Shale Gas: Basics and Definitions

Shale gas is natural gas, one of several forms of unconventional gas and is trapped within shale formations with low permeability.

Shale Gas

Shale is a clastic fine-grained sedimentary rock, and is generally a combination of as primarily clay, silica (quartz), carbonate (calcite or dolomite), and organic material, as secondarily some of elements such as uranium, iron, vanadium, nickel, and molybdenum are present. Bituminous/black shale is a composite of a large amount of kerogen, which is a mixture of organic compounds. From this rock, the shale hydrocarbons (liquid oil and gas) are extracted.

Shale acts as source rock only or both source rock and reservoir rock. Shale samples may have different colors due to their different organic contents, clay contents, and other minerals. Organic materials deposited in shale were buried with time. With the increase of temperature and pressure, organic materials, such as lipids from animal tissue and plant matter, or lignin from plant cells, were transformed into kerogen.

Depending on organic materials, pressure, and temperature, kerogen was converted to oil, wet gas, and dry gas. In some shales, gas migrated from shale through fractures, faults, etc. due to expansion. However, gas did not migrate in some shale rocks. In that case, shale is defined both as source rock and reservoir rock, which is the case of shale gas reservoirs.

Shale gas is natural gas, one of several forms of unconventional gas and is trapped within shale formations with low permeability. Shale gas reservoir is defined as an organic-rich and fine-grained sedimentary rock in which gas is self-sourced, and some of the gas is stored in the sorbed state. Shale gas is not just ‘shale’. Gas is of thermogenic or biogenic origin and stored as sorbed hydrocarbons, as free gas within the rock pores and natural fractures and inter- granular porosity, as gas adsorbed on organic material, clay and as gas dissolved in kerogen and bitumen.

Three distinct processes result in the formation of thermogenic gas within shale: (1) the decomposition of kerogen to gas and bitumen; (2) the decomposition of bitumen to oil and gas (steps 1 and 2 are primary cracking); and (3) the decomposition of oil to gas and a carbon-rich coke or pyrobitumen residue (secondary cracking). The latter process depends on the retention or adsorption of oil in the system. Although industry parlance commonly describes these as shale plays, these are truly mudstone. Shale gas reservoirs can also be composed of shale (fissile), mudstone (non-fissile), siltstone, fine-grained sandstone interlaminated with shale or mudstone, carbonate rocks, clay minerals, and other minerals such as calcite and quartz.

Shale gas reservoirs consist of matrix and natural fracture systems. They have also layered structures. Gas in shale gas reservoirs is stored as a free gas phase in the pore spaces of shale matrix and natural fractures. Moreover, gas is stored as an adsorbed phase on the surface of shale matrix, especially on the organic materials (kerogen) and clay minerals, and also a small amount of gas dissolves in water and/or oil. Adsorption capacities of shale gas reservoirs range from 20 to 85%. Total organic carbon (TOC) content increases adsorption capacity.

Moreover, Ross and Bustin stated that clay content of shale increases adsorption capacity. Shale gas reservoirs have dual porosities, in the rock matrix and the natural fractures. Due to overburden pressure, natural fractures are generally closed. Hence, shale gas reservoirs have low porosity values. At micron scale, it was shown by several authors that the shale organics are nanoporous materials. Shale matrix generally has micro- (pores less than 2 nm in diameter) to mesopores (pores with 2–50 nm diameters).

These small pores on shale matrix are due to clay content and organic content. Moreover, shale gas reservoirs have extremely low matrix permeability values typically ranging from 10 to 100 nanodarcies (10^-6 mD). Reservoirs with permeability values greater than 0.1 mD are defined as conventional reservoirs. Hence, shale gas reservoirs are in the classification of unconventional reservoirs.

Two basic types of producible shale resource systems exist: gas- and oil-producing systems with overlap in the amount of gas versus oil. Although dry gas resource systems produce almost exclusively methane, wet gas systems produce some liquids and oil systems produce some gas. These are commonly described as either shale gas or shale oil, depending on which product predominates production.

Shale-gas resource systems vary considerably system to system, yet do share some commonalities with the best systems, which are, to date, marine shales with good to excellent total organic carbon (TOC) values, gas window thermal maturity, mixed organic-rich and organic-lean lithofacies, and brittle rock fabric. A general classification scheme for these systems includes gas type, organic richness, thermal maturity, and juxtaposition of organic-lean, nonclay lithofacies.

Such a classification scheme is very basic, having four continuous shale-gas resource types: (1) biogenic systems, (2) organic-rich mudstone systems at low thermal maturity, (3) organic-rich mudstone systems at a high thermal maturity, and (4) hybrid systems that contain juxtaposed source and nonsource intervals.

The general methodology for conducting the basin- and formation-level assessments of shale gas and shale oil resources includes the following five topics (given directly from Jarvie, 2012):

1. Conducting preliminary geologic and reservoir characterization of shale basins and formations (depositional environment of shale (marine vs non-marine), depth (to top and base of shale interval), structure, including major faults, gross shale interval, organically-rich gross and net shale thickness, Total organic content (TOC, by wt.), thermal maturity (Ro)).
2. Establishing the areal extent of the major shale gas and shale oil formations.
3. Defining the prospective area for each shale gas and shale oil formation.
4. Estimating the risked shale gas and shale oil in-place (OIP/GIP) (Oil In-Place, Free Gas In-Place and Adsorbed Gas In-Place; net organically-rich shale thickness, oil- and gas-filled porosity, pressure, temperature. Play success probability factor, prospective area success (risk) factor).
5. Calculating the technically recoverable shale gas and shale oil resource (Favorable oil recovery, average oil recovery, less favorable gas recovery. Two key oil recovery technologies, importance of mineralogy on recoverable resources, significance of geologic complexity.

Global shale gas resource

Interest in shale gas potential has spread to countries such as Canada, Europe, Asia and Australia, especially in the United States and shale gas has become an important source of energy since the beginning of this century. Although Europe is in its infancy of shale gas business due to the legal restricts, economical issues, lack of geological information and industrial infrastructure, there are some substantial activites and future production plans in some countries like France, Poland, Norway, Poland, Ukraine, Sweden and Turkey.

The latest estimate of technically recoverable shale gas resource in 42 evaluated countries is 7795 trillion cubic feet (tcf), whereas more than 60% of the assessed shale gas resource is in the Asian-Pacific region, e.g., China and Australia, and in the Americas, e.g., the United States, Canada, Mexico, and Argentina. Today, the United States supplies more than a third of its gas needs from shales. It is estimated that in 2035, natural gas production from shale gas will meet 46% of gas production.

In addition, it is thought that shale gas production, which is ranked first in the world’s energy plans, will increase global gas resources by about 40% , and it is mentioned that it may cause an industrial revolution in the coming years and change the balance of energy resources in the world. Therefore, it cannot be overlooked that this situation may also be reflected in geopolitical positions. Exploration activities have been underway in many countries. Nevertheless, currently, the United States and Canada are the only major producers of commercially viable natural gas from black shale formations in the world.

Table 1.1 lists the main countries that contain large amounts of recoverable shale gas. China holds the first rank, followed by Argentina, Algeria, the USA, and Canada. In recent years, the development of unconventional shale gas resources in North America has had a major impact on the overall energy landscape of the region. This has all become possible due to significant advances in hydraulic fracturing and horizontal drilling technology, which allows access and recovery of unconventional resources that were considered economically and technically nonrecoverable just a few years ago.

Table 1.1 : Top 10 countries with recoverable shale gas resources

Production mechanisms in shale gas reservoirs

Horizontal drilling and hydraulic fracturing are the primary enabling technologies behind the recent surge in effective and economic shale gas production. In order to produce the natural gas from a reservoir, typically a drill pipe is sent down vertically a distance underground and then turned at a ninety degree angle horizontally into the target shale formation. In horizontal drilling the drill cuts down vertically for up to 7,000 meters and then continues horizontally for up to 2,000 meters.

According to industry results, horizontal wells produce, on the average, three to five times of the amount of natural gas that vertical ones do. Also, it is detected that six to eight horizontal wells drilled from only one well pad can access the same reservoir volume as sixteen vertical wells. Using multi-well pads can also significantly reduce the overall number of well pads, access roads, pipeline routes, and production facilities required, thus minimizing habitat disturbance, impacts to the public, and the overall environmental footprint.

Hydroulic fracturing can be defined as the pumping of a fracturing fluid into the shale formation in target under high pressures to fissures to maintain the gas flow through well bore. This method makes gas to go out of the rock to the well in economic quantities. Ground water can be protected during the shale gas fracturing process by a successfull casing and cementing design. Fracture fluids are primarily water based mixed with additives that help the water to carry sand proppant into the fractures.

Water and sand make up over 98% of the fracture fluid, and the remaining 2% consists of various chemical additives that improve the effectiveness of the fracture job. In addition to water and sand, other additives are used to allow hydraulic fracturing to be performed in a safe and effective manner. This fluid is injected into deep shale natural gas or oil formations and is typically confined by many thousands of feet of rock layers.