A New Tree-Type Fracturing Method for Stimulating Coal Seam Gas Reservoirs

Abstract

Hydraulic fracturing is used widely to stimulate coalbed methane production in coal mines. However, some factors associated with conventional hydraulic fracturing, such as the simple morphology of the fractures it generates and inhomogeneous stress relief, limit its scope of application in coal mines. These problems mean that gas extraction efficiency is low. Conventional fracturing may leave hidden pockets of gas, which will be safety hazards for subsequent coal mining operations. Based on a new drilling technique applicable to drilling boreholes in coal seams, this paper proposes a tree-type fracturing technique for stimulating reservoir volumes. Tree-type fracturing simulation experiments using a large-scale triaxial testing apparatus were conducted in the laboratory. In contrast to the single hole drilled for conventional hydraulic fracturing, the tree-type sub-boreholes induce radial and tangential fractures that form complex fracture networks. These fracture networks can eliminate the "blank area" that may host dangerous gas pockets. Gas seepage in tree-type fractures was analyzed, and gas seepage tests after tree-type fracturing showed that permeability was greatly enhanced. The equipment developed for tree-type fracturing was tested in the Fengchun underground coal mine in China. After implementing tree-type fracturing, the gas extraction rate was around 2.3 times greater than that for traditional fracturing, and the extraction rate remained high for a long time during a 30-day test. This shortened the gas drainage time and improved gas extraction efficiency.

Investigation on Coal Fragmentation by High-Velocity Water Jet in Drilling: Size Distributions and Fractal Characteristics

“…Figure 12 illustrates the relationship between the relative mass and relative fragment size of coal fragments. The fractal characteristics of fragments for various impact velocities and coal mass strengths are revealed, and the fractal dimensions can be obtained according to Equation (7). It can be seen that the size distributions of coal fragments have the fractal characteristic.…”

“…Therefore, the effective extraction of CBM is not only of considerable significance for meeting the global energy demand for sustainable development, but also necessary for improving coal mine safety and reducing greenhouse gas emissions [2,4,5]. Unfortunately, CBM reservoirs in China are characterized by low saturation, low permeability, low reservoir pressure, and relatively high metamorphic grade, resulting in a low CBM production rate [6][7][8]. Thus, some measures must be taken to improve the permeability of coal seams by increasing the number of cracks and amount of gas desorption in coal.…”

Structural Optimization of Slotting Nozzle Used to Improve Coal-Seam Permeability

“…In 2018, China became the world's largest importer of natural gas, with an import volume of 125.4 billion m 3 [2][3][4]. CBM occurrence conditions in China are complex, mainly due to three factors: (i) low gas saturation, with an average gas content of 45%; (ii) low reservoir pressure, with a pressure coefficient of generally less than 0.8; and (iii) high degree of coal-seam metamorphism [5]. Therefore, CBM is mainly exploited by underground extraction in China.…”

Uncertainty Analysis of Factors Influencing Stimulated Fracture Volume in Layered Formation

“…On the basis of models, such as Khristianovich-Geertsma-DeKlerk (KGD) model [2], Perkins-Kern-Nordgren (PKN) model [3], penny-shaped fracture model [4], pseudo 3D model [5] and fully 3D hydraulic fracture model [6], the numerical simulation of hydraulic fracturing technology has developed vigorously. Recent researches have expanded the complexity of hydraulic fracturing modeling and improved the mathematical model in simulation in three folds, including: (1) three-dimensional fractures developed near the wellbore [7][8][9]; (2) the interaction between a hydraulic fracture (HF) and natural fracture (NF) [10][11][12]; (3) the stimulated reservoir volume (SRV) of fractured formations [13][14][15][16].…”