Hydration Characteristics Of Hard Brittle Shale And Its Influence On Collapse Pressure

Hard brittle shale is the geological body of unconventional oil and gas resources,such as shale gas and shale oil,and it is also the geological body which drilling accidents frequently occur in deep resources exploration and exploitation.The wellbore stability of hard brittle shale formation has always been an key problem in drilling engineering.Hydration is one of the most important factors in the instability of hard brittle shale formation.Hydration could bring changes to the mechanical property of rock,then affect collapse pressure and produce serious difficulties to establish the technology of wellbore stability.In order to drill into the hard brittle shale formation safely,it is necessary to study the hydration characteristics of hard brittle shale,to explore the hydration mechanism of hard brittle shale and analyze the effect of hydration on collapse pressure.This paper takes the hard brittle shale of Shihezi formation in Ordos Basin as the study object and takes the effect of drilling fluid soak on shale properties as thread to conduct experimental study of rock mechanics,including direct shear test,triaxial compression experiment and hardness test,systematically presenting the rock mechanics characteristics of hydration under the action of drilling fluids and studying the acoustic features of hydration on the basis of ultrasonic transmission experiment.Upon the recognition of the physical properties of foundation rocks of hard brittle shale,the paper reveals the hydration mechanism of hard brittle shale and investigates the effect of hydration on collapse pressure.According to the experimental research and theoretical analysis:Hydration of hard brittle shale directly brings changes to the mechanical properties of rock,after the hydration,strength of rock decreases,hardness of rock becomes lower.External structure features,water absorption,elastic modulus and Poisson’s ratio are difficult to evaluate the hydration characteristics of hard brittle shale.Ultrasonic signal,such as acoustic travel time,attenuation coefficient,time domain curve and frequency curve can be used as the efficient methods for studying the hydration characteristics of hard brittle shale.There is no smectite in hard brittle shale,so the hydration swelling is not obvious,microfissures generally exist in hard brittle shale.The natural structure characteristics and mineral composition of brittle shale are the determinant factors of hydration reaction and are the root causes affecting hydration reaction,while cation exchange capacity,specific surface area and wetting property affect the hydration characteristics of hard brittle shale to varying degrees.Hydration of hard brittle shale is one of the major reasons of causing increased collapse pressure and it poses a potential threat to wellbore stability.Therefore,it is imperative to fully recognize the hydration characteristics of hard brittle shale when carrying out wellbore stability design to avoid wellbore collapse and make sure the safe processing of drilling.