Studying the Kaiser Effect During Modeling of Rock Loading Conditions Using the NX-borehole Jack

The paper deals with modeling the stress state experienced by rocks in a loaded around-borehole volume to justify a promising method of in-situ stress measurements based on the Kaiser effect. Mathematical modeling of loading the horizontal borehole walls with a NX-borehole jack in a direction of principal components σ₁ or σ₃ of the natural stress field was performed for conditions of the unperturbed salt rock mass of the Verkhnekamskoe potassium salt deposit. As a result of the numerical calculation of stresses, it was determined that recovery of radial component σ_r in compression regions of the rock volume around the borehole occurs when the jack's pressure reaches the initial value of the principal stress. At the same time, the concentration of tangential component σ_θ in these regions is retained. The rock salt specimens were tested under modes similar to the stress states of the rocks in the around-borehole volume to study the Kaiser effect under these conditions using a triaxial compression vessel. It was found that in the second loading cycle, the recovery of the initial values of axial component σ_axial of the specimen stress field, unloaded after the first cycle, stimulates acoustic emission. This regularity is true for the case when there is no increase in confining pressure σ_conf on the specimen between the cycles. In the conditions with an increase in confining pressure σ_conf between the first and second cycles, the Kaiser effect is found at higher values of σ_axial component than the initial ones. It is shown that the nature of the effect depends on a type of a specimen's initial stress state and is caused by the growth resumption of longitudinal microcracks, if σ_axial > σ_conf in the first loading cycle, or by a closure of lateral microcracks when σ_axial < σ_conf in the first loading cycle.