| Muli area is the only land permafrost area where gas hydrates are found in China at present.Hydrates occur in the pores of rock formations as disseminated or in the fissures of rock formations as flakes.The lithology of hydrate reservoirs is mainly mudstone,oil shale,siltstone,fine sandstone,etc.The occurrence environment,occurrence form and reservoir formation mechanism differs significantly from those of hydrates in sea areas and polar permafrost areas.In view of the study area of low degree of exploration,a set of identification and evaluation for permafrost hydrate reservoir method is supposed to carry out through the analysis of response characteristics and rock physical experiment logging hydrate reservoir,and to study the formation mechanism and main controlling factors and research anomalies.In this paper,the logging response characteristics of hydrate reservoir were discussed by using conventional logging and ultrasonic imaging logging data in terms of lithology and occurrence state.Combining with core testing and electrical data,a reservoir parameter calculation model was established.Digital cores under various conditions were established,and the electrical characteristics of hydrate reservoir were studied by using numerical simulation method.It provided a link to study the relationship between electrical characteristics of hydrate reservoir and factors like hydrate distribution and saturation.The main research process and achievements are as follows:1.It analyzed the log response characteristics of hydrate and different lithologies.The analysis showed that the gas hydrate in Muli area was categorized as high resistivity,high acoustic velocity and low density;resistivity is sensitive to porous hydrate,resistivity and acoustic logging are sensitive to fractured hydrate,and the fractured hydrate reservoir is the main reservoir in the study area.2.It carried out the rock physics experiments and formation parameters were obtained by rock electricity,physical properties,and sound velocity measurement for further collation and analysis.3.It established the pore-type digital core model of hydrate reservoir by X-ray CT scanning method,and then established the fractal discrete fracture network was constructed by random simulation method based on fractal theory,and obtained the three-dimensional fracture network digital core of fractured reservoir by superimposing with the digital core of porous reservoir,which laid a foundation for numerical simulation of hydrate reservoir.4.A pore network model with the same topological structure and geometric characteristics as the digital core was established by using the maximum sphere algorithm.The correlation between the pore structure parameters of hydrate and the physical properties of hydrate reservoir rocks was studied qualitatively and quantitatively.The parameters of pore structure which characterize the geometric and topological characteristics were consistent with the calculation results of the digital core.5.By finite element method it calculated the digital core resistivity under different hydrate distribution and saturation conditions,the electrical response characteristics and main controlling factors under different factors,and it discussed the generation mechanism and the law of influence on the macro-physical characteristics of rocks.Based on the simulation results,it inferred that the hydrate distribution in the study area presented mainly as contact hydrate.6.It examined the calculation model of hydrate reservoir parameters(shale content,porosity,saturation,etc.).The study showed that the saturation evaluation models of Archie,Modified Archie and Indonesia can still be used to calculate the water saturation of sandstone porous hydrate reservoirs and mudstone and shale fractured hydrate reservoirs.The objective function for optimum analysis of Muli hydrate reservoir was established.The reservoir parameters were obtained with back calculation through SR1,DFP and BFGS variable scale optimization methods. |
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