--> Lithological Assemblage of Coal Measures and Its Influences on Coalbed Methane Development in Shouyang Block, Qinshui Basin, Central China

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Lithological Assemblage of Coal Measures and Its Influences on Coalbed Methane Development in Shouyang Block, Qinshui Basin, Central China

Abstract

In China, it is estimated that the original in-place coalbed methane resources are of 36.81×1012 m3 within a depth of 2000 m. The Qinshui Basin, situated in Central China, is the most mature basin for coalbed methane exploration and development in China, with original in-place resources being of 3.85×1012 m3 in Pennsylvanian-Permian coalbeds. The main targets coals for coalbed methane development are coals #15 and #9 of Pennsylvanian Taiyuan formation, and coal #3 of Permian Shanxi formation. Coalbed methane development needs firstly dewater the target coals for desorbing the gas and hydraulic fracturing is a commonly used technology to stimulate the coalbed reservoirs. A successful hydraulic fracturing requires that strata separating the coals from water-bearing sandstones be of sufficient strength or thickness to constrain fracture-height growth. In Shouyang Block, northern Qinshui Basin, single or multiple stimulations are practiced for all wells with almost the same standard fracturing volume, where the problem emerges, i.e., whether the fracturing volume is appropriate for all wells. Combining well performance data, hydrochemical data and detailed lithological assemblage analysis of 30 coalbed methane wells in Shouyang Block, northern Qinshui Basin, it is concluded that: (1) The aquifers potentially influencing the dewatering process of coalbed methane wells are, sandstones K1 and lenticular sandstones K2s below and above the coal #15 respectively, sandstones K5 and sandstones K7 below and above the coal #9 respectively, and, sandstones K7' above the coal #3; (2) Three wells with extremely high water production (superior to 100 m3/d) are inferred to dewater coals in communication with aquifers through faults, and (3) 18 wells, i.e., 60% of the total 30 wells, are considered to be appropriately fractured, but 9 wells, i.e., 30% of the total 30 wells, are considered to be overly fractured. Water production diagnostic of coalbed methane wells at the early dewatering stage provides a way to identify the aquifers influencing the production performance of wells and to diagnose the appropriateness of the fracturing volume. It is suggested from this study that: (1) The precautionary selection of well locations/coalbeds should be based on faults interpretation; (2) Fracturing volume design should be based on the lithological assemblage of target coals.