Gas-Bearing And Rock Mechanical Properties Logging Comprehensive Evaluation Of Shale Gas Reservoirs In A Area

Shale gas reservoirs usually have a number of organic matters,more complicated mineral components and pore structures,more nanoscale pores,low porosity and permeability.The water saturation and gas content are difficult to calculate.At the same time,the anisotropy and heterogeneity are strong.Due to the presence of bedding and other structural weakness planes,borehole collapse,mud loss,and other drilling accidents are easy to occur during drilling.The A area has abundant shale gas reservoirs,and has good potential for prospecting and exploitation.However,there are also problems including gas content evaluation and poor production performance after hydraulic fracturing.Fully utilizing a variety of logging evaluation techniques in shale gas reservoirs is the focus of this study.From the actual reservoir,comprehensive utilization of the logging data of new technologies and experimental results based on seven properties evaluation,gas-bearing and rock mechanical properties are well studied.This research can lay the foundation for the effective exploration and development of shale gas reservoirs.In this paper,an accurate and effective petrophysical volumetric model is established based on the actual condition of interest interval,and the content of organic and inorganic matters are calculated separately.Then,the pore structure is studied.The TOC content ranges from 1%to 5%with high maturity.And the mineral components are very complex dominated by illite and quartz.The pore type includes organic pores and in-organic pores.The pore shape is mainly long and narrow,the BET average pore diameter is 44nm,and the BET average specific surface area is 22.6m~2/g.The porosity is evaluated by the petrophysical volumetric model and NMR log,and the results show that the petrophysical volumetric model method is better.The non-Darcy flow and adsorption effect will increase the apparent permeability.Flow regime is analysised by Knudsen number,and the apparent permeability coefficients are calculated taking into the slip-flow and adsorption effect.Then,the absolute permeability can be corrected.From the perspective of sonic logging,based on BISQ model,the correlation factors affecting Vp-to-Vs ratio are studied,and the results show that the main influencing factors are water saturation and porosity under the condition that the squirt-flow lengths are the same.Then,the new water saturation calculation model is eatabilished.The new model is not affected by organic matter and conductive heavy minerals,and improves the calculation accuracy of water saturation compared with resistivity method.Conducting the isothermal adsorption and acoustic amplitude joint experiment.Under different pore pressures,the relationship between the amplitude attenuation coefficient and the gas content is studied.And the results show that the free-to-adsorbed ratio increases with the increase of amplitude attenuation coefficient and porosity.Then,a new model of free-to-adsorbed ratio is established.The practical applications are found to be in good agreement with the results of the traditional methods,which provides a flexible and effective method to evaluate gas content using less parameters.Based on the array sonic logging data,2D anisotropy and 3D anisotropy are characterized in shale gas reservoirs.And the results show that 2D anisotropy is weaker,however,3D anisotropy is stronger showing that the shale formation is TIV media.Then,the calculation methods of elastic parameters and strength parameters are systematically interpreted.The Biot coefficient is estimated with the shear modulus of the solid mixture,and the results are found to be in good agreement with the static Biot coefficients calculated by permeability,providing a new method for calculting Biot coefficient using mineral compositions and wave velocity.During the prediction of pore pressure,the compression wave slowness is corrected for organic matters according to the petrophysical volumetric model,and the gas-bearing correction is carried out based on the empirical formula.Then,the mudstone layers have the same criteria for estabilishing normal compaction trend line and the estimation accuracy of pore pressure is improved.Small fracturing tests is used to constrain the minimum horizontal stress.Stress polygon method and borehole collapse images are used to constrain the maximun horizontal stress,showing that the calcualtion of in-situ stress is reliable.The influence of in-situ stress,intact rock strength,weak plane strength,and weak plane occurrence on safe drilling mud density are analyzed systematically under arbitrary azimuth(0~360~o)and inclination(0~90~o).Then,the evaluation process of wellbore stability is established,which provides a theoretical support for the wellbore trajectory optimization and drilling process successfully.The brittleness difference between different mineral components is highlighted using the elastic parameters to correct the mineral components.Then a new brittleness calculation model is established.Compared with the acoustic method and the mineral constituent method,the new model improves the brittleness accuracy.Based on the uniaxial compressive strength,new brittleness and fracture toughness,a new fracability index is established and the formation fracability is divided into three categories.The evaluation results are in good agreement with the actual test results,and the new fracability index provides the corresponding technical support for the development of shale gas reservoirs.