Multi-scale Analysis Methods And Predictive Models For Seepage And Heat Transfer Properties Of Hydrate Reservoirs

Natural gas hydrate has the advantages of the huge energy reserve,low carbon,and low pollution,and is a new type of clean energy with great development potential.Hydrate reserves in the South China Sea are very abundant,efficient exploitation of which can ease China’s energy tension and improve energy composition,while helping to achieve the goal of carbon peaking and carbon neutrality.Hydrate formation,development,and decomposition within hydrate reservoirs are mainly controlled by temperature and pressure conditions.The main reservoir properties that determine the evolutions of temperature and pressure are thermal conductivity,permeability,and relative permeability.Meanwhile,they are also important parameters for numerical models of hydrate exploitation.Therefore,the effective prediction of these seepage and heat transfer properties of hydrate reservoirs is one of the key tasks in hydrate research.The macroscopic heat transfer and seepage properties of hydrate reservoirs are essentially dependent on the relative content and distribution of multiple phases(gas,water,and hydrate)within the pore space.On the one hand,hydrates show different pore habits due to different hydrate formation conditions;on the other hand,hydrate formation and decomposition processes change the distribution of gas-water-hydrate,as well as their relative volumes.As a result,the heat transfer and seepage properties of hydrate reservoirs show diverse and dynamic changes with hydrate phase change.This also determines that the model construction requires the micro-to macroscopic correlation analysis,which starts from understanding hydrate morphology and multiphase distribution at the microscopic scale and subsequently correlates them to macroscopic heat transfer and seepage properties,so that the predictive models that meet the real characteristics of hydrate reservoirs and has the high predictive accuracy can be constructed.This paper combines various methods of numerical simulation,theoretical analysis,and analytical derivation to conduct a multi-scale study on the seepage and heat transfer properties of hydrate reservoirs.The main research contents and results are as follows.(1)A multicomponent mass transfer and hydrate phase-change kinetics coupling model applicable to brine system in the marine condition is established,whose predictions of hydrate conversion rate agree well with experimental measurements with errors within 9%,to explain the coupling control mechanism of hydrate phase change and multi-component mass transfer on hydrate phase change process;a numerical model for the evolution of pore structure in porous media during hydrate phase change is established to simulate the evolution of pore structure under different hydrate formation conditions,and the change rules of pore structure and permeability of porous media with hydrate phase change are quantitatively analyzed.(2)A quantitative method of correlating hybrid hydrate pore morphology(i.e.,coexistence of grain-coating and pore-filling habits)evolution with permeability variation is proposed,and,based on it,a permeability model applicable to different hydrate formation conditions(excess-gas,excess-water and dissolved-gas conditions)is established.The model not only can be widely applied to the prediction of the published permeability of pressure cores and laboratory synthetic cores(basically all NRMSE<0.1),but also explains the influence of different hybrid hydrate pore morphology evolutions on permeability variation.(3)The relative permeability of hydrate-bearing porous media under different hydrate formation conditions is calculated.Based on the calculated results,how different hydrate pore morphologies affect residual gas saturation and residual water saturation,as well as the change of relative permeability,are explained.Further,the quantitative equations correlating residual gas saturation and residual water saturation with hydrate saturation are established so that the relative permeability models are constructed.(4)A pore-scale model of hydrate-bearing porous medium describing the multiphase distribution pattern is developed,which is used to model the heat conduction process and derive the analytical solution for the medium thermal conductivity.Using the analytical solution,the change rules of the thermal conductivity with multiphase saturations and distributions under different hydrate formation conditions is analyzed.Combining with Pythagoras mixing model,a thermal conductivity predictive model is proposed,which can offer predictions matching well with experimental measurements and explain the intrinsic correlation of multiphase thermal properties,relative volume,and pore distribution patterns with the bulk thermal conductivity.(5)A thermal-hydro-mechanical-chemical coupling model for hydrate exploitation is constructed based on the proposed permeability and thermal conductivity models.Taking the hydrate reservoir in Shenhu area,South China Sea as an example,long-term gas production for different hydrate formation conditions is predicted using the coupling model,and results show that in three-year gas production,cumulative gas volume under excess-water/dissolved-gas conditions is～25%higher than that under excess-gas condition.Gas production performance and multi-physical field evolution of the layered hydrate reservoir in Nankai Trough,Japan,were also predicted.The gas production rate(～1.15×10~4m~3/d)was almost constant after 200days.Temperature is the main cause of the differences in the dissociation characteristics of the various hydrate layers.The lower hydrate layer dissociates at the fastest rate and produces the most gas contribution due to the inflow of the water bringing heat from the underburden.