Simulation Studies On Multilateral-Well Production Behaviors Of Clayey Silt Hydrate Reservoirs In The Shenhu Area,South China Sea

Natural gas hydrates are becoming a strategic high spot in the energy competition among major countries around the world.They are widely distributed in clayey silt sediments on the seafloor.There are abundant hydrate resources in the clayey silt reservoirs in the Shenhu area,South China Sea.However,the actual gas production rate in the Shenhu area is significantly lower than the industrial gas production standard.It is an inevitable requirement to employ new production measures and substantially increase gas recovery to realize the commercial production of clayey silt hydrate reservoir in the Shenhu area.The joint application of complex structure wells represented by multilateral wells,well network strategies,and reservoir modification technologies is expected to achieve a magnitude increase in hydrate produciton capacity.It is critical to establish and optimize the composition of the multilateral well networks and to clarify its relationship to production capacity before realizing the practical application of multilateral well networks in the reservoirs mentioned above.However,simulation studies using multiple multilateral wells to produce hydrate reservoirs have high requirements for experimental simulation conditions and numerical simulation techniques.The advantages of combined application of multilateral wells and well networks in clayey silt hydrate reservoirs have not been proven.The law of synergistic production enhancement and interference when multiple multilateral wells synergistically produce clayey silt hydrate reservoirs has not been clarified.The influence of key geological factors,including strata dip and reservoir physical properties,on the production capacity of multilateral wells is also unclear.In this study,experimental and numerical simulations were performed on the key issue of “production enhancement in clayey silt hydrate reservoirs by multilateral wells”.The following results were obtained:1)Experimental simulations of methane hydrate production using a vertically distributed two-branch horizontal well(i.e.,the angle of two branches is 90°)were firstly conducted with a customized horizontal multilateral well experimental simulation device.The production capacity of the horizontal two-branch well and a vertical well were compared experimentally.Based on the experimental simulator parameters,a three-dimensional numerical model of the hydrate reservoir consistent with the experimental setup was established.Meanwhile,the accuracy of this new method for modeling multilateral wells was verified by comparing the production performance of experimental and numerical simulations.The results of numerical simulations at the experimental-scale showed that the highest production efficiency is achieved by multilateral wells with four horizontal branches.The symmetrical placement of horizontal branches in the lower part of the hydrate reservoir is more conducive to the extraction of gas.2)A 3D hydrate reservoir model with dip angle of 10°～15°,which is consistent with the seafloor conditions,was established based on the geological and topographic parameters of hydrate-bearing reservoir at site W11 in the Shenhu area.Combined with experimental-scale simulation results,a reservoir-scale production simulation study was conducted when the number of branches of multilateral wells ≤4.The results show that placing spiral multilateral wells in horizontal positions in the tectonic low part of hydrate reservoirs with isotropic permeability is better for long-term gas production.For multilateral wells with vertical main wells,spiral branches with a phase angle of 90° are more advantageous for gas recovery.The more uniform the spiral branches distribution and the lower the production pressure,the higher the gas production rate.Within the range of 3 to 9 MPa,the gas recovery rate can be increased by around 8 % for a reduction of 1 MPa in pressure.Moreover,spiral branches is recommended to be deployed uniformly in the lower part of the ultra-low permeability(<1 m D)hydrate reservoirs.While in the low-permeability(>1m D)hydrate reservoirs,spiral branches are suggested to be employed uniformly throughout the whole reservoir.3)The results showed that,under the criteria estabilised in this thesis,the larger the well spacing,the higher the production efficiency of the spiral multilateral well network.However,the placement of multilateral wells within this well network has almost no impact on production capacity.There is almost no “blind zone” in the hydrate-bearing reservoirs when multiple spiral multilateral wells are jointly employed to produce gas hydrates,effectively avoids the drawbacks of multiple vertical wells.The combination of multilateral well networks and reservoir modification strategies(e.g.,cover blocking method and depressurization combined with cubic fracturing method)is highly effective in enhancing gas production.However,production times and reservoir permeability need to be considered to determine whether clayey silt hydrate reservoirs are produced by the depressurization method alone.4)There are three stages in the hydrate production process: no effect stage,synergistic effect(1+1>2)stage and “negative” effect stage,which are controlled by well spacing of production wells,reservoirs permeability and production time.Due to the synergistic effect,spiral multilateral wells with more branches show outstanding performance in increasing production in ultra-low permeability reservoirs,especially when the ratio of single branch length to reservoir width exceeds 0.15～0.25.The incremental gas production of spiral multilateral wells also increases with the number of branches and wells in the ultra-low permeability reservoirs.However,in the low-permeability hydrate reservoirs,the increase in gas production is limited once the length of branches is overlong or the number of branches is too high.The increase in the number of multilateral wells will enhance the overall recovery performance of the well networks.However,the increment in gas production volume decreases as the number of wells increases.The research results of this thesis deepen the understanding of the influence of reservoir characteristics and well network parameters on gas hydrate production.This study provides a new perspective for the application of spiral multilateral well network in clayey silt hydrate reservoirs in the Shenhu area.It also provides new ideas for enhancing the production of gas hydrates,and provides an important guidance and application value for future commercial production of gas hydrate.