water production by hydrate decomposition.In addition,the splitting grouting experiments were carried out with the foamed superfine cement type reservoir reforming grout,and it was found that the grout has good splitting and grout diffusion ability under the condition that the axial pressure is 5 MPa,the confining pressure is 4 MPa,and the hydrate saturation does not exceed 0.30.Based on the formulated foamed ultra-fine cement-based reservoir stimulation grout,the splitting grouting process parameters were optimized,i.e.,the preference for the grouting area with low hydrate saturation（not more than 0.30）,the medium grouting flow rate（20-40 ml/min）and the maximum grouting well diameter（10 mm）,and an efficient splitting grouting stimulation method was established at the laboratory scale.In summary,experimental and simulation studies have been carried out in this paper on the seepage characteristics of clayey silt hydrate reservoir,splitting and seam formation behavior,grout diffusion behavior,and the performance of reservoir stimulation and splitting grouting modification process.The related conclusions can provide references in the field of clayey silt hydrate reservoir stimulation,and provide theoretical and technical support for the efficient development of marine NGH in China.