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

Abstract

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 σr 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 σθ 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 σaxial 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 σconf on the specimen between the cycles. In the conditions with an increase in confining pressure σconf between the first and second cycles, the Kaiser effect is found at higher values of σaxial 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 σaxial > σconf in the first loading cycle, or by a closure of lateral microcracks when σaxial < σconf in the first loading cycle.