Factors Controlling Shale Gas Production: Geological Perspective

Abstract

Abstract Trapped gas within the matrix and fractures of source rock shales is termed as "shale gas." Geologically, the shale-gas-bearing formations are fine-grained, organic-matter-rich (0.5 to 25%), porous, but impervious sedimentary strata with natural gas trapped in the pores, natural fractures, and/or adsorbed onto the clay surface. The primary factors related to the geology and geochemistry of the formation that govern shale gas production potential would be porosity, permeability, natural fracturing, their original hydrocarbon-generating potential, and the amount of gas still present in the formation. Hydrocarbon-generating potential includes the amount of organic matter originally deposited or the total organic carbon content (TOC), types of source organic matter or kerogen type, thermal maturity and hydrocarbon-generating capacity or gas yield, and the extent of conversion of source organic matter to hydrocarbon. These factors again depend largely on the availability of organic matter, depositional environments, depth of burial, local geothermal gradient, and degree of metamorphism. Hydrocarbon-producing potential further depends on the network of natural fractures, microporosity, adsorption, etc. The impervious nature of shale gas formations requires extreme natural or artificial fracturing (fracture stimulation) to recover commercial quantities of gas. Hydraulic fracturing and horizontal drilling are successful techniques recently used for commercial shale gas production, which depend on geomechanical properties, such as mineralogy and brittleness/ductility of the formation. However, selection of drilling and stimulation methodology requires knowledge of other geological parameters, such as the arrangement of the bedding planes, stratigraphy, natural fracture porosity and intensity, clay content, shale-water absorption, fluid-water sensitivity, shale capillary, shale fractal pattern, shale hydration, gas-shale fracture conductivity, relation of geological features of the formation with regional geological settings, and spacial and temporal variation of reservoir parameters. These parameters govern the direction of fracture propagation, gas recovery rate, and wellbore stability of the drilled well, and also aid in determining base-fluid salinity and fluid-type selection for a fracturing treatment. The aim of this paper is to elucidate the geological processes related to shale gas formations for the development of the proper technology to aid hydrocarbon recovery.