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The oil system consists of a series of geological processes and elements: rock, seal rock, reservoir rock, trap and overload, each acting synchronously, resulting in the accumulation of volumes of hydrocarbons.
The fraction of the trap, which is saturated oil and / or gas, provided in the presence of water, is called the reservoir of oil.

A hydrocarbon reservoir is a system that, by being confined, efforts are under strain and pressure of the overlying load. Hydrocarbons are in the form of liquid and gas. The distribution of the phases due to density differences, as well as the vertical permeability of the reservoir. There are sites in which only liquid hydrocarbons or gases.

The naturally fractured reservoirs are defined as those hydrocarbon deposits whose production is influenced by the presence of fractures.

They are known as fractures, those flat discontinuities in the material that makes up the rock, can be microscopic or macroscopic, they appear due to the efforts of confinement to which has been under a rock, are superior to the efforts of rupture. Natural fractures influence the behavior of both the production and the decline in reservoir pressure. This influence can be both negative and positive in the flow of fluids. For example, open fractures, ie that are not cement or partially mineralized (with good permeability), usually have a positive effect on the flow of oil, but adverse happens to the flow of water and gas, as high permeability for those who have problems tend to occur in conificación. Furthermore, fractures can be completely mineralized cement or permeability barriers to all types of flow.

How to identify a naturally fractured reservoir?
There are several ways to detect if you are in the presence or absence of a naturally fractured reservoir:
It often happens that a well produces a very high rate and registered a sharp decline of the latter, apparently, without any explanation of this problem, this is one indicator of the presence of fractures in a reservoir.
One of the most striking aspects of these deposits is their dual-porosity, which is defined as primary and secondary
Other indicators of the presence of fractures are the problems of loss of movement in a borehole drilled to, as well as changes in the rate of penetration, and information received from the nuclei.

Geometry of naturally fractured reservoirs
The naturally fractured reservoir is mathematically represented by the dual porosity model, which consists of the superposition of two systems with different pore characteristics. The porosity in the matrix model, is called primary porosity, while the inherent network of fractures, vacuoles, and other estilolitas; is called secondary porosity.

Production behavior of naturally fractured Yacimientos
Here are the basic features of the behavior of production for the type of deposits under consideration.
Absence of the transition zone
The transition zone, the key feature of the deposits matrix with low permeability, is not present in naturally fractured reservoirs, in cases where the matrix is zero or very low permeability, so that the effects of capillary pressure does not occur in the area or intergranular porous matrix due to the absence of displacement of fluids through it. All this is because in the open spaces of networks of fractures capillary pressure is almost zero because of the size of the pore space and thus the balance between phases: gas, water, oil, is defined solely by the gravitational forces, ie, forming a horizontal interface and well defined in your contacts without transition zone.
Therefore, the original water-oil contact and gas-oil can be found through the analysis of fluid levels in monitoring wells along the production history of the site.

PVT properties with respect to the depth
When describing a fractured reservoir, if the fractures are good continuity both horizontally and vertically, it is possible that a process of convection resulting from the combination of thermal expansion of the fluid and the gravitational compression. In the course of geological time, this process of convection reaches promote uniformity in the composition of the hydrocarbon through the reservoir, keeping constant pressure bubbling like the rest of the PVT properties no matter what the depth at which is being taken.

Relationship between pressure drop around the producing wells and the rate.
In a fractured reservoir pressure drop in the vicinity of producing wells is small when compared with the high pressure differential in those areas where there are no fractures, due to low permeability of the matrix. This phenomenon is due to:
The fluid flow toward wells in a fractured reservoir occurs only through the network of fractures due to the blocks of the matrix only feed the fractures with fluid.
The vast intrinsic permeability of fractures compared to the matrix, which can achieve high rates with minimal pressure drops.

Gas layer in the network of fractures
As the oil flows through the fractures is a very low pressure gradient, which facilitates the segregation of the gas released into the upper part of the network of fractures, where it ends, forming a layer of gas in its top.

Pattern of decline in pressure
Previously, it was announced that oil production unit, the rate of decline in pressure is really low when it is saturated fractured reservoir, compared to a reservoir with no fractures.

Gas-Oil Ratio
Again in fractured reservoirs recognizes an improvement over the matrix fields. In this case is to be noted that the gas-oil in fractured reservoirs, is less according to the production sites in the matrix. This difference is mainly due to the low capillary pressure which the pore spaces of the fractures, and consequently the free gas separated accomplished quickly and easily into the upper zones of the fracture and the reservoir instead of flowing into the producing wells.
Water-Oil Ratio
This parameter, in naturally fractured reservoirs, is a function of production rate, while the matrix deposits will depend on various factors such as characteristics of the rock and fluids, and the behavior of fluid displacement, this once more with the production rate.

Based on information from: Chapter 9 Predicting Reservoir System Quality and Performance by Dan J. Hartmann and Edward A. Beaumont