laterally to give the wellbores extended contact with the formation. The advantage of horizontal wells is the long and constant exposure to source rocks. For example, a vertical well piercing a 30m-thick layer would only have 30m of exposure to the oil and gas interval, whereas a horizontal well would have several hundreds of exposures.

Figure. The cartoon shows the difference between conventional (right) and unconventional (left) drilling methods. The branches extended on the horizontal well are the representatives of hydraulic fracturing.

https://www.croftsystems.net/oil-gas-blog/conventional-vs.-unconventional

Hydraulic fracturing, commonly known as fracking, further extends the drainage pattern around horizontal wellbores by creating fracture patterns that facilitate flow. Fracking injects a high-pressure ‘fracking fluid’ (primarily water, sand and chemicals) into a wellbore to create cracks in the rock formations which release the oil and gas inside. (https://ec.europa.eu/jrc/en/publication/eur-scientific-and-technical-research-reports/overview-hydraulic-fracturing-and-other-formation-stimulation-technologies-shale-gas-0) This method is now widely used world-wild, ensuring the US and Canada to have constant gas supply for 100 years and has presented an opportunity to generate electricity at half the CO2 emissions of coal. (https://www.bbc.co.uk/news/uk-14432401)

Steam-assisted gravity drainage (SAGD) is another method used to open up large deposits below the surface and produce heavy crude oil and bitumen. It is an advanced form of steam stimulation in which a pair of horizontal wells are drilled into the oil reservoir, one a few meters above the other. High-pressure steam is continuously injected into the upper wellbore to heat the oil and reduce its viscosity, causing the he