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Classification of treating pressures in coal fracturing
Published in W.A. Hustrulid, G.A. Johnson, Rock Mechanics Contributions and Challenges: Proceedings of the 31st U.S. Symposium, 2020
R.H. Morales, J.D. McLennan, A.H. Jones, R.A. Schraufnagel
Coal formations are characterized by a natural microfracture network of face and butt cleats, usually oriented parallel to the maximum and minimum principal in-situ stresses, respectively. Bedding planes are well defined and are generally horizontal. Young’s modulus and fracture toughness values in these friable materials tend to be an order of magnitude smaller than for sandstones. As the mechanical properties of coal differ from most conventional reservoirs, new approaches to stimulation design and fracturing analysis are required. As a first approach to this, the following sections use the pressure decline behavior, after shut-in following hydraulic fracture treatments, to classify fractures into generic treatment categories. Pressure data from wells in the Black Warrior Basin (Alabama), tabulated in the COMPAS (Coalbed Methane Production and Stimulation) Database (Bell, et al, 1987), were used for this purpose. For the reservoirs considered, the minimum horizontal stress gradients (0.70 to 0.85 psi/ft), obtained from in-situ stress measurement (Jones & Gajewsky, 1988), are smaller than the overburden gradient (0.95 to 1.1 psi/ft). Fracture mechanics theory predicts that fluid treatment pressures slightly higher than the minimum in-situ stress are sufficient to propagate a fracture. On this basis, fluid treating pressure gradients greater than 1 psi/ft are considered abnormally high for vertical fractures. However, for the Black Warrior Basin and elsewhere (Palmer, et al, 1989), such high pressures appear to be observed in greater number of treatments. Numerous fracturing mechanisms have been hypothesized to explain these high pressures. These are: tip plugging/fracture blocking (Jones, et al, 1989), parallel fractures (Jeffrey, et al, 1989), increase of fracture toughness (Shlypobersky & Wong, 1988), complex fracture branches interacting with the natural fracture network (Palmer, et al, 1989), poroelasticity (Palmer, et al, 1989; Jeffrey, et al, 1989), tip plasticity, etc. However, controversy remains in distinguishing, with certainty, the actual mechanisms active under field conditions. Nolte (1979), have demonstrated that important parameters quantifying a fracture and a fracturing process can be estimated from evaluation of excess pressure during a treatment and by pressure decline analysis following fluid injection. In this paper, the pressure decline responses associated with specific fracturing mechanisms (such as tip plugging and fracture blockage) are determined and used to classify fractures.
Optimal selection of coal seam pressure-relief gas extraction technologies: a typical case of the Panyi Coal Mine, Huainan coalfield, China
Published in Energy Sources, Part A: Recovery, Utilization, and Environmental Effects, 2022
Zheng Shang, Haifeng Wang, Yuanping Cheng, Bing Li, Jun Dong, Qingquan Liu
The method of mining a protective coal seam and performing methane pre-drainage in a pressure-relief coal seam can effectively eliminate the danger from gas outburst (Cheng, Yu, and Yuan 2004; Karacan et al. 2011). Most of the coal seams in China influenced by geological tectonic movement are considered to have developed into complex and soft structures (Cheng and Yu 2007). As a result, the permeability of the coal seams which is not more than 0.1 × 10−3μm2 (the lowest in the Beipiao coal field is 0.7 × 10−4μm2; the highest in the Fushun coal field is only 1.8 × 10−3μm2), are significantly lower than those in the San Juan and Black Warrior basin of the USA (Hu, Jiang, and Su 2000). Consequently, protective seam mining is the most effective method for achieving the safe and economic exploitation of an outburst coal seam (Dong et al. 2015). The coal seam located above the protective coal seam is called the upper protective coal seam, and the lower protective coal seam is located below the protected layer (Liu et al. 2010). Vertical cracks and delamination cracks in the protected layer will significantly increase the permeability of the overlying coal strata (Figure 1) (Karacan et al. 2006). In recent years, the main method for performing pressure-relief gas extraction in China is surface well drilling and net-like penetrating boreholes (NPB) extraction and depends on the different coal seams, gas occurrence, and geological conditions long-term exploration (Jin et al. 2016; Kong et al. 2014; Liu et al. 2011; Yuan 2013c).
Numerical analysis of drainage rate in multi-layer coalbed methane development in Western Guizhou, Southern China
Published in Petroleum Science and Technology, 2023
Yong Shu, Shuxun Sang, Xiaozhi Zhou, Fuping Zhao
Globally, coal seams in a considerable number of coal-rich areas are vertically distributed in multiple layers, such as Powder River Basin, San Juan Basin, Black Warrior Basin in the United States, Horseshoe Canyon in Canada, Sura Basin in Australia, and Western Guizhou-Eastern Yunnan in China (Clarkson 2009; Towler et al. 2016; Yang et al. 2019a; Zhao et al. 2019). Guizhou Province is the main coal-rich area in South China, with CBM resources of 3.15 × 1012m3, ranking third in China (Yang et al. 2019a). As an important measure to improve the productivity of CBM wells, multi-layer CBM development is an inevitable choice to realize the efficient development of CBM in Western Guizhou. However, the production of some multi-layer CBM development wells in Western Guizhou is not significantly higher than that of single coal seam development wells, and is even lower, indicating that some coal seams are hardly helpful to the production of some multi-layer CBM development wells, which seriously violates the original intention of multi-layer CBM development (Zhao et al. 2019). Therefore, under the same drainage rate, not only the time of dewatering and gas production between coal seams, but also the stress-sensitive damage, velocity-sensitive damage, and Jamin damage of the reservoir permeability are also different, resulting in disharmonic gas and water production between coal seams, that is, known as interlayer interference or wellbore interference, which is very likely to limit the production capacity of some coal seams (Yang et al. 2015; Zhao et al. 2019). Previous studies have shown that by optimizing the drainage rate, interlayer interference can be alleviated to improve the production of multi-layer CBM development wells (Yang et al. 2019a). Therefore, optimizing the drainage rate is a significant issue for multi-layer CBM development in Western Guizhou.