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# Production geology

Production Geology is a branch of petroleum engineering which studies the geology of reservoirs in order to manage the production of a field and get more hydrocarbons out of it. The work of production geologists includes understanding the reservoir from a geological point of view and creating a model for it by using computer software. This model provides all the information needed for hydrocarbons production. Rock type may affect production rate. When the porosity and the permeability change from one rock type to another, the production rate will change. During production, the reservoir may be damaged and this causes changes in porosity and permeability of the reservoir which in turn causes a change in production rate. This article discusses these points in brief.

## Determination of Petroleum Reserves and Its Relationship with Geology

After confirming the presence of oil reservoir from exploration wells, reserve estimation must be done in order to know the amount of oil which will be produced. The volume of rock which is located between the oil/water contact and the gas/oil contact is determined by 3D seismic and logging. However, oil/water contact determination depends on the correct identification of depth using seismic travel times. It is difficult to determine the average porosity. So, it is determined in a range by using porosity logs and core data analysis. The saturation of oil or gas is determined from logging interpretation and core data analysis as well.[1]

The volume of oil or gas in place (Vp ) is calculated from: Vp = Vr · N/G · ϕa · Sat

In which:

(Vp) = The total initial volume of oil in place.

(Vr) = The volume of rock above the oil/water contact (OWC) and below the cap rock.

(N/G) = Net to gross volume ,the ratio between the volume which have sufficient amount of oil to produce (N) and the total volume of rock (G).

(ϕa) = The average porosity.

## Effect of Rock Type on Production Rate

The change in rock type is an important reason for changing the production rate. In other words, rock type controls rock properties such as porosity and permeability which in turn control the production rate. The effect of many types of rock on hydrocarbons production is as the follows :

### Sandstone rocks

Sandstone affects production rate through the following two main factors:

Reduction in porosity with depth
Porosity decreases with depth resulting in a decrease in production rate. However, this reduction depends on sandstone type as follows:
1. Immature sands lose porosity faster than well sorted sands
1. Poorly sorted sands lose porosity faster than well sorted sands.
Pressure and temperature effect on porosity
1. When the temperature increases ,it causes faster cementation and a decrease in porosity.
1. When the pressure increases ,it reduces compaction and increases porosity.

### Carbonate rocks

The depositional environment may change the quality of carbonate reservoirs as follows:

Reefs
Reefs have high porosity as they are initially cemented ,and it is necessary that hydrocarbons migrate into the pore space early to keep the porosity.
Carbonate mud rock
The common deposits are aragonite .When aragonite re-crystallizes into calcite, the porosity is destroyed by the resulting calcites. In addition, chalk is a type of carbonate mud rocks which has high porosity and low permeability because of its small size .When they are fractured , they can make good reservoir rocks.
Dolomite rock
Good reservoirs are formed by dolomites. The volume of Mg atoms is 13% smaller than Ca atoms. After dolomitization, the total volume of material decreases by 13% and generates 13% incremental porosity.[2]

### Other types of rocks

The two major reservoir rock types around the world are sandstones and carbonates. They represent around 90% of the world's production of hydrocarbons. Other types of rocks contain around 10% of the world's production. The mechanisms that help other types of rock to work as reservoir rocks include:

Dissolution
The dissolution of feldspars in granites forms a good reservoir rock and is called granite wash.
Fracturing
Making fractures in igneous and metamorphic rocks generates good porosity and permeability in these rocks making them good reservoirs.[2]

## Changes of rock properties During Production

During production, changes in reservoir rock properties occur which may result in reservoir damage. This takes place in two ways:

### Chemical reactions

During drilling or water injection, the minerals and drilling liquids may react and damage the reservoir. This damage is treated by chemicals injection to improve the permeability.

### Mechanical damage

Clay minerals or small grains are carried towards the well by fluids and block the pores around the well. This damage is treated by lowering the production rate.[1]

## Monitoring Production

During production, the information about fluid properties in reservoirs is provided from the comparison between production and injection pressures. This data is very important for reservoir simulation models. [1]

## Conclusions

Determination of petroleum reserves depends on the geology of reservoir as it depends on its porosity and saturation of rock. The change in production rate depends on rock properties such as porosity and permeability which change from one rock type to another. During production, the properties of reservoir rock may be changed by chemical reactions or mechanical damage. So, production rate will change in turn. Monitoring production supplies important data which is important for reservoir simulation models.

## References

1. Knut, B. 2010. Petroleum Geoscience:from sedimentary environment to rock physics:Springer-Verlag Berlin Heidelberg.
2. Martin, Jonathan. Petroleum Geology Reservoirs. http://users.clas.ufl.edu/jbmartin/petroleum_geology/reservoirs.html