Study On Characteristics Of Che Mo-mechanical Coupling Of Coal Shale And Its Borehole Stability

Instability of borehole is a long-standing worldwide problem in geological drilling, most notably concerned with with the hole wall stability of clay shale. During the process of drilling, most problems of borehole instability(as many as 86%) happen in shale formation because of its co mplicated physical properties. Soon after drilling begins, mud would make contac t with clay shale, which produces an effect of chemo-mechanical coupling, resulting in the decreasing of rock strength and further borehole collapse or hole shrinkage. Many academics at home and abroad have already made long-term studies on wellbore stability in large-diameterpetroleum drilling. Models of borehole stability have been established from aspects of mechanics, chemistry as well as chemo- mechanical coupling and abundant research results have been achieved. However, there are few researches aiming at slim hole stability of shale. Therefore, based on the previous studies, this paper aims at slim hole stability of shale and makes a deep study in its influencing factors of chemo- mechanical coupling through the methods of experimental study, mathematical models building and numerical simulation.Research contents and corresponding conclusions are as follows:(1)This study takes Luoping coal shale as basic research object, micro- fabric characters of coal shale and its characteristics of hydration are we ll-studied through X-ray diffraction test and Scanning Electron Microscope test. X-ray diffraction test shows that the clay minerals contentof Luoping coal shale exceed 50%, among which illite- montmorillonite mixed- layer occupies a fairly large proportion.Montmorillonite of clay minerals is the microcosmic cause of the swelling of the shale whilethe high-contentillite- montmorillonite mixed-layer possesses less ionic bonds and characteristics of heterogeneity hydration, which may rocks produce inhomogeneous expansion and further result in decreasing of rock structural strength. SEM test shows Plenty of microcracks on the surface of rock samples and damaged cements between cracks. Those filters are mainly flaky illite and fold lamellar montmorillonite layers with developed pores. Besides, fine-grained and laminated clay particles of rock samples mix with each other, which form the scaly and zonal distribution characters. This structure character sharply increases contact areas between clay particles and water molecules, enlarging hydration area of rocks, which decreases hydrogen bonds and Van der Waals forces and increases the lubrication between cracks, further decreasing the rock strength.(2)soaking through shale and shale mechanics experiments, the aging law shale strength is affected by hydration. After soaking the apparent core visible cracks along the crack invade liquid shale. Deep cracks under the action of capillary forces weaken the rock molecular force, while the cracks lubrication, causing rock cohesion and internal friction is reduced. Soaking solution for crack shale weak surface erosion and gum erosion, goes through the internal micro-cracks and the main crack, The intrusive liquid lowers the intensity of the surrounding rock of the weak cell wall structure, resulting in the wall collapse in drilling holes. In uni-axial compression experiments shales with cracks are likely to split along the worst fracture, but overall or shearing fracture seldom happens. When there are multiple groups of cracks in the core of shale, the worst cracks control the damaging mode of the core of shale. With the extension of immersion time, rock compressive strength and elastic modulus have been reduced, the compressive strength of rock affected by the soaking time is particularly evident.(3)Though the further study of the calculation model of chemo-mechanical coupling,the experiment method of model parameter and the theoretical thought of chemo-mechanical coupling, the mud system suitable for the stability of shale ho le wall was designed as follows: 1. Aiming at the complete stratum, the polypropylene amine system with the properties of broad spectrum and anti-sloughing was designed; 2. the polysulfide drilling fluid system through physical filling, plugging hole wall cracks and preventing mud infiltration at the greatest extent was designed; 3. Potassium lime anti-sloughing mud system was designed, based on the chemo-mechanical coupling theory.(4)An indoor evaluation method of the mud system of shale hole wall stability was designed, according to the theory of chemo-mechanical coupling, which includes the tests of density(ρ), funnel viscosity(FV), initial gel(G1), the final shear(G10), plastic viscosity(PV), dynamic shear(YP), water loss(FL), filter cake(K), acidity(pH) and other conventional mud performance, and the chemo- mechanical coupling strength test, sample recovery test, inhibition of swelling test and other anti-sloughing evaluation experiment. Experiments prove that the mud system can effectively achieve the performance parameters of anti-sloughing, which was designed by the guiding thought of chemo- mechanical coupling and anti-sloughing.(5)Evaluation of borehole stability under chemo- mechanical couplingin shale was carried out by using numerical simulation of FLAC3 D software. It turned out that: The mud system parameters Designed under the guidance of anti-sloughing through what used in chemo-mechanical coupling in the model played an effective role of borehole stability.(6)Mud system compoundedon the evaluation method of borehole stabilitybased on chemo-mechanical coupling acquires a good production effect when It is applied to the geological drilling on Luoping certain diggings.Annual total footage reaches 9000 m and the depth of biggest hole is 1200 m. 1#mud system is cheaper and have simply technology, it is more suitable for drilling in stable stratum; 2# mud system’s function mainly is plugging the hole, suitable for collapsed ground with weak water sensitivity caused by micro- fracture. 3# mud system reduces the hydraulic potential difference, chemistry potential and electricity potential difference between mud and fluid in shale stratum pore, and improves the interaction with shale and stratum fluid, and keeps borehole stability.