Hole wall’s stability is a complex problem which exploration engineering has been working to solve. There are many reasons which may lead to hole wall’s instability. Aiming at hole wall’s stability in coal exploration, this paper takes shale as objects of study and uses the MPC method (for example: analyzing on rock’s microstructure characteristics, physical and mechanical properties, and chemical characteristics) to make a thorough research on unstable coal shale formations in a work area in LuoPing district.
Zou Jiea, Chen Liyia*, Wang Shenga, Zhang Chuana
aChengdu University of Technology, Chengdu,Sichuan, 610000
Geological Engineering Drilling Technology Conference (IGEDTC), New International Convention Exposition Center Chengdu Century City on 23rd-25th May 2014.
The results show that the shale in the area has high content of the mixed-layer of illite and smectite, water intrusion dramatically decreases rock strength and elastic modulus and leads to poor mechanical properties. Considering the coupling effects of mechanical properties as well as the drilling fluid sealing characteristics of drilling fluid must be improved and its osmotic pressure should be reduced.
Hole wall stability is one of the issues which drilling engineers pay most attention to. Related research started from the solution of borehole stability problems in oil drilling. Early, H.M. Westergard, Bradley and other specialists did some researches on the borehole stability by mechanical method, described the wall stress distribution law, and analyzed the borehole sloughing, and mud losses caused by stress. In 1969, Gray, Darley and Chenever [2,3] pointed out, wellbore instability was not just purely mechanical problems, Mud shale hydration could be an important reason. In the early period of 1980sˈa research groups represented by Professor Huang Rongdun and Chen Mian did a systematic research and analysis on the stress of hole rock wall, in-situ stress measurement technique, measuring method of rock strength and the distribution determination & relevant calculate method of Shale hydration stress, based on geological mechanics, porous elastic medium mechanics, rock mechanics and acoustics theory.
Until 90s, the mechanical and chemical factors are coupled for mathematical quantitative research. In 1989,C.H.Yew and M.E. cheneverttook the first step of quantitative research on coupled mechanics and chemistry. They assumed the anisotropy of shale permeability, compared the movement of water-to-mud shale to the thermal diffusion, and then used the thermal diffusion model to simulate the diffusion of adsorbed water, combined with the mass conservation equation, finally obtained the water absorbing capacity equation under cylindrical coordinate system.
Then they associated the mechanical property of mud shale with its total water content, worked out a mathematical model of solving the coupled mechanical & chemical stress, strain and displacement. Huang Rongduntried M.E. Chenevert’s method to conduct a similar analysis, Hale and Mody  did an experimental study on the chemical potential influence on the stability of the shale. In the article the author firstly proposed that shale osmosis is the result of joint action of pressure potential and chemical potential.
They thought the shale and water-based mud surface should have a semi-permeable membrane, however it might not be an ideal one. They proposed a simplified model, which added the osmotic pressure to the solution of the elastic stress distribution model. R. F. T. Lomba, etc. [7-9]applied the coupled flow combination of water & solutes to the wellbore stability calculations, studied the chemical potential efficiency of water in mud shale and drilling fluid on wellbore stability. C. P. Tan etc. believed the total water potential (sum of pore pressure and chemical potential) is the root cause which leads to the water movement. Based on Biotporous elastic theory, Nguyen and Abousleiman discussed the chemical percolation coupling parameters, the physical meaning and the value range of chemical & mechanical coupling parameters, and analyzed the influence of the coupling term on the near borehole wall stress and pore pressure. C.H. Yangtook advantage of the coupling mechanical and chemical research method, proposed the quantitative calculation method of hole collapse cycle. Wang Qian etc.considered the fluid flow and ion transport as well as the combined effects of solid deformation generated by electrochemical potential in shale drilling fluid system, and proposed a new shale wellbore stability coupled model. Through the finite element calculation of mud shale pore pressure field and stress field around borehole wall, analyzing the influence of the mud shale and drilling fluid performance coefficient on the parameters of strata around the borehole wall collapse as well as collapse pressure.
There have been many domestic and foreign related researches on borehole stability in oil drilling, but studies aimed at the hole wall stability of small aperture exploration is relatively less. This article is based on the hole wall stability problem in coal exploration, using the M-P-L method to analyze the microstructure, physical and chemical properties of coal formations’ hole rock, to explore the mechanism of hole wall strata instability.
2. Experimental Methods and Results
2.1. Sample Collection
Luoping coal exploration site, from new to old, Quaternary, Lower Triassic Guanling groups, Yongning town groups, Feixianguan groups, Kayitou groups; Permian Changxing groups, Longtan groups and Emei mountain basalt groups. Samples collected from the Longtan groups coal formations field in ZK1-2 and ZK4-2, the depth is about 700m.The thickness of Longtan groups is from 106 to 300m, mainly consists of gray, sallow siltstone, silty shale, argillaceous siltstone and shale, the top contains calcareous concretions, while the bottom contains aluminum soil shale or basalt conglomerate, has 12 to 13 coal-bearing layers.
Longtan groups is a complicated section of formation, where hole wall collapse and other problems mainly occur. Using XY-6 drilling rig, adopting rope core technique, we took out the core segment. The core segment is mainly made of shale and a small amount of coal rock, and there hasa complex geological structure in the area, where many normal faults and reverse faults intricate.The formation is influenced by the fault dislocation and uplift and the stress extrusion, the gum damage can be seen clearly from the taken-out core segment, which mostly has not yet cemented or is just semi-cemented, and cracks are welldeveloped. Coring methods and coring results shown in Fig. 1 and Fig. 2.
2.2. Sample Analysis of Microscopic Structure
Dividing the collected mudstone samples into 3 groups and using the experimental X-ray diffraction, analyzing the content of whole rock mineral. The Science DMAX-3C diffractometer is used as the experimental equipment. Analysis conditions: Cu target (λCuKα1 = 15.4056nm), Ni filters. Experimental Method: The sample was dried to be formed into pieces, placed under the diffraction into observation. Sample 1#'s X-ray diffraction pattern was shown in Fig. 3.
Fig. 3 X-ray Diffraction Patterns of Sample 1
As can be seen from Fig. 3, the main line of the sample is 3.3446, 3.5160, 7.1499,4.4758, 3.5160, and 3.1923. The rock mineral composition of the whole rock samples can be launched, as shown in Fig. 4. Clay mineral content of three samples is shown in Table. 1. As can be seen from the X-ray diffraction data graph, the total amount of clay minerals is more than 50%, of which the illite mixed layer has a higher proportion.
Table. 1 ZK4-2 Clay Mineral Analysis.
The montmorillonite in clay minerals is the microscopic reason of the clay-shale's swelling, the high levels of the illite mixed layer has few strong ionic bonds, it causes inhomogeneous hydration and uneven swelling degree of rock, and it also reduces the overall structural strength of shale.
Fig. 4 Whole Rock Mineral Content.
The CEC‘s exchanged larger capacity can be further proof that the shale in this stratum is easy to swell, the cell wall is vulnerable to water-sensitive damage.
Drying some rock samples and forming them into slices, then using SEM electron microscope (Hitachi, Japan) to reveal its arrangement structure of clay minerals and cemented manner, we can clearly see the sample microstructure, arrangement and distribution of the mineral components, are shown in Fig. 5 and Fig. 6.
Fig.5 SEM Scan Results.
As we can see from the SEM results in Fig. 5, the rock sample surface has many micro-cracks and pores, and there are glial damage and cemented incompletion phenomenon between the cracks. The filling thing is flaky illite and crapy flaky montmorillonite layer, part of it has porosity development, and the pores have soluble residues.
Because of the fault's uplift and extrusion, it can be observed in figure 6 that there has trace of refilled by mineral clay after some early calcareous filler sheding, and it affect the stratification level, and micro-cracks development. This microscopic structure and layering characteristics make shale internal cracks' stress intensity increase, critical fracture toughness reduce and crack propagation, under external pressure. If the micro-fracture develops or tectonic stress concentrates, it is quite easy to lead to shale cracking, spalling and instability.
On the other hand, in the drilling process of this area, drilling filtrate along the micro-cracks or joints surface to the deep strata intrusive, it will swell and soften quickly, reduce the mechanical its own strength, at the same time, increase hydration and dispersion of the shale, expand shale hydration area, weaken the hydrogen bonds and van der Waals forces and molecules of shale and lubricate the cracks. It caused the shale's reduction of cohesion and internal friction, cements' dissolution, etc. This destroyed the strata's original equilibrium pressure.
Once the drilling fluid filtration get higher, it is easy to cause the hole wall's block off and collapse or other complex situations.
Fig. 6 Rock Sample Stress-strain Diagram.
2.3. Uniaxial Compressive Experiments
The test equipment adopts the MTS-815 testing machine introduced from the United States MTS company. Full device consists of five parts: hosts, Test star controller, hydraulic source, servo control valves and control computers, which has the characteristics of high fuselage frame stiffness, high accuracy, stable control and powerful features. The design indicators: maximum axial compressive static pressure 3000kN, dynamic pressure 2400kN; accommodating rock sample size: diameter 50 ~ 200mm, height 100 ~ 400mm; displacement measuring range (LVDT): ± 50mm; Strain measurement range: ± 0.01mm .The sample was prepared as standard specimens with 50mm diameter and 100mm height, then respectively used these groups of specimens to carry on uniaxial compression test and conventional tri-axial experiments.
The sample was prepared as standard specimens with 50mm diameter and 100mm height. The specimens were divided into five groups based on the soaking time, namely soaking 5h, 10h, 15h, 20h, and a group of specimens which was not soaked, each group has 3specimens.Place each good specimen on the press machine to carry on compression test. The experimental results are shown in Fig. 6.
We can see from the Fig.8 that the cracks appear on core appearance after soaking, and liquid invades into shale along fractures under the action of capillary forces, weaken the rock molecular forces while lubricate cracks, causing the decline of rock cohesion and internal friction. Soaking liquid leads to corrosion and erosion of pectin of the weak faces of the cracks, which breakthrough of the internal micro-cracks and the main cracks, and the drilling fluid invasion makes the rock strength decrease, result in drilling hole wall collapse.
Fig.7 Rock Parameters vs. Time.
Observing in Fig.9, the integral fracturing or shear failure of the shale is nearly occurs in the fracture system of uniaxial compression experiments, it occurs along the most unfavorable crack instead. When the core has multiple groups of cracks, the core damage model is determined by the group of crack which has larger extent of breakthrough. The mechanical testing in Fig.6 and Fig.7 shows that collapsed rock formations has low elastic modulus, relatively high poisson's ration and low compressive strength, therefore the rock is quite brittle, fragile and susceptible to compression, easy to lead to instability under strata horizontal stress.
Fig. 9 Core Damage.
With the elongation of immersed hours in the experiment, rock compressive strength and elastic modulus are successively reduced, especially rock compressive strength.
Shale swells easily, invasion of the drilling fluids filtrate leads to stress intensity of internal cracks increase, critical fracture toughness decrease and crack propagate, which is the root cause of shale intensity decrease. Micro fissures of the coal-bearing strata developed, which contain much more hydrophilic surfaces and have good water- absorption ability, at the same time, drilling fluids filtration to coal rock is inevitable, and the filtrate changes mineral composition of the filler of bedding plane, the shale micro-cracks in coal-bearing strata or coal ingredients that are not completely carbonized may cause the water swelling, or by the action of capillary pressure and pore dissolving of cementation, reduce the friction coefficient of the crack surfaces, exacerbate rock strength decrease, destroy the original balance of the formation pressure, result in loss of the original stability in coal-bearing strata.
Coal and shale often inter bed in the coal-bearing strata. After the collapse of shale under the coal, the coal will lose support and collapse, which is similar to the collapse of the upper shale, creating a vicious cycle. Expansion pressure generated by shale hydration will even increase extrusion effect on the adjacent coal seam, which in turn increases the hole walls stress of the coal rocks, so that the already broken coal rocks peel off or even collapse. Therefore, the stability of shale is closely related to stability of coal, the intercalation of the shale should be considered when taking into account the stability of coal rocks.
This article analyzed the microstructure, physical and chemical properties and uniaxial stress properties of rocks of the coal-bearing strata, based on the hole wall stability problem in coal exploration, and obtains the following conclusion:
- Using the M-P-L method to analyze and study the microstructure, physical and chemical properties of instable shale formation in Luoping working site.
- Shale has high content of clay, the main latent factor of water swelling derives are from the clay content, especially the contact with the drilling fluid and the mixed-layer minerals of illite and smectite, will leads to heterogeneous inflation ,which is an important reason of the rock strength decrease.
- The elastic modulus and the compressive strength of shale rocks in coal-bearing strata are low, which are decreased significantly increased with the effect of drilling fluids. Under the effect of the drilling fluid for a long time, the sharp decline of mechanical stability will result in the hole wall instability.
- The coupling effect of the mechanical properties of shale with drilling fluids should be considered when taking into account the stability of shale hole walk. Improve the sealing of drilling fluids and reduce its osmotic pressure is beneficial to hole wall stabilization.
I’d appreciate it that the 137 Coalfield Geology Team and Ms. Fu Xiaomin of Chengdu University of Technology has provided me with a lot of help and guidance. Many thanks for their great support.
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