Shale Gas Reservoirs: Estimation and Recoverable Volumes

What Is Shale Gas?

Shale gas is best described as rich in organic content and fine-grained (Bustin, 2006). Shale is, however, a very broad term, and gas found in any reserve is trapped in layers of sediment alternating between clastic and sandstone or carbonates. Shale usually denotes fissile mudstone containing millimeter-scale laminations of more than 50% silts and clays below 1/16 mm (<63 µm) grain size (Aplin et al., 1999; Khattab, 2012). There are different methods by which free or tight formations of gas may be found in the pores, which also vary in size. Adsorption is another storage method found in shale gas reservoirs. The layered structure both helps trap gas and helps transmit desorbed gas to percolate into the well.

There are interleaved layers of clay and limestone that trap the gas. The gas is formed and adsorbed from organic matter. Fine particles of clay mix with the shale, and the whole rock formation thus contains trapped hydrocarbons in what is called shale gas. The Total Organic Content (TOC) is a measure of the shale gas content that could be recovered from the layers of shale gas deposits. These formations take decades, often centuries, to form, during which time the porous structure hardens. The layers have low permeability that must be loosened by high-pressure water jets, which create a muddy mixture from which gas can more easily escape into the drills (BGS, 2013, p. 8).

Oil and gas are hydrocarbons produced from centuries of compression of organic substances. The main structure is grainy clay, interleaved with limestone, sandstone, and similar materials, into which gas percolates and becomes trapped. Physically, these traps occur at the elevated boundaries between two intervening layers of shale gas basins (BGS, 2013, p. 7).

Estimation and Recovery of Shale Gas

Shale gas occurs in a free state, adsorbed and trapped within porous kerogen, in the microscopic spaces of shale (microfractures), or even in larger spaces between layers called macrofractures (Bust et al., 2013, p. 95). The reserve estimate for shale gas is the amount of gas possibly available in the reservoir under consideration. The extractable amount of shale gas is called the recovery estimate. The reserve is the approximate estimation, and the recovery estimate is the useful extractable quantity. This leads to the recovery fraction — the proportion of the extractable, useful gas to the gross resource estimated in the reservoir — stated as a percentage value determined by time, money, and technical resources and constraints (BGS, 2013, p. 5).

The degree of production uncertainty and the stage of exploration determine the reserve and resource figures for any reservoir. GIP, OGIP, and GIIP — respectively, gas in place, original gas in place, and gas initially in place — are alternative names for the estimations acquired before extraction operations begin. These are rough estimates that enable investors and shareholders to make informed decisions. TRR (Technically Recoverable Resources) is a revised estimate methodology designed and adopted by the USGS (U.S. Geological Survey) specifically for estimating coalbed methane and shale gas. These figures provide a much clearer method of recovery and reserve estimation for extractable gas from shale gas basins (BGS, 2013, p. 6).

With the integration of the TRR methodology, the estimate figures offered by the U.S. Energy Information Administration (EIA) have improved, as they now take into account actual exploration figures accumulated over the years. This is still an exercise in its infancy, and different methods are being used by other agencies. With growing commercial awareness and technological advances in the field, both estimates and extraction capabilities are poised for improvement (BGS, 2013, p. 6).

Estimation measurements used until recently were based on the methodology deployed for coalbed methane. Shale gas basins vary significantly from such reservoirs, causing improper estimates when the same methods are applied to shale gas reservoirs. Additionally, shale gas is heterogeneous, unlike coalbed methane basins. Moreover, each shale gas reservoir may require its own specific methodology for estimation and extraction to achieve near-accurate levels. Unlike coalbed methane basins, there is a need to approximate the adsorption capacity of the kerogen that traps gas, as well as the inter-granular grain space that holds gas (Bust et al., 2013, p. 98). As such, various parameters — including local shale-play data, well-specific core and log data, or regional analogue information — must be incorporated into the design of estimates. After such calculation and estimation using the constraints of the "fractured volume," alternatively called the "stimulated volume," the traditionally used equations for GIP (gas in place) along with gross rock volume (GRV) can be applied (Economides & Wang, 2010).