WHAT IS A SHALE?
Author: Emanuel Martin
Shale is a sedimentary clastic fine-grained rock rich in organic matter composed of silt and clay with the distinctive feature of break easily in thin layers.
It belongs to the family of the mudrocks and although these are the 60% of the Earth’s crust just a group of them are of interest to the oil industry. The two main features of a shale to contain hydrocarbons is that they come from depositional environments formed by sediments rich in organic matter deposited under anoxic conditions. (The Black Sea is a current example of this type of basins).
Figure 1: image of a black shale rock, taken by James St. John.
How was generated the hydrocarbons inside the shale?
This organic matter that was deposited under anoxia conditions in ancient seas and lakes has undergone several transformations to measure that were buried. The sediments through a process called litification became a rock and part of the organic matter within them has become hydrocarbons.
Figure 2: surface equipment for hydraulic fracturing treatment.
This process where the organic matter is transformed in hydrocarbons can be divided in three stages: First to increase the burial the organic matter slowly and partially is cooking and transforming into kerogen during a period known as Diagenesis (temperatures less than 50°C). To continue the burial the pressure and temperature increase begin the second stage known as Catagenesis (temperature from 50°C to 150°C), kerogen decomposes generating oil along with the wet gas generation with increasing temperature. In the third stage known as Metagenesis further increases in the burial of sediment increase even more the temperature (temperatures greater than 150°C) and the pressure, where chemical changes of kerogen transform it in dry gas, H2S and CO2.
What kind of fluid can contain a shale?
As a result the shale according to its depth, temperature and maturation degree can contain petroleum, gas wet and/or dry gas. According to the fluid that is in greater proportion inside the shale are classified as shale gas or shale oil. Eagle Ford play in South Texas is a good example of how the same shale can contain the three fluids from oil, gas wet to dry gas with increases the depth of the shale rock.
Figure 3: Eagle Ford Shale Play in south of Texas, source: EIA.
Which is the difference between a conventional and unconventional oilfield?
In the conventional deposits the rock generator of oil is the mother rock (shale), in which the hydrocarbon is generated, is expelled (due to the pressure and temperature) and begins to migrate until reach an impermeable barrier or seal that block its movement making that the hydrocarbons (HC) are accumulated. This rock where the HC are accumulated is called reservoir rock , usually belongs to the group of sandstones, conglomerates, or carbonates, and has good porosity (dimensionless relationship between the volume of pores and the total rock volume) and good permeability (measure of the rock ability to let a fluid can flow through it).
Figure 4: Conventional reservoir is formed by a source rock, a reservoir rock and an entrapment.
In this way a conventional reservoir is formed by a mother rock (source rock) a reservoir rock (where the hydrocarbons are accumulated) and a seal rock or stratigraphic trap that doesn’t allow to the hydrocarbons continue flowing, allowing their accumulation. Thus the search and exploitation of conventional oilfield focuses on reservoirs rock, which contains stored large quantities of oil and/or gas and its petrophysical properties allow to obtain a profitable flow of hydrocarbons to the inside of the well.
Figure 5: in an unconventional field the mother rock, reservoir rock and seal rock are the same shale.
On the other hand in an unconventional field the mother rock, reservoir rock and seal rock are the same shale. This is the rock that generated the hydrocarbons found in the conventional reservoir associated with it and still contains a large part of the HC generated in their interior.
What is hydraulic fracturing?
Hydraulic fracturing is a well stimulation technique designed in 1947 to increase the drainage area of the same. How does it work? The basic operating principle consists in A mixture of water, proppant (sand or ceramic pellets) and chemicals is then pumped into the well at high pressure to generate microfractures and reopen natural microfissures in the rock. This produces a considerable increase in the drainage area of the well with microchannels of great permeability that increase the production flow.
Figure 6: surface equipment of hydraulic fracturing and a cross section of an horizontal well.
Hydraulic fracturing development
Since 1980 the fracking suffered a strong development in USA where managed to increase the fracture size, were developed multiple stages of fractures in the same well, were perfected fluid additives with a technological advance in the necessary machinery for their injection, (pumps and control systems). In addition the progress made in the horizontal drilling combined with the hydraulic fracturing transformed the shale in a new kind of reservoir rocks thus allowing its profitable exploitation on an industrial scale.
What is the importance of these resources?
As another source of hydrocarbons and energy, the fact of knowing their existence and economic viability for their exploitation generates a great economic and geopolitical impact to global scale. For example U.S. success with the production of this kind of resources managed to supply the internal energy consumption in the country after decades of imports and even more achievement to convert it into an exporting country of the same.
There are 95 geological basins in 41 countries around the world which have hydrocarbon reserves in shale/tight formations with volumes of gas reaching 7,299 trillion cubic feet and petroleum volumes that are around the 345 billion barrels. This means an increase of 47% of the world gas reserves estimated (22,882tcf) and an increase of 11% of global oil reserves estimated (3,357 billion of barrels) according to information provided by the U.S. Energy Information and Administration 2013.
Figure 7: Shale basins around the world, source: EIA.
We can conclude then that the role played by the shale in the global balance of hydrocarbon reserves is of vital importance and are a key element in the long term for the World supply.
Author: Emanuel Martin, Petroleum Engineer.
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