Chapter 8 - Hydraulic Fracking of Coal

Abstract

Hydraulic fracking is by far the most-used technique for coalbed methane production today. Under ideal circumstances it yields good gas production. The process involves drilling into the coal seam and casing the gas well; minifrac of the coal to determine its hydrofracking parameters; and finally pumping a calculated quantity of a fluid (water, gelled water, or cross-linked gel) mixed with sand to create a vertical, 500–1000ft long, bilateral fracture that produces gas. Minifrac is a small-scale version of hydrofracking that yields important information, such as instantaneous shut-in pressure, frac gradient, reservoir (pore) pressure, permeability, and fracture extension pressure. Theoretical calculations of the length, width, and height of fracture as well as its direction are done to estimate the fracture volume and efficiency. Three different models of fracture growth are discussed: (1) the Perkins and Kern model for water fracture of coal, (2) the Geertsma and deKlerck model for a cross-linked gel (highly viscous) fluid fracture, and (3) a radial model for horizontal fractures. Fracture designs for three cases are presented: (1) water fracture of coal at medium depth, (2) N2-foam fracture of coal at medium depth, and (3) slick water fracture of deep and thick coal seams that require a high rate of fluid flow (approaching 100bpm). Finally, fracture pressure analysis is presented. It generally has four modes (only three in coal seams). Mode I with a slope of ⅛ to ¼ indicates unrestricted linear growth with restricted height. Mode II is flat and indicates fracture extension with moderate height growth. Mode III with a steep slope of 45–63 degrees indicates restricted extension, mainly due to the dissipation of fluid (leak-off) or net pressure that creates the fracture.