Hydration Of The Fractured Shale Stability Research And Its Applications

Shale instability while drilling is a grand problem of hindering the development of drilling technology. Since C.H.Chenevert brought up the conception of shale stability coupling with mechanics and chemistry in the late of 1980s, the research about this issue entered into a stage of quantitative math model description from the stage of qualitative experiments. In recent years, the quantitative analysis theory of shale stability based on continuous medium (no fractures and micro-fractures) has taken the great development.But with brittle shale that widely exist in the world, fractures and micro-fractures are ubiquitous, its effect on shale stability cannot be neglected. On the base of the continuous medium quantitative analysis, this paper mainly focused on the treatment of the effects of fractures and capillary forces on shale hydration stability, deepened the correlative theory of quantitative analysis, and finished a great deal experimental study, discussed potential application of the correlative technology.The main matters studied in this paper and correlative results are follows:1. Deepened the theory of the continuous medium quantitative analysis. Based on conduct of energy and material, absorption and diffuse of water molecule and hydration ion in porous medium, electric field and repulsion of electric double layer, this paper analyzed the rules of water and components of mud filtrate transport and move in shale formation, the essence of hydrated stress and the rule of shale strength vary with the degree of hydration. Finally, stayed to stress, strain tensor and strength criteria, developed the analytical model about shale hydration.2. Took into account fractures and micro-fractures based on continuous medium, developed fracture-pore dual medium analytical model about shale hydration stability. This paper gave emphasis to discuss the process drilling fluid dash in deep part of shale along fractures and micro-fractures driven by pressure potential and adsorption potential, and the deep part hydration phenomena along facture face.3. Studied pressure penetration phenomena and its effect under the action of positive differential pressure in shale formation, capillary adsorption and its effect when negative differential pressure exist between hydraulic column pressure and pore pressure. The results show:(1) Shale considered non-permeable traditionally is micro-permeable in fact, positive differential pressure will bring "pressure penetration" and "filtrate push-in". This is essential cause of shale instability under the action of heavy mud.(2) Capillary absorption of water-based drilling fluid under the action of negative differential pressure is essential cause of shale hydration instability while under-balanced drilling, especial for "dry formation" saturated by gas.4. On the base of theoretical study of shale hydration, by the conditional simulation and similarity principle, a suite of experimental apparatus for shale hydration study was designed and set up, and the corresponding experiments (pressure transmission tests, end surface sopped tests, soaking tests and so on) was finished. The results of these experiments show that the theoretical model developed in this study is valid and reliable.5. By theoretical and experimental study, four basic measures were presented for stabilizing shale: increasing mud density, chemical constraint, physical blockage and wetting reversion. The four basic measures can be quantitative analyzed and predicted by core experiments and model simulation proposed in this paper. The correlative technology will play an important role in following fields:(1) Reducing the loss caused by shale instability while conventional mud drilling. When drilling in new developed area, the traditional method of searching the technical proposal to stabilize shale is "try and err". Before enough well be tried, the result can't be found, and the result found finally possibly isn't best proposal. But by the technology developed in this paper, the best technical proposal may be found after finished tw...