Evaluation Method And It's Application For Tight Gas Reservoirs

Unconventional gas resources are extensively distributed and of large reserve in China,there is a good reality for exploration and development of tight gas resources.Hydraulic fracturing is an important stimulation technique for low permeability reservoirs,however the working fluids entering into the tight reservoir greatly decrease the productivity.Damage evaluation is the foundation for efficient development of tight gas reservoir.Currently,there is no standard for tight gas reservoirs on damage evaluation.In our country,the techniques of damage experimental evaluation for tight gas reservoir rely on the industry standard SY/T5358-2010.The current methods have problems of difficult testing,high experimental equipment requirements,large experimental errors,stresssensitive damage to the core,and incomplete evaluation.The main types of damage to tight gas reservoirs are not clear enough.The main focus is on macro-evaluation.Test and evaluation methods cannot reveal the evaluation of micro-mechanisms and processes.In this study,the tight gas reservoirs in Jilin and Changqing regions were studied.The water-sensitivity,acid-sensitivity,and stress-sensitive damage assessments of tight gas reservoirs were systematically performed.The evaluation methods of reservoir damage and the research on water lock damage were focused on.For the characteristics of tight gas reservoirs with ultra-low permeability and high sensitivity to water saturation,the pressure decay permeability test is used as a basic method for reservoir sensitivity assessment,solving the problem of ultra-low permeability reservoir testing difficulties and sensitivity.The parameters of the current industry standards for sexuality evaluation were refined.The new method uses nitrogen as the test medium,the experiment meets the actual results of gas reservoir damage,and the combination of spontaneous infiltration-nuclear magnetic monitoring,spontaneous infiltration-pulse permeability test,and high-speed centrifugation.A set of evaluation methods for water lock damage in tight gas reservoirs was established to solve the problem of evaluation limitations in gas flooding methods.The results show that water lock,water sensitivity,and stress sensitivity are the main types of damage in tight gas reservoirs.Among them,the main mechanism of the water lock is the self-priming and retention of the fracturing fluid caused by the strong capillary force.In the process of self-priming,the micro pores preferentially absorb water.As the water saturation increases,the gas seepage is severely impeded,the critical water saturation of the reservoir is high,and the gas liquid cannot form continuous flow under a wide saturation condition.The overall fluidity is severely constrained.The smaller the reservoir permeability,the greater the difference between the initial water saturation and the critical water saturation,and the more severe the water lock damage.The self-priming ability of the reservoir is related to the self-priming rate and permeability,the initial water saturation,and the development of microfissures.The water locks in the eastern Ordos Basin study area are seriously damaged.Permeability drops rapidly after the fluids enter the reservoir,and they are mainly distributed in micro-throat throats with a radius of less than 0.1 ?m.It is difficult to move and reduce the contact time between the reservoir and the fracturing fluid.Rapid flooding is an important means of preventing water locks;the water sensitivity of reservoirs in the Yingtai Fault Depression of the Songliao Basin is generally moderate and strong,the liquid seepage channels caused by clay expansion are shrinking,and the particle migration caused by the expansion.Blockage damage is the cause of water sensitivity;adding clay stabilizer to the working fluid has a certain effect on the prevention of water sensitivity;for strong water sensitive reservoirs,the nature of fluid entering the well and the degree of mineralization should be further controlled.