Study On Mechanical Characteristics And Conductivity Of Acid Etch Soften Layer In Limestone

The worldwide 40% of the total proven reserves of oil(gas)is still in occupied by the carbonate reservoir today,and after a long period of development on carbonate reservoir which related to acidizing fracturing technology and gradually formed a set of primarily early phase reservoir stimulation technology.Study of conductivity in acid etched fracture must consider the situation of stress,so the stress conditions and the etched surface mechanical properties become the core issue,in this paper.After acid etch the surface of the rock in fractures the microstructure of rock been destroyed by acid dissolution,both the dissolution on mineral grains and intergranular cement all will reduce the rock strength,give expression to the phenomenon of rock softening on surface.On the surface soften rock has higher plasticity and lower embedding strength compared with inner rock,and it will have a certain influence on the contact shape of the acid etched surface at the micro scale.The macroscopic mechanical characteristics of acid soften rock were compared and the edge morphology of acid etched rock was observed then qualitative conclusion is summarized.By acid etching core triaxial compression mechanics experiment and observations of the etched rock sample wafer edge,corrosion of acid rock softening phenomenon is verified,summarizes the acid rock macro mechanical properties and the influence of the microstructure.The results showes that the maximum compressive strength and elastic modulus of all the acid etched cores were significantly lower than ordinary samples.The edge of the ordinary sample is found to be straight line at low magnification,and the whole boundary was complete and continuous.With the low magnification of the boundary of the acid etching sample,it can be observed that there are obvious jagged and estuarine corrosion line,and the mineral particles in the high magnification are in a fragmented and discrete form.In this paper the relationship between the acid etching condition and the surface vickers hardness is studied to further study the thickness of the softening layer under different acid etching condition.The thickness and the basic mechanical parameters of the rock samples under different acid etching conditions were obtained by the measurement of vickers hardness on the surface.In the process of pressure test,it is found that the indenter of vickers hardness tester shows the phenomenon of sample hardening under the large load,which proves that the acid corrosion softening has a certain thickness and can be broken by the diamond indenter.During the observation of electron microscope,the increase of acid concentration and acid etching time is found to increase the thickness of acid etching.The thickness of the softening layer of all the samples shows the obvious spatial distribution inhomogeneity,which is mainly affected by the large particles and the insoluble minerals.The conditions of acid etched fracture surface under stress is calculated through FEM,and the transformation law of distance between fracture surfaces is obtained.Conclusion shows that the distance is influenced by the soften layer thickness and initial fracture width,and in some special conditions the distance could be 0 which indicated that the fracture is closed completely.through statistics found that as the initial fracture width decreases gradually,70 MPa initial stress with 4 mm wide and 300?m soften layer is first appeared completely closed,the condition of the preliminary determination 4 mm for crack closed completely critical initial seam width,The condition of less than 4mm also depends on the combination of the closed stress and the thickness of the softening layer.Base the softening layer considered conductivity model is established,take the conditions which corresponding to 0 value of conductivity as standards,divides the boundary conditions on the impact of soft layer should be considered as: on the left side of the initial fracture width and at the same time the thickness of softened layer fall on the above of boundary to consider the influence of softening layer.It is pointed out that no matter the thickness of the softening layer and the width of the initial seam,the effect of the softening layer should not be considered.The engineering conditions conductivity has been established,and through its equivalent conversion to soften layer conductivity the soften layer thickness and conductivity can be calculated on the direction of fracture length under specify treatment parameters and reservoir information,then combine with the mentioned soften layer affecting boundary determine standard plot the real effective length of conductivity in fracture can be obtained.The result of calculation shows that the real effective length of conductivity is much more shoter than the acid effective distance which is the current standards and widely been used in oil field,this explained that in some cases the effect of acid etch soften layer must been considered to make correct estimate of conductivity in acid etched fracture.