Shale Gas Reservoirs, Resource Estimation and Recoverable Literature Review

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Shale Gas Reservoirs, Resource 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 the layers of sediment alternating between clastic and sandstone or carbonates. Shale usually denotes fissile mudstone containing mm scale laminations of >50% silts and clays below 1/16mm (

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Porosity

An estimation of porosity of the terrain of the basin is proving to be a very important factor in estimation and extraction technologies as the trapped gas within layers that are more porous have proven to yield shale gas ranging up to10x those of tighter and hence lesser porous layers. (Economides & Wang 2010).

Porosity is a function of the organic and inorganic composition of the reservoir and the thermal pressures it has sustained (diagenesis) (Jarvie et al. 2007; Katsube et al. 2000; Kuila and Prasad 2010). The original intergrain pores decrease in shale as diagenesis increases; the same process also results in formation of the intra-grain organic nanopores. Jarvie et al. (2007) proved that, the organic nano-pores are formed as shale matures into the level of oil window because of generation and expulsion of the hydrocarbon, resulting in further expansion of pore sizes which in turn yields larger volumes and the process continues improving extraction and recovery capabilities (Zhou et al. 2014).

The fundamental requirements of successful shale gas plays have >1% TOC, high Si%, low clay %, >40m thick, >1000km2, >1km deep, gas window thermal maturity levels, low expulsion potential and have a natural fracture network (Jarvie et al. 2007; Bouhlel and Bryant 2012). Shales, however are very heterogeneous and hence every play needs independent methodologies and estimations. That makes replication of one successful attempt to another play difficult (Khattab 2012).

The analysis of systemic variations in the mudstone architecture is measured on the parasequence scale using Sequence stratigraphy (Passey et al. 2010). Each parasequence corresponds to a 1-3m thick shale deposit strata depicting the organic and inorganic matter composition at the m-cm scale (Passey et al. 2010). The factors that play decisive role in formation of shale gas comprise organic production, their breakdown by bacteria and oxidization. This is accompanied by sediment dilution of the organic matter (Passey et al. 2010). The factor that ultimately controls all these processes and factors is the amount of dissolved oxygen on the n-benthic activity, bioturbation, colour, pyrite and TOC% in the mudstone. The interface is called the 'reduction-oxidation front' (Potter et al. 2005).

Extraction Methods

The two basic methodologies in prevalence for estimation are:

1. GIP reserve estimates based on a geological strata studies, volume anlysis and gas contents ('bottom-up approach'), and

2. Well Estimates based on available technology, performance, density ('top-down approach'). (BGS 2013, p.10)

Shale gas is extracted from organically-rich, marine transgressive to highstand mud-rocks layered composition of carbonate-clastic deposits. The GIP and transportability of gas in the deposits is determined by factors like adsorptive, Porosity, and permeability. Transportation properties help decide the course of exploration of the reserve. The measurements of porosity, permeability, inter-granular permeability, and specific surface area offer information about the level of extraction from a particular basin, reserve or play. One important method for allowing liquid petroleum to escape into the drills for extraction is the fracturing process. This becomes a necessity as shale gas plays are found around thermal gas and liquid petroleum (Horsfield & Schulz 2012).

Fracturing is done to improve the porosity of gas and is carried out in stages. The process is called hydraulic fracturing where about 99.5% water, and chemicals (0.5%>) are pumped into the plays at very high pressure fractures the shale gas layers increasing its porosity (U.S. Shale Gas Primer, 2009). The clay swells under certain chemicals and hence each reservoir may require a different chemical. Clay is capable of standing higher pressures than silica which may crack under pressures where clay may only bend (Canadian National Energy Board 2009).

Commercial drilling and fracturing process involves drilling up to sixteen horizontal wells from the same vertical drill as it reaches the shale gas deposit under the surface. The vertical shaft then turns at angles of 900 . The drilling extends horizontally for about a couple of kilometers. Thus, a larger cross sectional area can be fractured simultaneously which is seen to yield better results, commercially and in terms of technical efficiency (Bouhlel and Bryant 2012). This method is not applicable in case a shallow shale gas reservoir is encountered (Khattab 2012).

The bottom-up methodology is applied to virgin locations for estimating oil reserves and resources in untapped topographical regions. this method estimates the OGIP. This is followed by applying the TRR estimates through percentage recovery factor estimate. The various factors that contribute to these factors are the surface area.....

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