Today, there is much that is talked about and discussed in the oil and economic world of unconventional reservoirs. But what are these unconventional reservoirs? Giving an overview of what unconventional reservoirs are from the point of view of operations, geology and petroleum engineering, it leads us to look for a definition about what these are. One of the main parameters that these type of reservoirs will define is their permeability, so we would be referring to that: they are very low permeability reservoirs, less than one microdarcy (Figure 1), but that they have the possibility to produce hydrocarbons, through the application of different techniques related to hydraulic fracturing. Much of the unconventional resources are found around the world, and have particular characteristics such as their rapid depletion (Figure 2).
The fact that they are geological formations of low permeability requires that drilling alternatives based on the development of horizontal wells with the generation of hydraulic fracturing of multiples stages be generated. It is also important to know, among other things, the physical and transport properties of the type of rocks that make up unconventional reservoirs. Likewise, reservoirs known as unconventional are of great controversy worldwide for environmental impacts that the development of such reservoirs could lead, as well as, the use of induced seismicity, which is widely used in the stimulations and studies of such reservoirs.
Over the past 10 years, there has been an increase in oil and gas production, in this type of unconventional reservoirs, making countries such as the U.S. and Canada exporters of this type of hydrocarbons. Similarly, the oil industry in general has benefited from the set of technologies implemented in the development of this type of reserves.
The discovery of many of these types of reserves could not be possible in the world, without the development of horizontal wells with multiple fractures (Figure 3), which allowed giving economic interest to the geological formations of low permeability. Since traditional or conventional discoveries and developments were intended for rocks with other types of properties: highlighting techniques, where the so-called best petrophysical properties were sought: high porosities, high permeabilities, high oil or gas saturations, low water saturations.
Unconventional reservoirs are located all over the world. Its large-scale development begins in the U.S. and Canada, as well as countries such as China and Australia, have pioneered the development of such reservoirs. However, there are great potentials around the world for the development of unconventional reservoirs, such as Mexico, Venezuela, Brazil, Argentina, France, Poland, Algeria, Ukraine, Iran, Libya, Egypt, Qatar, Saudi Arabia, Pakistan, United Arab Emirates and South Africa. This, if we refer to its horizontal spatial location; but if we speak in their vertical location, that is, in the geological column, we will find them embedded among the so-called conventional reservoirs, since many of them represent in the conventional system of reservoir distribution, characteristics of source rock and rock seal, as well as could have been limiting the permeability of the source rock.
The importance of unconventional reservoirs is that for the next 50 years and more, the planet will still require energy from this type of resource; and the global oil industry, must ensure that the whole world is provided with this type of energy in an economically cost-effective manner, which guarantees all the national security principles of the countries involved, generating the least possible environmental impact and in harmony with the living conditions of society. In this sense, one of the advantages of this type of deposit is the production of natural gas that is generated in the world, in order to reduce CO2 emissions into the atmosphere. Much of the gas production can come from unconventional deposits in the world.
In terms of its classification, we can find different types of unconventional deposits, but there is basically a classification in three types:
- Rich in Organic Matter- Source Rock
- Conventional Unconventional
- Hybrid plays
1. Rich in organic matter- Source Rock: They are made up of those rocks that in previous explorations, or in the description of traditional oil systems, constitute the Source Rock, however, there are accumulations of hydrocarbon in this type of rocks, which turn them into storage rock; and their accumulations are considered unconventional.
2. Conventional Unconventional: They represent the group of conventional reservoirs in sandstone or limestone, whose accumulations in organic matter are not very significant, but whose low permeability requires the design of horizontal wells with multiple fractures to ensure the hydrocarbon flow from formation to wells. Given their treatment and their very low permeability they are considered unconventional.
3. Hybrid Plays: Represent the combination of the two above types, i.e., they have large organic content and at the same time were considered conventional resources.
Organic-rich deposits are generally formed in platform environments (Figure 6), where some organic sediments are deposited in combination with very fine grain sediments, specifically on a continental margin, where in geological eras, could coexist algae, phytoplankton, and organic matter die and fall to the sea floor in relatively deep water, additionally it is buried by clay in a reducing environment (So the organic matter doesn’t oxidize).
In a geological context we must say that over time this accumulation of organic matter has much to do with the relative variation of sea level, which will give the precise depths for the deposit of fine sediments, combined with the generation or the number of living organisms, which when they die, can settle in the basins. This organic matter reaches the maturation necessary to become oil or gas.
Thanks to this combination of fine sediments and organic matter, today we can find the reservoir of shale, rich in organic matter. We can find deposits with great lithological variation, where they are present:
- Combination of shale sand, carbonates and shale
- Basal shale, which correspond to areas of high clay content and high organic content
- Areas of high organic content with presence of clays
When talking about hybrid play and conventional unconventional deposits, it is necessary to refer to the terms of production and drilling modes in order to achieve the best results in these types of reservoirs. They are precisely called unconventional because their permeabilities are so low that they require horizontal drilling and multiple fracturing, as we have already mentioned. Also, its rapid depletion is a typical characteristic of this type of reservoirs, even if we are talking about sandstone or carbonate type storage rock.
Observing the hybrids’ play and conventional unconventional deposits, the characteristic that differentiates them is their organic content, since hybrid plays, even though they are made up of sandstone or limestone source rock, are very related to the processes that gave rise to the source rock, or there is an important content of organic accumulations, which point to these types of formations the character of hybrids.
As for their production characteristic, let us remember that there are deposits in which induced fractures are generated, or what we know as hydraulic fracturing, in order to generate the channels that can allow the communication of the fluid from the training to the well. Accumulations that may be contained in fractures are temporary and the formation matrix must refill these fractures. So, their production behavior will be fast depletion depending on the time, requiring subsequent stimulations that could reactivate the productive state of the wells. Among the most well-known types of stimulations is multi-stage fracturing and induced seismicity.