In-depth knowledge on petroleum reservoirs usually determines the credibility of geologists, petroleum engineer, geophysicists, and reservoir engineer. These professionals gather vital evidence acquired from seismic readings or logging of formations. Once the evidences are processed, the formations will then be classified as non-potential or potential reservoir. Potential reservoirs could be swollen with hydrocarbons. Following this classification, further investigations need to be done to measure the porosity and permeability of the surrounding rock. Porosity and permeability of the rock surrounding the reservoir will normally affect the cost of oil extraction and production. To understand more on petroleum reservoirs, this article will explain the topography of petroleum reservoirs, and the concepts of porosity and permeability. Besides that, this article will also introduce some factors that will affect the porosity of a rock and how it impacts on porosity calculation that is commonly done by petroleum engineers to estimate the spaces that can filled by potential hydrocarbons.
Petroleum reservoirs are generally a source of hydrocarbons that is beneath the earth crust. It may be under land or under water (offshore). The hydrocarbons in the reservoir are most commonly found inside a layer of porous rock or sedimentary rocks, just like water inside a sponge. Sedimentary rocks that holds hydrocarbons include sandstone, limestone and dolomite. Other than layers of porous rocks, petroleum reservoirs also require a layer of impermeable rock that functions as a seal, trapping the hydrocarbons in place.
Figure 1: An example of a petroleum reservoir containing Oil, Gas and Water
Porosity and Permeability
Porosity is defined as the amount of pore spaces that can accommodate fluids such as hydrocarbons and water. To put it simply, it is the capacity of the rock to hold fluids. This parameter is essential for reservoir experts to quantify the amount of hydrocarbons in place that determines whether that particular reservoir is profitable or not. However, rocks with high porosity can also be a problem if the pore spaces are not interconnected with each other. This is because reservoir fluids has difficulty flowing through the rock. Porosity is of no use if the rock is impermeable.
Permeability is defined as a measure of the ability of the rock to act as a medium that transmits fluids. The unit measurement used for permeability is Darcy or usually represented as ‘k’. A porous formation is not necessarily permeable, but highly porous formations are often highly permeable as well. Generally, sandstones and carbonates such as limestone and dolomites are the most common reservoir rocks as they are usually both porous and permeable. On the other hand, impermeable rock like shale which has low permeability usually serves as a good seal that traps hydrocarbons when petroleum accumulation takes place as shown in Figure 1.
Types of porosity
In terms of deposition, porosity is classified into two main categories which are primary and secondary. Primary porosity is the porosity that is initially developed in the reservoir rock ever during its deposition time while secondary porosity is a porosity that is developed after the initial porosity has been formed. Usually it is due to various geological activity and geochemical processes that impacts significant alteration in the reservoir rock characteristic. Some examples are grain dissolution in carbonates and sandstones, formation of vugs and fractures developed in some sedimentary rocks. Vugs are cavities that varies with size inside rock that results from dissolution or earth tectonic activity. When there is cavity, there will be more space for fluids to accumulate inside that particular rock. Hence, the rock will have higher porosity. The same concept is also applied to fractures developed in the rock.
In terms of connectivity, there is effective and absolute porosity. Generally, not all pore spaces are interconnected throughout the reservoir. However, some of them are well interconnected that it forms channels for fluid to flow easily. This porosity is called effective porosity and it is somewhat closely related to the rock’s permeability. Absolute porosity however is defined as the ratio of the total pore volume in a rock to the bulk volume of the rock. This parameters consider all of the pore spaces available in the rock let it be interconnected or isolated from each other.
Figure 2: Porosity and Permeability
Image source: http://petroleum101.com/what-is-a-petroleum-reservoir/
The right side of Figure 2 shows the rock is only considered as porous and permeable if there are spaces for fluid to accumulate in the rock and it is interconnected with each other which gives better fluid flow. This is an ideal characteristic of a good reservoir rock sought by petroleum engineers after simple porosity calculations is done. Porosity of a rock is calculated by finding the ratio of void spaces to total rock volume. As mentioned before, there are two types of porosity which are effective and absolute. Effective porosity, φeff is typically calculated using the total volume of pore spaces that is interconnected divided by the total volume of the rock or what engineers refer to as bulk volume. Similarly, Absolute porosity, φabs is then calculated using the total volume of pore spaces whether it is interconnected or not divided with the bulk volume. Refer to Figure 3 and Figure 4.
Figure 3: Formula used to calculate effective porosity of a rock
Figure 4: Formula used to calculate absolute porosity of a rock
Factors affecting porosity
Porosity can be affected by:
1.The shape of the grain particles
Particle shapes are generally classified into two types which are rounded and angular. Rocks with rounded grain particles has more porosity compared to the angular ones
2.Grain sorting arrangement
Rocks with well sorted arrangement usually gives a higher porosity. As shown in Figure 5, grains that are poorly sorted has less space for fluid to accumulate when compared to well sorted grains.
Figure 5: Sorting arrangement of grain particles and its shape
Image source: http://www.oakton.edu/user/4/billtong/eas100/sorting.htm
Porosity decreases as the amount of interstitial (in between the rock grains) and cementing material increases. Other than fluids, the pore spaces between the grains of rock can also be accommodate by smaller materials. These small interstitial materials or minerals that accommodate in between the rock such as quartz or cement can reduce the void spaces that are ideally meant for fluids.
4.Vugs and fractures
These are commonly non-interconnected pore spaces that is caused by dissolution of soluble materials after the formation of the rock. Some fractures of the rock however can be very useful as it is an important source of good permeability for low porosity rock.
Porosity and permeability is one of the main key aspect for petroleum engineers to roughly estimate the capacity of oil, gas or both that is available in a discovered reservoir. Even though it is just an estimation, these parameters play a crucial role for all oil and gas companies in determining whether the discovered petroleum reservoirs are worth investing with the costly current production technology.
© 2016 MSSA Malaysian Structural Steel Association All Rights Reserved.