WHERE IS THE GAS STORED?
Gas in an unconventional field is found in shale which is a rock rich in organic matter belonging to the family of the mudrocks with the distinctive feature of breaking easily in thin layers. It´s constituted by particles of fine grains that have one size of less than 62,5µm, giving us a low porosity and a pore size extremely small, with an average diameter ranging from the 4 to the 200nm, and a ultra low permeability that rounds between the 54nd to 150nd.
To visualize quickly the dimensions that we're talking about we show below an image of a shale that appear in surface, then making a zoom we can observe in microscale the matrix and microfractures network, and finally reaching a nanoscale we can see the kerogen, the microfractures and gas molecules.
PORES AND MICROFRACTURES.
The gas inside the shale is found stored in pores and natural microfractures. The pores we can clasificate in two groups: pores inside the non organic matter and pores inside the organic matter.
PORES WITHIN THE ORGANIC MATTER
These type of pores generally are the most abundant in the shale and are associated to the generation of hydrocarbons in-situ from kerogen, their sizes ranging from 1nm to 10µm and may arrive to have to 1000 pores in a small portion of organic matter.
Reed estimated that the porosity within the organic matter ranges from 0 to 25% in weight, it can become even 5 times bigger than the porosity of the matrix and is dependent of the pore pressure and stress. In the Barnett play for example more than 70% of the pore volume comes from the pores present in the organic matter.
The Kerogen has the feature to be able to store a large amount of gas due to its volume pore, inside which the gas is found like free gas, and their ability to adsorb molecules of gas in the pore walls. In the majority of the shale has been found a directly proportional relationship between increasing of matter organic contents (TOC) and the gas storage capacity.
In the majority of the shale has been found a directly proportional relationship between increasing of matter organic contents (TOC) and the gas storage capacity.
The next picture shows the pores in the organic matter and pores in the matrix taken for the Longmaxi Formation from China by a emission scanning electron microscope (FE-SEM). The organic matter is seen in black color and the matrix in grey color.
PORES WITHIN THE MATRIX:
The pores within the matrix may be classified as intercrystalline, intraparticle, intergranular and pores formed by mineral dissolution.
Intercrystalline pores are among clay flakes and other particles of the matrix. Their sizes range from 0.1nm to 2µm. These types of pores are more abundant in compacted shale and highly pressurized (Fayteville) or shale rich in clays (New Albany).
a-Intercrystalline porosity (Haynesville, Curtis et al 2010). b-Detail of pore system showing intercristaline pores predominant.
Intraparticle pores are found within nanofossil fragments, calcareous mudstone or within framboidal pyrite. They have a pore size ranging from 0, 1nm to µm.
b - Intraparticule porosity (fecal pellet, Haynesville, Milner et al 2010). c-(FE-SEM) images of a black shale from the Longmaxi Formation (TOC 1.30%) showing the Pyrite Framboids.
Intergranular pores are associated with grain-supported silty laminae or beds within shale. (Montney and Colorado Shales): these pores are larger than the pores found in organic matter, with sizes ranging from the 10µm the 200µm.
f - Intergranular porosity in silt lamination (Colorado, Sliwinski et al 2010).
Pores formed by dissolution: they are produced by the dissolution of carbonate, dolomite and/or pyrite. They are generally present in smaller quantities (Milner et al 2010) associated with secondary porosity with sizes of pores ranging from the 2µm the 200µm.
g - Carbonate dissolution porosity (New Albany, Schieber 2010). H-Dissolution along margins of dolomite grains in the New Albany Shale.
The natural microfracture can harbor relatively large quantities of gas and generate important flow networks to connect among themselves and with pores present in the kerogen and matrix. These microfracture and microfissures were generated during the transformation of organic matter into hydrocarbons when these reached the thermal maturity needed they cracked.
This abrupt change of volume caused an increase in the differential pressure sufficiently large, greater than the pressure of overburden, to fissure the rock and allow escape to hydrocarbons from shale (mother rock) toward a conventional reservoir. Then many of these natural microfractures were closed by deposition of natural cements and crystallizations.
In the images A and B, we can observe how the microfractures extend between two fragments of kerogeno with lengths of 200um and 2um thickness. (Taken from Engelder et al. 2008).
COMPARISON OF THE PORE SIZES
In the following graph we can see how it distributes the pore size according to their origin and classification. The larger pores are provided by pores intergranular and generated by dissolution of minerals and pores smaller come from organic matter and pores intraparticle.
In each “shale plays” are produced different configurations between these types of pores in accordance to its lithology and geological events post-depositional , reaching a porosity that is 3% to Horn River, 6% to Barnett, 8% for Marcellus, Haynesville 10% and 11% for Eagle Ford.
HOW SMALL IS A NANOMETER PORE?
To quickly view the small size that are the nanopores we´re going to analyze the molecule of methane gas and thus we can understand easily that we are talking about pores whose sizes are of the molecular order.
The methane molecule has only 0,4nm of size and in an average organic pore of 10nm fit approximately 30 molecules, it is to say, their ability to store gas is too small. This has forced the Petroleum Engineering to develop a new area of study, the movement of gas and oil in nanopore with nanodarcies permeability and flow mechanism in part different from the conventional.
So we could say that the era of the shale is the era of nano petroleum engineering; it´s the study of the movement of small groups of hydrocarbons molecules in tiny sizes with tiny flow capacities but that they have contributed an increase the gas world reserves in 7299Tcf according to data provided by the Energy Information Administration (EIA) in 2013.
AUTHOR: Emanuel Martin, Petroleum Engineer.
Written by the author:
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