Candela Rocío Barbisan
Chemical engineer
Conventional reservoirs are those where the hydrocarbon is stored in the “Reservoir Rock” while unconventional are those where the hydrocarbon is stored in the “Source Rock” or a low permeability rock.
We have heard a lot about conventional and unconventional oil and gas fields. Also, regarding the latter, you will have heard about the increase in production, in association with terms such as “shale”, “tight” or “fracking”. Well, first of all, we must understand how a reservoir arises and where the hydrocarbons are stored.
Hydrocarbons arise from the anaerobic decomposition of microorganisms, therefore, they are organic compounds based on a carbon chain structure. Over the years, sedimentation continues above this layer and due to the pressure that is exerted, the liquid generated tends to migrate towards a reservoir. That is to say, the first formation of crude occurs in “Mother Rock” while its definitive storage occurs in the “Reservoir Rock”, from the upward migration. This “Reservoir Rock” has particular characteristics that allow the extraction of hydrocarbons, such as its permeability and porosity.
What do we mean by both concepts? The permeability has to do with the ability of the rock to allow the fluidity of the circulating fluid, while the porosity is the amount of interconnected pores available to store the hydrocarbon.
Well, the main difference lies in where the hydrocarbon is lodged, while in a conventional well the hydrocarbon is found in the Reservoir Rock, permeable and porous that acts as a “sponge”. In the unconventional well, the hydrocarbon is found in a rock with low permeability or compact rock that does not allow it to rise, it may even be the rock where it was generated, the Mother Rock.
That is why, as the fluid in a conventional reservoir tends to migrate towards the surface, it encounters a Seal Rock that prevents it from doing so due to its construction characteristics, while the Mother Rock, in an unconventional reservoir acts as its own seal. .
Modern technologies and processes
Until a few years ago, unconventional exploitation had not been developed. A few years ago, these new deposits began to be discovered, forcing the Oil and Gas industry to reconsider and discover new technologies for extraction.
This is when the concept of “Fracking” is introduced. As we mentioned before, the hydrocarbon from an unconventional reservoir is stored in a sealed reservoir, so it needs some stimulation to migrate to the surface, and this is done through hydraulic fractures that generate channels so that it can rise and flow. In addition, depending on the difficulty of migration to the surface, “horizontal” extraction methods are developed in order to flow, instead, a conventional well is drilled vertically.
There are many other differences between the two reservoirs, from the composition of the fluid (water cut in the hydrocarbon), maturity of the reservoir, pressures, depletion of the wells and number of perforations based on it, among others. However, the drilling methodology in both is similar, with a bit from surface to reservoir rock.
Types of unconventional deposits
As the requirements for the formation of an unconventional type reservoir are much lower than those of a conventional one, companies have turned their eyes to the potential of exploiting this type of formation that contains hydrocarbons, which as we know is a non-renewable resource. In addition, it is known that there are many more deposits of this type that have not yet been explored, and can greatly exceed the reserves of conventional deposits.
It is time to explain two more concepts that we mentioned, they are: “tight” and “shale”. In these two cases we refer to the type of rock: low permeability or non-existent or null permeability, respectively. And as we already said, the quality of the reservoir is given by its permeability.
Although there are different types of unconventional deposits, these two mentioned are the ones that have taken the most boom in recent times. In the “tight” ones, the reservoir rock is very compact but it is not the source rock or parent rock, rather the hydrocarbon flowed until it found this impermeable layer. In the case of the “shale” the hydrocarbon was trapped in pores, it is necessary to generate fissures in the rock so that it moves through the interior of the formations and thus improve permeability. Through fracking, fluid is injected at high pressure (typically: water (95%), sand (4.5%) and additives (0.5%), generating the channels necessary for extraction.
It should be noted that today, hydraulic stimulation has significantly improved the performance of wells and their lifetime in service, therefore, this technique is being widely implemented, not only in unconventional reservoirs.
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