| The physicochemical and mechanical interaction between deep hard brittle shale and drilling fluid could lead to the deterioration of mechanical properties of hard brittle shale,and then cause wellbore collapse which seriously restricts the development of oil and gas resources.Clarifying the strength deterioration mechanism of hard brittle shale under the action of drilling fluid is the key point to adjust drilling fluid properties to control shale strength reduction and then sustain wellbore stability.However,the previous study on strength deterioration mechanism of hard brittle shale after hydration cannot provide theoretical support for the optimization of drilling fluid.In this paper,an evaluation method of mechanical properties of hard brittle shale after hydration in deep well drilling condition is developed.Furthermore,a mathematical model which could describe the initiation of microcrack in hard brittle shale under the action of drilling fluid is constructed and used to reveal the strength deterioration mechanism of hard brittle shale after hydration from the view of chemical-induced microcrack propagation.Finally,a method controlling strength reduction of hard brittle shale after hydration is achieved through adjustment of drilling fluid density and chemical properties(interface property coefficient).The above works provide theoretical and methodological support for evaluation and optimization of drilling fluid performance in maintaining wellbore stability of hard brittle shale.The main works are as follows:(1)By developing a HPHT(high temperature,high pressure)fluid-rock interaction experimental apparatus and establishing a method for testing rock shear strength parameters based on continuous scratch experiment,a new experimental platform for evaluating strength deterioration effect of hard brittle shale after hydration in deep well drilling conditions is constructed.In addition,the experimental study on strength deterioration effect of hard brittle shale after hydration in deep well drilling condition and ambient condition.It is found that: deep well drilling condition would aggravate the degradation of shale strength and the new proposed method could reflect the real strength degradation characteristic of deep hard brittle shale.(2)Based on thermodynamic theory,fracture mechanics theory and geochemical reaction model,a mathematical model which could describe the initiation of microcrack in hard brittle shale with the interaction of fluid-rock is constructed,and the quantitative relations between drilling fluid property and fracture parameters of microcracks initiation,is obtained.In addition,the strength deterioration mechanism of hard brittle shale is revealed by quantitatively exploring the detailed physicochemical and mechanical interactions between fluid and rock under different environmental factors(environmental fluid composition,concentration,p H and temperature)and their effects on the initiation and propagation of microcrack of hard brittle shale.It is found that:(1)Compared with quartz and calcite,microcracks in clay minerals are more likely to expand under the action of fluid,which results in the strength properties deterioration of hard brittle shale.(2)The increase of fluid concentration could inhibit the initiation and propagation of microcrack,and thus control the strength reduction of hard brittle shale.This is mainly attributed to the change of adsorption of functional ions and hydrogen ions on the surface of shale minerals,and there are fundamental differences in the detailed mechanisms for different shale minerals.(3)Compared with Na Cl,the performance of KCl solution to inhibit the initiation and propagation of microcracks in hard brittle shale is more better,which means higher rock strength.It is related to the adsorption capacity of functional ions and hydrogen ions and the interfacial tension of fluid.(4)The increase of fluid p H aggravates the initiation and propagation of microcracks(for illite),and thus aggravates rock strength reduction.The fundamental reason is the adsorption of functional ions in the fluid is changed by the adsorption of hydrogen ions.(5)The increase of environmental temperature induces the initiation of microcracks under the illite-fluid interaction,and thus results in rock strength deterioration.It is mainly due to the change of adsorption of functional ions and the interfacial tension of fluid.(6)The fluid-rock interface property coefficient could reflect the strength reduction of hard brittle shale after hydration.(3)Firstly,rock cohesive strength is used as the mechanical representation of the chemical properties of drilling fluid.By establishing a wellbore stability model considering the strength deterioration effect of hard brittle shale under deep drilling conditions,the relationship between the equivalent density of borehole collapse pressure and rock cohesion is clarified and the required cohesion index of hard brittle shale for wellbore stability could be obtained(evaluation standard).Furthermore,the fluid interface property coefficient is proposed as the chemical property index of drilling fluid reflecting the strength deterioration of hard brittle shale.A quantitative evaluation method of shale-fluid interface property is constructed to guide the preliminary optimization of chemical property of drilling fluid.Combined with the developed experimental platform for evaluating cohesive strength of hard brittle shale under deep drilling conditions,an optimization method for drilling fluid properties which could sustain wellbore stability is proposed(optimization method).Finally,as for the condition that other properties of drilling fluid meet certain requirements,a new method which could inhibit strength deterioration effect of deep hard brittle shale by adjustment of drilling fluid densitychemical properties(interface property coefficient)is developed.Field application shows that the new method can effectively guide the optimization of drilling fluid properties and realize the wellbore stability of deep hard brittle shale. |
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