Shale gas is defined as natural gas occurring in shale formations. It is an unconventional energy resource, which has become an increasingly important source of natural gas globally and has the potential to grow as a major energy source in the next decade. However, production of shale gas remains technically and economically challenging.
Shale acts both as source rock and reservoir rock. Having high total organic content and falling in the gas window (302°F–392°F), shale has sufficient potential to generate huge amounts of natural gas. Generally, natural gas is stored in a shale matrix, which is highly porous but has very poor permeability. This makes it difficult for gas to move into the wellbore. Gas production in commercial quantities requires the presence of fractures to provide permeability. Some shale formations have natural fractures, which act both as storage space for natural gas and as a transport medium from the matrix interior to the wellbore. The recent boom in shale gas production was made possible by the application of hydraulic fracturing, which creates extensive artificial permeable fracture paths in the shale matrix—acting as highways for natural gas transport to the wellbores.
The storage mechanism of natural gas within the shale matrix is complex. Approximately 15% of the gas produced is held in natural fractures and some in pore spaces. The remaining 85% of the gas is stuck to the shale matrix by a process called adsorption. Wellbore production of adsorbed shale gas is enabled by the process of desorption, which occurs with reservoir pressure reduction. Many recent works suggest that desorption of the adsorbed natural gas may significantly affect the production behavior of gas wells. Previously, adsorption was considered an unconventional mode of gas storage and the effects of gas desorption were usually ignored in conventional reservoir engineering.
Major shale gas producers in the United States include the Barnett Shale in north central Texas, the Fayetteville Shale in northern Arkansas, the Haynesville Shale in northern Louisiana, the Marcellus Shale in Pennsylvania, the Woodford Shale in Oklahoma, the Eagle Ford Shale in south Texas, and others.
Generally, there are two types of adsorption processes:
- Physical adsorption or van der Waals adsorption. Primarily, gas is stored by the process of physical adsorption. Natural gas is adsorbed on the organic matter present in shales and in some cases on certain clay minerals. Physical adsorption is the result of intermolecular forces of attraction between clay particles and natural gas and is a readily reversible phenomenon. The intermolecular attractive forces between clay particles and natural gas is greater than those between gas molecules themselves, so gas condenses on the surface of the solid (clay particles) even though its pressure may be lower than the vapor pressure corresponding to the prevailing reservoir temperature. When the adsorbed substance (natural gas) remains attached to the clay surface, the partial pressure of the adsorbed substance equals that of the contacting gas phase. The natural gas attached to the matrix can be removed or desorbed by lowering the pressure of the gas phase or by increasing the temperature of adsorbed gas.
- Chemical adsorption or chemisorption. Chemisorption or activated adsorption is the chemical interaction between the gas and the adsorbed substance. Here, the adhesive force is generally much greater than that found in physical adsorption, and a large amount of heat is also liberated. This means that there is little chance of chemisorption of natural gas in the shale matrix. Also, the process is frequently irreversible and on desorption, the original substance undergoes a chemical change. Therefore, chemically adsorbed natural gas cannot be produced in its original form. We should thus limit our focus to physical adsorption.
Shale mineralogy plays a great role in determining the amount of gas that is physically adsorbed to shale surfaces. Because most of the shale gas reservoirs are isothermal in nature, temperature has little effect on desorption, whereas pressure plays a dominant role. This type of adsorption/desorption process can be explained by the Langmuir, Brunauer-Emmett-Teller, and Freundlich isotherms.
Shale gas is an important unconventional energy source for our future needs. Commercial production of shale gas began in the past 10 years. The first commercial shale gas production was realized from the Barnett Shale by Mitchell Energy. The breakthrough technologies that stimulate shale gas reservoirs and have reshaped economic profitability are horizontal well drilling and multistage hydraulic fracturing. Hydraulic fracturing creates paths for gas migration from reservoir to wellbore and simultaneously enhances desorption of natural gas from shale surfaces. A closer investigation of desorbed gas release and long-term well productivity will give a clearer picture for putting greater numbers of shale gas wells into production.
Shale gas production cost stands at USD 5 to USD 6 per MMBtu, which is slightly higher than conventional gas production. Today, 20% of total gas consumption in the US is shale gas. Based on today’s shale gas recoverable reserves and present rate of production, it is likely that this unconventional energy source will ramp up and be capable of meeting energy needs over the next 100 years.
