Chapter 5 - Pore Pressure and Stress Field in Coal Reservoirs

Abstract

Every single point in a CBM reservoir is exposed to four different stresses, namely, the reservoir (pore) pressure, the vertical stress, the major horizontal stress, and the minor horizontal stress. The magnitude of these stresses has a great impact on the selection of the production technique and its success or failure. The pore pressure in most coal seams in the world is about 70% of the hydrostatic head or 0.33psi/ft of depth. There are some exceptions where the coal seam has a reservoir pressure that is 1–1.2 times the hydrostatic head. Such fields are highly productive. The vertical stress is typically 1.1 × depth, where depth is in feet and pressure is in psi. Horizontal stresses are best derived from equations created from massive data banks collected around the world by research organizations and the world stress map (WSM). Horizontal stress in the rocks containing coal to a depth of 10,000ft is created by “plate tectonics.” The impact of the stress field on production techniques (vertical wells with hydrofracking and horizontal wells drilled from surface) was discussed for major coal fields around the world including Western United States, Eastern United States, Western Europe, Eastern Europe, India, Australia, and South Africa. Estimation of elastic modulus and poisson ratio of coal seams by sonic logging is discussed. Finally, derivation of bulk modulus, shear modulus, and relationship between various elastic properties is presented. Elastic modulus and shear modulus are needed to design a successful hydrofracturing job.