Natural gas of the same quality is often found in different underground locations. Traditional resources are often concentrated in discrete subsurface accumulation zones (reservoirs) with permeability values above a certain minimum value. Such natural gas is obtained from the geological structure through the expansion process. Natural gas stored in high-pressure reservoirs expands in drilled wells in a controlled manner for capture, processing and transport to the surface. This expansion process helps improve recovery from conventional, high-quality natural gas reservoirs. For example, when the average pressure of a natural gas reservoir drops from an initial 30 MPa to 6 MPa during the life of the gas field, approximately 80% of the natural gas originally formed in the reservoir will be recovered.
In contrast, unconventional resources are stored in accumulation zones with very low permeability values. These accumulation zones include compact sandstone structures, coal seams [coalbed methane (CBM)], and shale structures. Unconventional reservoirs are more continuous and often require advanced technologies such as horizontal wells or artificial stimulation for cost-effective production. In addition, its recovery rate is also lower than that of conventional reservoirs – usually only 15% to 20% of the stored natural gas can be obtained. A comparison of different forms of resources is shown in Figure 1.
The term “reserves” refers to the total amount of natural gas that can be extracted in the future under specified technological and economic conditions. Natural gas reserves can be broken down into the following categories based on characteristics: “Proved Reserves”, “Reserve Growth” (by assessing the long-term future development of known gas fields), and “Unproved Reserves”. Namely, natural gas reserves expected to be discovered in the future through exploration (MIT report, 2011). Different technologies are used to extract different types of natural gas.
Hydraulic fracturing, a common technique for extracting natural gas, has been tested for practicality for 60 years (see Figure 2). The fracturing process occurs after drilling is completed and the steel casing is inserted into the wellhead. After the casing is drilled into the target area where natural gas or oil is stored, the fracturing fluid is injected downhole into the target area through the casing.
Acidizing involves pumping acid (usually hydrochloric acid) into the formation to dissolve the rock in the borehole and allow gas and liquids to flow downhole more smoothly. Numerous studies have shown that 20% to 85% of fracturing fluids remain in the ground. The flow of fracturing fluids from downholes back to the surface is called flowback, and these flowback wastes are usually stored in open pits or ponds at the well site before being disposed of. Ideally, a hydraulic fracturing treatment is designed to form sufficiently long, well-accommodating fractures to achieve maximum productivity. With the aid of horizontal drilling, hydraulic fracturing can make shale gas extraction more economical. Without the help of these technologies, natural gas would not be able to flow out of wells as quickly, and shale gas would not be able to be produced commercially on such a large scale. Between 2000 and 2009, technological advances made possible single-pass multi-stage hydraulic fracturing systems and interlayer isolation, which helped to generate gains in the exploitation of difficult reservoirs.
Another technology, the horizontal drilling method, was originally put into drilling operations in Texas, USA in the 1930s. After further improvement, it became the application standard in the industry since the 1980s. This technology can increase the thickness of the rock that the drill penetrates, obtain the natural gas contained in the deeper formation, and allow a larger amount of natural gas to flow into the well. The horizontal drilling method first uses a vertical well drill to drill into the desired depth from the surface, and then rotates the direction of the drill bit by 90° to drill horizontally into the natural gas reservoir. In the late 1990s, horizontal well drilling capable of multi-stage lateral cracking greatly broadened the application range of horizontal drilling methods. For the first time, horizontal drilling technology has met the requirements of developing shale gas, reducing the number of wells on the surface of urban areas while drilling larger reservoirs (Saurez 2012). This technology greatly reduces the need for production equipment and minimizes the impact of the mining process on the public and the entire ecological environment.