Brittle Shale Hydration And Its Effects On The Stability Of The Wall

Wellbore instability problems will appear when drilling in shale formation. This paper evaluates the hydration process of hard brittle shale, studies the law of the hydration on borehole stability with the lab experiment and numerical simulation theoretically. Reference for drilling design and theoretical basis to drill safely can be provided in hard brittle shale formation with the study of this paper. All the research is based on the Xujiahe Formation in western Sichuan area. Main content includes:(1) Basic characteristic analysis on the mudstone is researched with the instrument in lab: measurement of clay mineral composition and component content, structure performance of the clay and clay plane. Physical and chemical properties of mudstone are analyzed with experiments to acquire basic parameters of the mudstone. Characteristics of the porosity and permeability of hard brittle shale are studies with electron microscope; micro-pore is the main pore type in the shale formation.(2) Water invaded models including base block and fissure water are built. Solution flow physical model in the mudstone matrix cracks is built with the COMSOL software. Permeability in fractures is far greater than in matrix and fracture is widely seen. The invasion near wellbore area was mainly the result of micro cracks in hard brittle shale.(3) With the aid of shale hydration expansion strain experiment device developed independently, largest shale hydration expansion is found in clean water. The greater the mineral ion concentration is, the weakest the hydration expansion is and the weakest the influence on the rock strength is. Sodium and potassium ions can inhibit the hydration of shale, but potassium ion makes a greater influence.(4) Micro unit and microstructure of hard brittle shale are observed with the scanning electron microscope and a stereoscopic microscope. Water will flow into rock deeply along the micro fracture in hard brittle shale with different ions in aqueous solution, which accelerates the shale hydration and dispersion. Micro cracks will further expand and extend, becoming big, main cracks.(5) Assessment results of hydration on borehole stability are reached with the simulation. Water intrudes into formation along the fissure network, leading to the pore pressure rising up in the area near the wellbore. Besides, formation collapse occurs increasingly. Exemplified is done finally.Theoretical basis and technical support during the drilling with hard brittle shale of Xujiahe Formation in western Sichuan area can be provided with the study of this paper.