Multi-Scale Structure Description And Borehole Instability Mechanism Of Organic Rich Shale

Shale gas has become a new bright spot in the global unconventional oil and gas resources exploration and development. Borehole instability of organic rich shale is a problem commonly encountered in the process of shale gas well drilling, seriously restricted the process of the exploration and development of shale gas. In view of the water-based drilling fluid drilling horizontal well is very difficult, so usually by oil-based drilling fluid to control borehole instability. However, oil-based drilling fluid is far did not achieve the desired effect.Organic rich shale contains a great number of clay minerals and micron-crystal quartz, high brittle, nano-microscale pores and fractures development, strong heterogeneity, borehole is easy to instability. Drilling fluid is alkaline, which pH value is typically great than 9. As we all know, lye can enhances the hydration ability of clay minerals, corrodes siliceous minerals as clay minerals, quartz and feldspar. But, organic rich shale be recognized as low swelling rock, we usually ignore inhibiting the swelling when use water-based drilling fluid drilling. Oil-based drilling fluid can inhibit clay minerals hydration swelling, so alkali corrosion maybe the important reason for borehole instability. We infer that may be alkali corrosion is one of the reasons of shale borehole instability. Therefore, on the basis of recognizing the structural characteristics of the organic rich shale by multi-scale structure description, researching the destroying of structure in different scale and changing of mechanical properties after lye effected, is new idea for researching the borehole instability mechanism, can provide a new method for borehole instability controlling.This thesis has chosen the Longmaxi group and Niutitang group outcrop shale for study, described the minerals composition, characteristics of porosity and permeability, structure characteristics from clay aggregate scale to layer scale of organic rich shale via modern core analytic techniques. Studies have shown that organic rich shale reservoir is low porosity and low permeability, quartz and non-swelling clay minerals are in a dominant positon, mainly nanoscale pores which poor connectivity. Shale contains a great number of micro-fractures and macro-fractures, and in poor structural stability.Base on clay, clay aggregate and core plug scale, this thesis has studied the impact of organic rich shale electric double layer repulsive force, hydration swelling and pores and fractures structure after effected. The study pointed out that with the pH value increasing, electric double layer repulsive force increasing, and reaches the maximum value when pH=12. When pH=11 or 12, the swelling ratio reaches the maximum value, and can increases 20%～75% as relative to distilled water. Lye promote shale become easy to swell, fractures easy to propagate, structure easy to destroy rock.Have experimental studied the law of lye corroding single mineral and organic rich shale. and the shale structure destroying and mechanical properties changing by alkaline corroded. Studies have shown that lye can corrodes clay minerals strongly, quartz also can be corroded when its grain size equal to clay minerals. Alkaline corrosion will causes a great number dissolved pores be produced, fractures expanded, compressive strength, cohesion and friction angle decreased significantly.Have experimental studied the effects of lye on self-absorption capacity of organic rich shale, analysed the dynamic variation rule of fracture width and the effects of lost circulation with computer simulation, theoretical calculated the changing of horizontal well formation collapse pressure before and after alkaline corrosion. The study reveals that the lye significantly improved the self-absorption capacity and reduced the rock strength of organic rich shale, and alkaline corrosion increased the increment of fracture dynamic width, pushed up the horizontal section formation collapse pressure.Synthesizing the experiment result and shale gas horizontal drilling engineering practice, have established organic rich shale borehole instability mode when high pH drilling fluid drilling, confirmed alkaline corrosion is one of the important causes of borehole instability. This thesis has gave one advice that on the premise of keeping drilling fluid density, good rheological properties and physical and chemical activity of additives, it is desirable to lower the pH value to alkalescent, strengthen the fracture sealing ability to maintain shale borehole stability.