Study On Effects Of Shale Hydration On Wellbore Stability

Hydration which may cause shale swelling,rock strength descent,pore pressure increaseand so on is one of dominant factors of borehole sloughing. By combining theoreticalknowledge of many branchs of learning the thesis has researched the mechanism of shalehydration as well as its dominant factors thoroughly and deeply. Through analysis ofmechanism of the coupling of chemistry and mechanics of wellbore instability, the paperpresents that controlling hydration stress is the key of inhibiting shale hydration of the shale.This paper has analysed and concluded four kinds of methods of measuring hydrationstress, exploratorily tested hydration swelling pressure on the base of reconstructing theoriginal HPHT wellbore stability simulation experimental equipment, established experimentevaluation method of shale hydration'effects on wellbore stability and researched theinfluence of potassium chloride , sodium chloride and calcium chloride on the lowpermeability of shale through the technology of pressure transmission.According to the chemistry and mechanics coupling model of wellbore instability, wehave performed software to calculate hydration and mechanics coupling safe density windowof shale. The software consists of rock mechanics parameters calculate, dynamostaticmechanics parameters transform, earth stress calculate and safe density windowcalculate.Utilizing experimental data and well log information, we have calculated andanalyzed the influence of shale hydration on sloughing pressure of formation and safetydensity windows. Through analyzing the solution of drilling sloughing-prevented technology,we think that by prohibiting and slowing down filtrate invasion,pressure transmission, shalehydration can be prevented and controlled, and borehole wall instability can be inprovedefficiently.It is the first time of domestic that SD-A and HA-Al synergetic effect was provedto be effective in preventing and slowing down pressure transmission by using pressuretransmission technology.A set of water-based sloughing-prevented drilling fluid system was optimized in the paper by using modified HPHT wellbore stability simulation experimentalequipment and doing pressure transmission experiment, which has excellent ability of sealingand preventing wellbore soughing.