Definition of Conventional and Unconventional Oil Reservoir

We have heard much about the deposits of Petroleum and gas conventional and unconventional. Also, regarding the latter, you may 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 decomposition anaerobic from microorganisms, therefore, they are organic compounds based on a carbon chain structure. Over the years, sedimentation above this layer continues and due to the pressure exerted, the generated liquid tends to migrate towards a reservoir. In other words, the first crude oil formation occurs in "Roca Madre" while its storage

definitive occurs in the "Reservoir Rock", from the migration in ascending form. This "Reservoir Rock" has particular characteristics that allow the extraction of hydrocarbons, such as their permeability and porosity.

What do we mean by both concepts? Permeability has to do with the ability of the rock to allow the fluidity of the circulating fluid, whereas, porosity is the number of interconnected pores available to store the hydrocarbon.

Well, the main difference lies in where the hydrocarbon is housed, while in a well Conventional 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 low permeability rock or compact rock that does not allow its rise, it can even be the rock where it was generated, Mother Rock.

That is why, as the fluid in a conventional reservoir tends to migrate towards the surface, it encounters a Seal Rock that it is prevented by its constructive characteristics, while the Mother Rock, in an unconventional deposit, acts as its own stamp.

Modern technologies and processes

Until a few years ago, unconventional exploitation had not developed. Some years ago these new deposits began to be discovered that forced the Oil and Gas industry to re-train and discover new technologies for extraction.

This is when the concept of "Fracking" is introduced. As we mentioned earlier, the hydrocarbon from an unconventional reservoir is stored in a sealed reservoir, so it needs to certain stimulation to migrate towards the surface, and this is carried out through hydraulic fractures that generate channels so that it can ascend and flow. In addition, depending on the difficulty of migration to the surface, methods of “horizontal” extraction in order to run, instead, a conventional well is drilled in a vertical.

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 wells and number of perforations based on it, among other. However, the methodology Drilling in both is similar, with a bit from the surface to the 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 set their sights on the potential of exploiting this type of formation that contains hydrocarbon, which, as we know, is a non-resource renewable. In addition, it is known that there are many more deposits of this type that have not yet been explored, being able to 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 zero 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", the reservoir rock is very compact but it is not the source rock or bedrock, but the hydrocarbon flowed until it met this impermeable layer. In the case of shale, the hydrocarbon was trapped in pores, it is necessary to generate cracks in the rock so that it moves through the interior of the formations and thus improve the 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 nowadays, hydraulic stimulation has notably improved the performance of wells and their time service life, therefore, this technique is being widely implemented not only in non-oil reservoirs conventional.

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