Microscopic Dynamics Mechanism Of Deep Shale Under Water-rock Interaction

During the drilling process,a series of physical and chemical reactions(water-rock interaction)will take place between shale and drilling fluid,which will induce shale crystalline layer to develop into large-scale fractures from micro-nano fractures,and then cause wellbore instability.In this paper,the microcosmic mechanism and control method of shale deterioration under water-rock interaction were explored from the perspective of molecular dynamics,which provides theoretical guidance for deep shale wellbore stability.The main work is as follows:(1)Microscopic mechanism of shale degradation caused by water-rock interaction under high temperature and high pressureThe substitution characteristics of clay minerals in deep shale are analyzed experimentally,and a molecular dynamics model of water-rock interaction of typical shale minerals considering the substitution characteristics of clay lattice was established,which overcomes the defects of the traditional model that can not reflect the intrinsic fabric and diversity characteristics of clay minerals.The conduction and distribution of water molecules and cations between layers was systematically evaluated,as well as the response rules of Micro-Mechanical properties of shale microcrystals to water content,temperature and pressure conditions and lattice substitution types.The microcosmic essence of shale deterioration under water-rock interaction is revealed,and the key to the control of the shale deterioration lies in the microscopic mechanism of water inhibition and wettability regulation.It is found that:(1)The microcosmic essence of shale deterioration under water-rock interaction: one is the expansion of crystal layer which increases the vertical compressibility of montmorillonite crystals;the other is the transfer of cations to the middle of interlayer domain,which absorbs more water molecules and weakens the electric neutralization of negatively charged clay layers,resulting in the increasing tendency of mutual exclusion of clay layers carrying the same charge.(2)Temperature and pressure have little effect on clay expansion while water content is the main factor of clay expansion.The control of water is the basis of controlling the shale deterioration under water-rock interaction,and the control of water performance and water content through the regulation of hydration inhibition and wettability is the key to control the deterioration.(2)Microscopic mechanism of chemical inhibition of deep shale water-rock interactionBased on molecular dynamics theory,a micro-molecular dynamics evaluation model of cations inhibiting shale water-rock interaction was established,which takes into account various occurrence modes such as complete cation replacement and different concentration gradients in interlayer domain,as well as the lattice substitution characteristics of clay.This breaks through the limitation of traditional macro-inhibition evaluation that can not reflect the micro-fabric characteristics of shale and the micro-inhibition process.The microscopic reaction process of typical inorganic salts inhibiting water-rock interaction in shale was explored.Then it reveals the micro-mechanism of the chemical inhibition effect of cations on shale water-rock interaction that the cations can promote the contraction of interlayer spacing,compress fluid intrusion channels,reduce the intrusion ability of water molecules,increase the negative charge balance ability,and reduce the interlayer electrostatic repulsion force.The response law of the inhibition of cationic on water-rock interaction to temperature-pressure conditions and ion types and concentrations was revealed,and the chemical control inhibition method for water-rock interaction was proposed.(3)Microscopic mechanism of wetting conduction of deep shale under water-rock interactionBased on the inhibition control of water-rock interaction,a molecular dynamics model of wetting spreading of typical minerals in deep shale was established.Through the analysis of ion hydration ability in the wetting process,the deficiency of traditional macro-evaluation that can not characterize the micro-process of shale wetting spreading was remedied.The wetting behavior of typical inhibiting inorganic salt solution on the surface of shale minerals was studied.The micro-dynamic mechanism of shale wetting spreading and the response law of temperature,pressure,salt solution type and concentration were revealed.(4)Inhibitory fluid optimization for degradation of water-rock interaction based on chemical inhibition and wetting regulationBased on the microcosmic mechanism of deep shale deterioration induced by water-rock interaction,an evaluation method of hydration inhibition of drilling fluid at microscopic scale was proposed.Considering that the key to controlling the deterioration of water-rock interaction is the regulation of water performance and water content,it is proposed to use water hydration inhibition and wetting properties to synergistically inhibit the progress of water-rock interaction,and prevent the loss of structural integrity and strength degradation of deep shale.The fluid regulation method for the deterioration inhibition of deep shale water-rock interaction has been developed and validated,and an optimization design method for controlling the wellbore stability of deep shale has been put forward.