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124

Introduction to Unconventional Shale Gas System

Byongcheon Yang

․Yunjung Nam․Jaeho Kim Korea National Oil Corporation, [email protected]

Shale is a fine-grained, clastic sedimentary rock composed of mud that is a mix of flakes of clay minerals and tiny fragments of other minerals. It is characterized by breaks along thin laminae or parallel layering or bedding due to clay grains that tend to lie flat. Shales are generally deposited in low-energy depositional environments, where the fine-grained clay particles fall out of suspension in these quiet waters. During the deposition of muddy sediments, there can also be deposition of organic matter in the form of algae-, plant-, and animal-derived organic debris, which generate natural gas during thermal cracking. Due to the very fine sheet-like clay mineral grains and laminated layers, typical unfractured shales have matrix permeabilities on the order of 0.01 to 0.00001 mD, resulting in generated gas that are trapped, and cannot move easily within the rock. Thus, shale is characterized by both reservoir and source for natural gas generated from the organic matter.

Shale gas is natural gas produced from shale. In terms of its chemical composition, shale gas is typically a dry gas primarily composed of methane, but some formations (e.g., Eagle Ford shale) do produce wet gas (also known as ‘condensate’).The gas volumes are significantly stored in a macro-/micro-porosity system, and it can also be adsorbed onto minerals or organic matter within the shale. The potential of a shale gas formation will be evaluated by identifying specific source rock characteristics. For development and production, wells may be drilled either vertically or horizontally with hydraulic stimulation (i.e., fracturing). Due to the commer- cial success of shale plays in the United States, shale gas has been a game changer, and interest has spread to potential gas shales in Canada, Europe, Asia and Australia.

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