1.1 Background
To better define the term unconventional resources, and to understand why they have been called unconventional, the history of oil should be reviewed.
Petroleum seeps are one of the earliest sources of oil, along with wood and coal, used for heating and illumination. Another source of oil for illumination was whales. Whaling, which is the term that describes hunting of whales, became a competitive industry in the eighteenth century. Factories were built to make ships, and wellāequipped fleets were sent for one purpose and one purpose only: hunting whales. Whale oil was mainly used in oil lamps (Jackson, 1998). The amount of oil from the above two sources was not sufficient enough for the high demand. With the industrial revolution, the need for an alternative source of energy, other than wood and coal, to maintain machinery and for transportation, had never been greater. Coal mining was a very dangerous occupation, and it resulted in a high number of casualties (Natgas, 2013). Hence, in the nineteenth century, oil exploration began, and it has changed the way the world operates in all aspects of life.
People started to dig wells using basic percussion tools, and the rubble was carried out by loading it into baskets and pulling it up to the surface. The drilling process improved a bit by using the cableātool drilling method, which helped drill wells faster. In the early twentieth century, rotary drilling was employed with the ability of cleaning the wellbore while drilling by using fluids. Rotary drilling has changed the whole petroleum industry and, over time, rotary drilling technology has been improved drastically (Hogg, 2016). With the ability to drill wells more conveniently using rotary drilling technology, there still exists a more challenging mission, which is where to drill. To this day, with the advancement in imaging the Earth's strata by using seismic waves, there is still uncertainty about whether hydrocarbon exists in a new area or not, and drilling is the only way to find out for sure.
The global demand for oil surged, and thousands of wells were drilled both onshore and offshore. Keep in mind that this whole process is an investment, and profit is the main motivation. To recover oil faster and without the need of drilling new wells from the start, horizontal drilling and directional drilling were employed. In addition to that, horizontal drilling also helped decrease the drilling footprint, which greatly helped minimizing the environmental impact from the drilling process. With directional drilling technique and the secondary and enhanced oil recovery methods (conventional methods), more hydrocarbons became accessible. However, even with directional drilling and the wellāestablished recovery methods, there is way more hydrocarbons that are trapped in certain reservoirs, and those hydrocarbons cannot be recovered by the conventional methods because of complex petrophysical properties of the reservoirs. Hence, the question now is how to extract the hydrocarbons that are trapped in those reservoirs. Those reservoirs are called unconventional reservoirs, and the hydrocarbons that are trapped in them are called unconventional resources. Thus, in general, the unconventional reservoirs can be defined as the reservoirs that contain a huge quantity of hydrocarbons that cannot be extracted by the conventional methods due to their petrophysical properties, which will be discussed in detail in Chapter 2. The unconventional resources are classified into different cate...
