Research On Calculation Method Of Shale Gas Loss Basing On The Whole Process Of Coring

Shale gas content is an important reference index for efficient shale gas exploration and development.According to the sequence of shale coring time,shale gas can be divided into shale gas loss,desorption gas and residual gas.Among them,shale gas loss is one of key variable for accurate determination of shale gas content.Thus improving the accurace of shale gas loss calculation is considered affects shale gas resource assessment.At present,the calculation of the shale gas loss methods mainly include improved USBM(United States Bureau of Mines)method and polynomial fitting method,etc.However,due to the relatively large burial depth of shale gas reservoirs,the confining pressure and temperature vary greatly with the change of the position of the core during the whole process of coring.During this process,shale gas desorption,seepage and escape rates will change significantly,resulting in large deviations in the estimated shale gas loss using some data under a single temperature and pressure condition.Therefore,it is of great significance to establish a calculation method for shale gas loss based on the whole process of coring.This paper analyze the whole process of shale coring and study the effects of confining pressure,temperature and shale gas occurrence state on the shale gas desorption law;Using theoretical analysis methods,etc.,the calculation model of the gas escape rate on the core surface is obtained.Combined with GCTS(Geotechnical Consulting and Testing Systems)experiments,the influence of different depths and soaking time on core permeability was studied,and a calculation method of shale gas loss based on the whole process of coring was established.Finally,the shale gas loss simulation experiment platform is independently developed and physical experiments are conducted to verify the theoretical method.The main results are as follows:(1)The effective stress of the shale core changed abruptly during the drilling stage,which leads to a sudden increase in the permeability of the core by orders magnitude,resulting in a significant increase in the shale gas escape rate in the early stage of desorption.Through GCTS experiment,the influence of all confining pressure,pore pressure,coring fluid immersion time,immersion pressure,etc.on core permeability was studied;Combined with the analysis of the core stress during the shale coring process,the effective stress of the core experienced a significant reduction during the process of coring,and there was a sudden change in the effective stress of the core during the initial drilling stage of coring;For a 1500 m deep shale gas well,the effective stress mutation of core can reach up to 10.7 MPa,and the corresponding core permeability has increased by orders of magnitude.As a result,the escape rate of shale gas increases significantly in the early stage of desorption,and it is believed that the gas loss of shale will increase significantly at this stage.(2)A calculation method is established for shale gas loss based on the whole coring processFirst,systematically analyzed the variation law of core temperature and pressure with depth,coring time,coring fluid density and other factors,and established a calculation model of pore pressure that changes from adsorbed gas to free gas during the whole coring process;Second,it is found that the shale will increase its permeability stress sensitivity coefficient after being soaked in coring fluid,and based on the experimental results,the shale core permeability calculation model obtained during the sampling process is revised;Third,using the equation of state,motion equation and mass conservation equation of the gas unsteady seepage process,combined with the initial conditions and boundary conditions of the core,the Boltzmann transformation is used to obtain the calculation model of the gas escape rate on the surface of the core during the coring process;Finally,as the integral of the shale gas loss based on the whole coring process cannot be directly calculated,the calculation of the shale gas loss is realized by combining with the secondary development of matlab software.(3)The simulation experiment method of shale gas loss in the process of variable temperature and pressure is proposed.In view of the fact that there is no direct reference to the measurement standard for shale gas loss,the physical simulation experiment method for shale gas loss is proposed,and the simulation experiment platform for shale gas loss is developed based on this.The platform is composed of five subsystems: water bath thermostat subsystem;main experiment subsystem;gas supply subsystem;gas measurement subsystem and vacuum pumping subsystem.The platform has realized the simulation of core inflation compensation and the simulation of temperature and pressure changes of the core during the whole process of coring.The relative error analysis of the experiment shows that the relative error: 3.92 %;2.54 %;3.41 % in the vacuum stage of the platform;the gas injection reference tank stage and the shale gas loss simulation stage respectively,which ensures the accuracy of the experiment.The shale gas loss simulation experiment is carried out to verify the theoretical method proposed in this paper.The results show that: The average error rate of shale gas loss calculated by the theoretical algorithm based on the whole coring process is 8.49 %,while the average error rate of polynomial fitting method is 29.93 %,which verifies the correctness and effectiveness of the calculation method of shale gas loss based on the whole coring process.The research paper provides a new perspective for the calculation of shale gas loss,and effectively compensate contribute to the existing shale gas loss calculation methods.