The impact of the coal macrolithotype on reservoir productivity, hydraulic fracture initiation and propagation

Abstract

Macrolithotypes control the pore-fracture distribution heterogeneity in coal, which impacts stimulation via hydrofracturing and coalbed methane production in the reservoir. Here, the aspects of coalbed methane production that are impacted by macrolithotype were determined from production data from 56 wells in the Hancheng Block, China. The macrolithotype contribution had a significant influence on the well productivity, with dull lithotype-rich (dull and semi-dull coal) coal having significantly lower productivity than wells with bright lithotype-rich (bright and semi-bright coal) coal. The main reason for this difference is that macrolithotypes impart a fracture distribution, which further impacts the hydrofracture stimulation and subsequent coalbed methane production. Here, the hydraulic fracture was evaluated using the appropriate mechanical behavior for each macrolithotype with PFC2D software, and the impact of the macrolithotype on hydraulic fracture initiation and propagation, which are controlled by stress and mechanical properties, was investigated systematically. The result showed that the hydraulic fracture propagation types are controlled by the pre-existing natural fractures. Furthermore, due to the well-developed natural fracture network, the hydraulic fractures in bright coal were mainly composed of the opening, steering propagation, and compound types, forming a lattice-like fracture network. Dull coal is denser, and its hydraulic fractures are characterized as an isolated fracture distribution. The conclusions were verified through monitoring of hydraulic fracture by the microseism wells which show that the fracture in the bright coal is more likely to match the “complex fracture network” description, and the “simple” case when the dull coal is fractured. Meanwhile, the hydraulic fractures mainly open at a shallow depth, but with the increase in depth, the hydraulic fractures are dominated by the crossing type. If the hydraulic fractures are the opening type, only at a higher liquid pressure can the natural fractures in dull coal be initiated. Thus, the difference between hydraulic fractures in bright and dull coal lead to different flow paths and ultimately result in a variation of coalbed methane production. Therefore, during coalbed methane output, a bright coal reservoir will have more potential than a dull coal reservoir, which provides a scientific base for coalbed methane exploration and development.