Research Of Water Imbibition Model In Shale Multiple Pores

The network fracturing with large-scale water-based fracturing fluid is the key technology to effectively unlock shale oil and gas.However,some mechanisms are still unclear,especially imbibition flow mechanism of fracturing fluid in shale micropores and the effect of imbibition on the microstructure and physical properties.These make the target of the fracturing design and flowback optimization is unknown,restricting the recovery enhancement of shale oil and gas.Therefore,focusing on the mechanism of water imbibition in shale,this paper has conducted a series of studies on shale microstructure characterization,imbibition model and engineering influence analysis.Shale is characteristics of multi-component,multiple pore.According to occurrence type,the pore type of shale can be divided into organic pore,brittle mineral pore and clay pore.Combined with field emission electron microscopy,high pressure mercury intrusion and low pressure nitrogen adsorption,the differences of pore shape,pore size and fractal characteristics of these three types of pores were analyzed.Based on contact angle test,mineral analysis,and mixed wettability model,the wettability of organic pore is calculated.Based on mineral analysis,porosity test and petrophysical model,the multiple porosity is calculated.The results show that the pore shape,pore size distribution,fractal characteristics,wettability,pore volume and porosity proportion of the three types of shale pores are significantly different.According to the difference of shale imbibition force,the three types of shale pore are divided into non-clay pore(including organic pore,brittle mineral pore)and clay pore.The imbibition force of non-clay pore is capillary pressure while the imbibition force of clay pore includes capillary pressure and osmotic pressure.For the clay pore,considering non-circular shape of capillary,microscale effect,based on the Navier-Stokes equation,Navier boundary slippage model,the water flow equation in elliptical capillary is established.Moreover,combined with capillary pressure equation,considering fractal features of the channel tortuosity and pore size,the non-clay pore fractal imbibition model is established by fractal theory.For the clay pore,the semipermeable membrane characteristics of the clay pore are analyzed.The results show that osmotic pressure in the shale is significant.Based on the Fritz semipermeable membrane efficiency model,Van’t hoff equation,equivalent salinity model,the osmotic pressure model of the clay pore is established.Then based on the NS equation of the plate flow,considering boundary slippage,considering fractal characteristic of the channel tortuosity and pore size,combined with capillary pressure and osmotic pressure equation,the clay pore fractal imbibition model is established.Based on the petrophysical model of shale multiple porosity,combined with the fractal imbibition models of organic pore,brittle mineral pore and clay pore,the shale multiple-pore imbibition model was finally established.The imbibition model is verified by theoretical model and imbibition test.By definition of imbibition coefficient in characterization of imbibition ability,water imbibition flow law irt shale is analyzed.The results show that the factors controlling imbibition volume mainly include imbibition area,proportion of three types of porosities,pore structure parameters,capillary pressure,osmotic pressure,boundary slippage and fluid properties.Shale imbibition and displacement experiment have been conducted,combined with porosity,permeability test,three-dimensional microscopic system,SEM,mercury intrusion test and CT scanning,the shale microstructure and properties before and after the experiment have been compared and analyzed.The results show that the imbibition process is accompanied by initiation of clay pore and bedding fracture,dissolution and fall off of minerals,and will result in increase of porosity and permeability.The porosity,permeability,mass loss,fracture initiation are positively correlated with imbibition volume of water.Based on elastic mechanics,an initiation model of imbibition induced fracture is developed.The results show that the induced fracture initiation is controlled by the shape of clay pore,orientation of fracture and stress,confining pressure,eapillary pressure,osmotic pressure,anisotropy tensile strength.Finally,based on the imbibition model and scale upgrade,flowback efficiency prediction,engineering parameters optimization,imbibition and flowback characteristics of fracturing wells have been analyzed.The shale multiple-pore imbibition model established in this paper effectively reveals the imbibition law of water in shale.it can provide effective guidance for the determination of shut-in time and the optimization of fracturing parameters.