| Natural gas hydrate is a new and clean alternative energy source with huge reserves,which is widely distributed in marine sediments and continental frozen soil areas.China has relatively abundant gas hydrate resources,which are mainly distributed in South China Sea and the frozen soil areas of Qilian Mountains,and a relatively large number of gas hydrate samples have been obtained during exploration and drilling.Compared with permafrost regions where gas hydrates are found in other countries,the mineral composition and pore structure of gas hydrate reservoirs in Muli area of Qilian Mountains are more complex.Reservoir rocks are compact,high in hardness,low in porosity and accompanied by fractures.These are different from the lithology and geological characteristics of other areas,resulting in significant changes in the well logging response characteristics of gas hydrate reservoirs in Muli area,and even no obvious increase in resistivity,which is one of the main identifying signs of gas hydrate reservoirs.Instesd,it presents unique low-resistance characteristics similar to low-resistance oil and gas layers,which leads to many difficulties in gas hydrate reservoir identification and saturation calculation in this area.Therefore,clarifying the main influencing factors of the electrical characteristics of gas hydrate reservoirs,combing the conductive mechanism and response characteristics of low-resistance gas hydrate,and establishing low-resistance gas hydrate petrophysical diagnosis and saturation calculation models have become key issues that need to be solved urgently.In this research,starting from the characteristics of the actual gas hydrate reservoir,this paper uses a combination of digital rock technology and petrophysical experiments to carry out research on the conductive mechanism of low-resistance gas hydrate,rock physical diagnosis and electrical identification,providing theoretical guidance and implementation methods for well logging identification,saturation calculation,and reserve estimation of low-resistance gas hydrate in the research area.This paper first systematically combed and analyzed the well logging response characteristics of pore-type gas hydrate reservoirs and fracture-type gas hydrate reservoirs based on the types of gas hydrate reservoirs in Muli area,Qilian Mountain.The characteristics of reservoir lithology,physical properties,pore structure and spatial distribution,gas hydrate and fluid distribution are systematically studied,and comprehensive analysis the correlation between four properties and low resistance characteristics.The results show that there exists a certain amount of low-resistance gas hydrate,which are found in both pore-type and fracturetype gas hdyrate reservoirs.Secondly,the conventional digital rock model construction methods are introduced in detail,including X-ray CT scanning and procedural method to construct pore-type digital rock and superimposed fractional Brownian motion model generated rough fracture with self-affine factal charactistics to construct fractured digital rock.Based on the characteristics of the gas hydrate formation process and the in-situ observation results of gas hydrate formation and distribution in the laboratory,a numerical simulation method of gas hydrate distribution based on the formation process is proposed,and realized the construction of gas hydrate digital rock model that effectively reflects the gas hydrate distribution and resistivity characteritics.In addition,combined with the microscopic distribution characteristics of gas hydrate,numerical simulation methods for idealized distribution and different cementation modes methods are proposed,which lays the foundation for the study of the influence of gas hydrate distribution and saturation on the electrical characteristics of reservoir rocks.Digital rock technology and fractal theory were used to analyze the characteristics of the pore structure of the reservoir from a qualitative and quantitative perspective.The results showed that the pores of the reservoir rock are mainly distributed on the micron scale and the throats are mainly distributed on the nanoscale.The permeability is mainly determined by the micron pore throats,the poorer the reservoir physical properties,the more abundant the development of nano pore throats.In addition,the reservoir rock present obvious segmented fractal characteristics,indicating that the pore structure of the rock is complex,the pore throats are mainly developed in different micro-nano scales and the spatial characteristics of many small throats surrounding the larger pores,which laid a foundation for the study of the characteristics of rock poreperties and pore structure on the reservoir conductive mechanism.Through comparing the electrical properties of gas hydrate-bearing sediments and idealized gas hydrate digital rocks,the microscopic distribution modes changes during the formation of gas hydrate are effectively identified.The results show that with the increase of gas hydrate saturation,the main distribution modes of gas hydrate present a dynamic change law from pore-filling to cementing and then to grain-coating.Based on the non-Archie characteristics of the electrical properties of gas hydrate-bearing sediments,considering that the saturation index is a function of gas hydrate saturation and its value is considered to be closely related to the degree of gas hydrate formation and distribution modes,then a saturation index calculation model is established,which lays the foundation for proposing a new gas hydrate saturation calculation model.More importantly,based on the research results of gas hydrate reservoir characteristics and the proposed numerical simulation method of gas hydrate generation and distribution,the method of combining rock electrical experiments and digital rock technology is used to carry out the study of reservoir conductive mechanism and low resistance causes from the perspective of reservoir intrinsic factors.The influence of rock physical properties and pore structure,mineral composition,formation water salinity,gas hydrate microscopic distribution modes and other factors on the electrical proeprties of rock are analyzed.The main influencing factors of rock electrical proeprties and the causes of lowresistance are clarified.The results show that the main factors that cause low-resistance gas hydrate are ranked according to the quantitative order of the degree of resistivity decline: rock physical properties,formation water salinity,pore structure,micro-fracture,clay minerals,heavy minerals and gas hydrate microscopic distribution modes.Finally,based on the research results of gas hydrate reservoir characteristics,conductive mechanism and low resistance causes,evaluation models for porosity,permeability and irreducible water saturation parameters of the reservoir have been established.Considering the conductive effects of clay,irreducible water and movable water,combined with the correlation between cementation index and pore structure complexity index,a new gas hydrate saturation calculation model is proposed.The actual application results show that the model’s logging identification of low-resistance gas hydrate is in good agreement with the logging data,which verifies the feasibility and accuracy of the model for logging identification and fine quantitative evaluation of low-resistance gas hydrate.In addition,a dynamic saturation index gas hydrate calculation model was established based on the non-Archie characteristics of gas hydratebearing sediments.The actual application results show that the model can improve the calculation accuracy of gas hydrate saturation.From the perspective of the division of petrophysical components,the skeleton type and fluid type gas hydrate petrophysical model and digital rock model are constructed and the equivalance relationship is discussed,providing reference for the application of digital rock upscaling.Gas hydrate petrophysical model and digital rock technology are used to realize gas hydrate petrophysical diagnosis and microscopic distribution modes discrimination. |
