Analysis Of Induced Remote Displacements And Its Inverse Problem Caused By Gas Injection Into Reservoir

In recent years,with the development of large-scale CO₂ geological storage operations,the magnitude and extent of formation pore pressure disturbances caused by gas injection have never been seen before,which has triggered a series of new challenges.Reservoir gas injection under great buried condition has the characteristic of concealment.If the seal fails,it will cause serious problems for the environment and humans.Therefore,it is necessary to strengthen the dynamic monitoring,simulation and prediction of the reservoir.In the risk analysis and long-term security of operations,the analysis of far-field effects is essential.Long-term gas injection in the reservoir may cause geological deformation and activate fracture networks.However,the current carbon dioxide storage technology is not mature enough and is still in the stage of demonstration research.The data detected by the inclinometer array only reaches the millimeter detection level,and the accuracy is not enough.Based on the boundary element method,this paper uses FORTRAN language for programming,combined with FLAC 3D and ABAQUS software,to analyze the far-field effect of the ground surface caused by gas injection in the reservoir through modeling and simulation calculation.The main innovation of this article is to introduce the inverse problem calculation into the analysis of far-field effects.For the stress field and displacement field caused by gas injection in the reservoir,the discontinuous displacement method and the fictitious stress method of the boundary element method are used to model the two-dimensional simplified example of coin-shaped hole and the three-dimensional simplified example of spherical cavity.Numerical examples show that the closer to the reservoir,the greater the displacement and stress fields generated by gas injection.As the distance increases,the displacement and stress fields gradually decrease,and finally tend to be gentle;the results of the fictitious stress method are always higher than the discontinuous displacement method,and in the two-dimensional calculation example,the error of the fictitious stress method is too large.According to the general law of perturbation on the surface caused by the above calculated gas injection in the reservoir,through the numerical simulation of the two-dimensional simplified model and the three-dimensional simplified model,the range of reservoir gas injection disturbance on the surface is determined,that is,the surface is four times away from the top of the reservoir.The determination of the disturbance range has a very important meaning in the process of numerical simulation.The determination of the approximate range of the surface value can reduce the calculation amount of the program,improve the calculation efficiency,and save the calculation resources.At the same time,in the research of the above two groups of forward problems,it is found that the far-field effect caused by the exaggerated gas injection in the two-dimensional simulation is not accurate enough and the error is large.Also inverse problem analysis is introduced,and the stress field and displacement field at the reservoir boundary are solved from the displacement field or stress field at the monitoring points.The effects of regularization parameters and errors on the solution of the inverse problem are discussed separately,at the same time,the influence of sensors location,reservoir modeling shape and reservoir dip on inverse problem analysis was also studied.And finally a stable and accurate solution can be obtained.It shows that even in the case of limited data and obvious errors,inverse problem analysis is feasible in the analysis of the far-field effect of gas injection in the reservoir.Through inverse problem analysis,the displacement and stress fields in the surface can be used to reverse the displacement and stress conditions in the reservoir,and the migration of injected carbon dioxide in the reservoir can also be monitored.The numerical simulation of the CSEMP pilot project in Alberta,Canada was conducted to study the surface deformation caused by CO₂ injection into the reservoir.Then,using the surface displacement data obtained by monitoring,the gas injection pressure in the reservoir was calculated by the inverse problem analysis.The simulation effect is good,which has laid a solid foundation for the complex geological model and three-dimensional inverse problems.