Experimental Study On Hydration Fracture Of Brittle Shale Based On DIC Technology

The wellbore instability of shale reservoirs is a key factor restricting the drilling cycle and cost in China.Engineering design or optimization of drilling fluid to restrain the dispersion of shale to maintain the strength of wallbore rock,thereby maintaining wellbore stability.Dispersion of brittle shale is due to fracture separation caused by fracture propagation and communication between shale and water in drilling fluid.The existing dispersion evaluation methods cannot effectively evaluate the fracture evolution characteristics of shale hydration process,so it is crucial to research the fracture characteristics of brittle shale hydration process.Based on this,this paper uses DIC technology to study the evolution law of micro fractures in shale during hydration.The specific research contents are as follows:1.Study on the change law of shale strength under the action of drilling fluidBy using water and 2 %,4 %,6 %,8 % KCl solution soaking 24 h,48h,72 h,to explore the influence of different concentrations of KCl solution and soaking time on shale physical properties.The experimental results show that with the increase of KCl concentration,the number of newly developed microcracks on the surface of shale after immersion decreases,its uniaxial compressive strength and elastic modulus increase to a certain extent,and the Poisson’s ratio decreases.With the increase of immersion time,the uniaxial compressive strength and elastic modulus tend to decrease,but the longer the immersion time,the rate of increase with the increase of concentration decreases.Poisson’s ratio increases slightly with time2.Study on shale hydration fracture based on DICDIC technology is applied to the hydration fracture experiment of shale.DIC technology is used to measure the displacement field and strain field of shale before and after deformation.By comparing the changes of displacement and strain,the influence law of different KCl concentration on shale fracture propagation is explored.The fracture energy of shale is calculated according to the measured displacement and the tensile strength in the previous chapter.Experiments show that the fracture displacement of dry shale is smaller than that of shale soaked in water.With the increase of KCl concentration,the displacement of shale fracture decreases gradually.When the concentration reaches between 6% and 8%,the displacement change is no longer obvious,that is,there is a saturated concentration.After reaching the saturated concentration,the influence of KCl will not change.3.Micro-mechanism explanation of shale hydration fracture under drilling fluidFrom the microscopic view,the microscopic mechanism of shale hydration fracture is explained from two aspects of physics and chemistry.From the physical point of view,we analyze the negative adsorption between potassium ions and the crack tip,resulting in the change of interface energy at the crack tip.From the chemical point of view,we analyze the chemical reactions between potassium ions and different clay minerals in shale,resulting in the change of shale properties.According to Gibbs adsorption equation and Langmuir isothermal adsorption formula,the variable of interfacial energy at the crack tip is deduced,and the influence law of adsorption on interfacial energy is proved.