Unconventional resources are stored underground in impervious rock formations such as coal seams, sandstones and shale. As mentioned earlier, three unconventional resources include shale gas, which is stored in fine-grained sedimentary rocks; coalbed methane (CBM), also known as coalbed methane (CSG), which is extracted from coal deposits; tight gas stored in impermeable rock structures.
Shale gas is tightly sealed in tight spaces in the reservoir rock, requiring advanced technology that can drill into and stimulate (frac) the gas storage area. Fractures in the reservoir are the key to ensuring the smooth flow of natural gas into the well. Once stimulated, shale gas reservoirs produce gas like conventional gas wells. The application of these advanced technologies has greatly increased shale gas production, especially in the United States.
We have found that global reserves of unconventional natural gas are enormous, roughly equivalent to conventional natural gas reserves (Rogner 1997). While unconventional natural gas reserves are abundant, most of these resources are difficult to extract commercially or are too costly. This is why all global oil and gas reserves assessments focus on conventional gas reserves (NPC2007). However, two developments have shifted the focus to unconventional gas reserves: rapidly growing world energy demand drives up oil and gas prices, and healthy and steady growth in unconventional gas production has contributed to a decline in U.S. natural gas prices and forecasts for continued declines over the next few years (ElA-IEO 2011). Assessing the total amount of recoverable shale gas globally is subject to multiple and large uncertainties. Even in North America where shale gas is commonly produced, estimates of total and recoverable resources are subject to considerable uncertainty and many variables. While many regions of the world are rich in resources, some regions are not even included in the assessment. Given the inexperience of production in most parts of the world, coupled with the enormous uncertainty, current resource assessments must be extra cautious.
The current global technically recoverable reserves assessment points out that the reserves of shale gas are slightly more than 200 trillion m3 (7064 trillion ft3), the reserves of tight gas are 45 trillion m2 (1589 trillion ft3), and the reserves of coalbed methane are 25 trillion m3 (883 trillion ft3). In contrast, the assessment also pointed out that the global technically recoverable conventional natural gas resource reserves are 425 trillion m3 (15010 trillion ft3), of which 190 trillion m3 (6710 trillion ft3) are proved reserves (Pearson et al., EU, 2012). But there are still “upper, optimum and lower bounds” assessments for shale gas. For example, in the United States, the upper/lower/lower limit assessment is 47/20/13 trillion m3, and in China, the assessment is 40/21/16 trillion m3. That is, in the United States, the upper and lower bound assessment values are 230% and 64% of the best assessment value, respectively. Uncertainty may be more pronounced in assessments of other parts of the world. Among the many causes of variability and uncertainty in assessments, the main ones are the use of different methods, ambiguous terminology, obtaining high recovery factors, or the use of regression curve analysis (Pearson, EU, 2012) in assessments derived from production experience. It is estimated that among the recoverable natural gas resources in the United States, the total potential natural gas resources in shale is between 10 trillion and 25 trillion m3 (353 trillion ft3 to 883 trillion ft3) (ElA2011), , while the country’s proven natural gas reserves are 7.7 trillion m3 (978 trillion ft3) (BP2011). U.S. domestic natural gas reserves have grown 40 percent since 2006, with most of that growth coming from unconventional natural gas resources previously considered unrecoverable. New advanced exploration, drilling and completion technologies provide more efficient and lower cost paths to unconventional natural gas production. In 1996, the United States had proven unconventional natural gas reserves of 48 trillion ft3 and undeveloped resource base reserves of 366 trillion ft3; by 2008, these figures had reached 140 trillion ft3 and 917 trillion ft3 respectively (Kuuskraa 2009).
In April 2011, a study sponsored by the U.S. Energy Information Administration assessed 48 shale gas basins in 32 countries, including nearly 70 subsurface structures containing shale gas. The assessment selected countries with the most promising near-term shale gas resources and basins with sufficient geological data for resource analysis, including the most promising shale gas reserves. The study’s preliminary report indicated that there are 5,760 trillion ft3 in technically recoverable shale gas resources in these countries, plus 862 trillion ft3 in the United States, for a total of 6,622 trillion ft3. (EIA2011a, 2011b). The total amount of resources in the table is 5,760 trillion. If the 862 trillion in the United States is included, the total is 6,622 trillion. To take a more objective look at these shale gas reserve estimates, we can compare the world’s proven natural gas reserves9 released on January 1, 2010, with a value of approximately 6,609 trillion ft3 gigabytes. In addition, in addition to shale gas, the global technically recoverable natural gas reserves are roughly 16,000 trillion ft3. As a result, adding the identified reserves of shale gas and other natural gas, the global technically recoverable natural gas reserves will increase by more than 40% to a total of 22,600 trillion ft3.
As many countries in other parts of the world are just beginning to understand how to assess recoverable shale gas resources, there is considerable uncertainty in their assessment of shale gas resources. Furthermore, the study excluded several categories of potential shale gas resources, such as: ①In addition to the 32 countries included in the research including Russia and the Middle East, Central Asia, Southeast Asia and Central Africa are also rich in conventional natural gas resources; ② Due to various reasons, some shale basins in candidate countries could not be evaluated; ③ Offshore resources. This makes this assessment more conservative. Not only that, but the potential coalbed methane, tight gas and other natural gas resources in these countries were not included in the assessment. Of the 32 countries included in the EIA-sponsored study, two groups of countries have the most dramatic shale gas development. The first group includes France, Poland, Turkey, Ukraine, South Africa, Morocco and Chile, which currently rely heavily on natural gas imports but have large domestic shale gas resources; another group includes the United States, Canada, Mexico, China, Australia, Libya, Algeria, Argentina and Brazil, which also have large shale gas reserves and are already capable of producing significant quantities of natural gas.
Read more: Global natural gas production and forecast
