Research On Post-Frac Flowback Of Shale Gas Reservoirs

With the recoverable reserve of conventional hydrocarbon decreasing progressively all over the world,shale gas has emerged as a potentially significant source of new and unconventional natural gas reserves.Shale gas has huge reserves in China.The goal that makes the production of shale gas effective and economic is strategically meaningful to China,which aims to solve the energy gap,optimize the energy structure and promote emission reduction.At present,studies have proven that multistage hydraulic fracturing in horizontal wells is the most effective technology for exploiting shale reservoirs.Compared with hydraulic fracturing in conventional reservoirs,hydraulic fracturing in shale reservoirs is characterized by the low flowback efficiency of fracturing fluid,resulting in the retention of large amounts of fracturing fluid in shale formation.Therefore,it is of great significance to study the mechanism of fracturing fluid migration and retention,contributing to a better understanding of slickwater fracturing technology in shale reservoirs.Firstly,in-depth analyses of the shale formation physical properties from Sichuan basin are conducted by tests of permeability,porosity,mineral composition,wettability and mercury injection.The results lay the foundation of subsequent experiments.Secondly,the liquid saturation distribution along rock samples at varied time points during spontaneous imbibition are measured by the most advanced nuclear magnetic resonance(NMR)technology.Compared with those of sandstone,the characteristics of spontaneous imbibition in shale are analyzed.Then the study systematically analyzes the factors having great influence on spontaneous imbibition in shale,which include clay mineral,salt concentration,surfactant solution and bedding direction.And then,the shale sample is put in core holder to displace from dry face by the integration equipment of high-temperature and high-pressure displacement and NMR to simulate flowback process in shale.By analyzing the measured NMR data during displacement,themechanism of fracturing fluid retention is examined in depth.In order to comprehensively evaluate the mechanism,the effect of clay mineral,salt concentration,surfactant solution and bedding direction on the flowback is examined in detail.At last,a numerical flowback model is constructed considering the characteristics of flowback in shale.Integrated with the experimental results above,liquid saturation distribution maps from experiments conducted by NMR are used as target-match data to get a fitted capillary pressure curve in the shale gas numerical simulation model.The fitted capillary pressure curve takes into account not only the capillary pressure but also the osmosis pressure,resulting in a more accurate prediction to shale flowback.Based on the established flowback numerical model,the phenomenon of fracturing fluid retention in shale reservoirs is studied from the perspective of numerical simulation.Furthermore,the effect of injection pressure,matrix permeability and absorption-desorption on the shale flowback is investigated as well.This paper mainly investigated the low flowback efficiency of fracturing fluid and the mechanism of fracturing fluid retention in shale on the basis of experimental studies and numerical simulation methods.The results of this paper provides theoretical basis for optimizing the treatment of the hydraulic fracturing technology and the evaluation of post-fracturing in shale.