Shale and tight gas is a form of natural gas found deep underground (between 2 – 5km below the surface) held in shale rock formations or tight gas sands – often known as the ‘source’ rock. Shale is a very fine-grained sedimentary rock that acts as both the source and reservoir for the natural gas. Shale gas deposits are generally dispersed over large areas and require well stimulation or other extraction or conversion technology in order to extract the resource.
Shale gas deposits were originally ignored as producers sought larger reserves with a higher resource concentration in order to maximise their investment returns. Throughout the 1980s, shale appraisal commenced but it wasn’t until the following two decades, through significant technological developments and improvements in the cost effectiveness of drilling, well stimulation and conversion, that shale gas become an increasingly viable and important source of natural gas.
The most critical requirement for economically exploiting shale gas reservoirs is the accurate determination of ‘gas in place’. Shale has low permeability, so gas production in commercial quantities requires fractures to provide permeability. Shale gas has been produced for years from shales with natural fractures. However, the shale gas boom in recent years has been due to the introduction of modern technology in hydraulic fracture stimulation to create artificial openings around the well bores.
While older shale gas wells were typically vertical, more recently wells have been drilled horizontally to run along the target formation and require artificial stimulation to produce. Shale gas tends to cost more to produce than gas from conventional wells due to the expense of hydraulic fracture stimulation and horizontal drilling.