STUDY ON EFFECT OF WORKING FLUID ON FRACTURE PROPAGATION AND MECHANICAL PROPERTIES OF SHALE: BASED ON RESPONSE SURFACE METHODOLOGY

Shale wellbore instability is a major cause of high cost and long time drilling and completion for shale gas. The effect of working fluid soaking on shale crack propagation and mechanical properties was analyzed to optimize drilling technology and protect wellbore stability. Response surface methodology is applied to study the influencing factors under working fluid soaking. The results show that radial holes play a certain guiding role in the directional propagation of hydraulic fractures in low permeability reservoir. Fractures start to crack and propagate a certain distance along radial holes, and then gradually divert to the maximum principal in situ stress direction of reservoir. The expansion of shale cracks after fluid immersion can be divided into two stages: the rapid formation of cracks and the slow extension of existing fractures; the shale rock strength is reduced after soaking of different fluids, and the shale rock mechanical parameters are immersed by drilling fluid. Significant changes have occurred, showing a decrease in elastic modulus and an increase in Poisson's ratio. After soaking with the same drilling fluid, the longer the immersion time, the greater the reduction in shale rock strength.