Sandstone Reservoirs - Research Paper Example

Lecturer Sandstone Reservoirs Introduction A reservoir is defined as a natural chamber below the earth’s surface comprising of rock material where fluids migrate and are stored. The underground rocks have different levels of permeability and porosity. These properties influence a reservoir’s ability to produce and accumulate fluids such as water and hydrocarbons. Porosity which refers to percentage of the pore space is what determines the storage capacity of the rock. Movement of fluids in the network of rock microscopic pores is controlled by fluid and rock properties. Important factors geologists consider in reservoir studies are structure, depth, stratigraphy, faults, reservoir pressure, fractures and aquifer system (Abdus, Iqbal and Buchwalter 27). It has been observed that reservoir rock properties vary widely from one location to another. Reservoirs allow transmission of fluids under the influence of certain forces. The origin and development of reservoir properties are the deposition processes that occurred during prehistoric times. The main types of reservoirs include sandstones, carbonate rocks, shales and conglomerates (Abdus, Iqbal and Buchwalter 40). Types of Reservoirs 1. Sandstone Reservoirs Sandstone refers to a mass of a sedimentary rock made up of small sand sized rock grains or minerals. A continuum of sandstone rocks is what makes up a sandstone reservoir. Sandstone reservoirs generally have high permeability and porosity distributions represented by stochastic reservoir heterogeneity. The textural features of these reservoirs give an indication of the environment where they are deposited. Their characteristic colors are dark brown, light beige and tan. Sandstones are classified based on amount of grains of a specific chemical composition. Examples are arkosic and quartz sandstones. The predominant grains in arkosic and quartz sandstones are feldspar and quartz respectively. Porosity of sandstone reservoirs ranges from 10% to 30% (Renpu 36). Their intergranular porosity is as a result of sorting. The best sandstone reservoirs are the ones in which grains are all of the same size, devoid of clay and well rounded. They should also be coarse so that immovable fluid films that stick to them do not take up much of the pore space available. Examples of sandstone reservoirs are found in Western Canada Sedimentary Basin, coastal basins of Equatorial Africa, United States’ Gulf Coast, Rocky Mountains and Southern North Sea (Stoneley 39). 2. Carbonate Rock Reservoirs Carbonate reservoirs comprise of geologic formation that are naturally fractured which are characterized by heterogeneous permeability and porosity distributions. These are the predominant types of reservoirs all over the world accounting for about 50% of all reservoirs. The distinctive aspect of carbonate reservoirs is their intrabasinal origin. Their mode of formation was primarily dependent on organic activities. They were formed through biochemical processes in special environments. Organisms involved in their formation also contributed in determination of their qualities. Carbonate reservoirs are highly susceptible to processes of modification as a result of post-depositional mechanisms. The variations observed among various carbonate reservoirs result from processes such as lithification and compaction (Ahr 50). A notable feature of these reservoirs is their high sensitivity to environmental changes. Changes in temperatures affect biogenic activities thus affecting sediment production. This aspect is what makes carbonate reservoir development depth dependent. Favorable conditions enhance organic productivity while in unfavorable conditions productivity ceases. Carbonate reservoirs are considered autochthonous since they develop close to their final depositional sites. Water energy and basin configuration influences deposition which leads to formation of carbonate reservoirs. Examples of carbonate reservoirs are found in Big Horn Basin in Wyoming and south Caspian basin which encompasses water areas of South Caspian Sea, land areas of Western Turkmenistan and eastern Azerbaijan (Ahr 63). 3. Shale Reservoirs A shale reservoir refers to a mass of rock comprising of siliceous brittle and fractured shale. The rock is fine grained as a result of compacted silt and small sized clay particles. An important characteristic of shale reservoirs is their laminated and fissile nature. These reservoirs exhibit high variability in composition. They have low natural permeability and elevated amounts of total organic carbon. Examples of areas with shale reservoirs are Santa Maria basin in Monterey and Barnett Shale Basin in Texas (Zou 309). 4. Conglomerates Reservoirs These reservoirs are formed in environments with enough transport energy that can move large grains. Cementing in these reservoirs is facilitated by minerals such as calcite, gypsum and clays. An important characteristic of conglomerate reservoirs is high permeability as a result of numerous pore spaces. This makes them good storage and transport media for fluids. An example of a conglomerate reservoir is found in Qizhong area of Tibet region of China (Zou 299). Differences between Sandstone Reservoirs and Other Types of Reservoirs Carbonate reservoirs differ from sandstone reservoir in that carbonate reservoirs are mainly composed of plants and animals remains that grew almost at the same place. Carbonate reservoirs are more brittle compared to sandstone. They develop better vugs and fractures that are relatively important for flow and storage of fluids. Carbonate reservoirs are also harder and tighter than sandstone reservoirs (Renpu 9). Primary porosity of sandstone reservoirs is exclusively interparticle while that of carbonate reservoirs can either be intraparticle, interparticle, intercrystalline, vuggy, fenestral or cavernous. Porosity-permeability relationships of sandstones are relatively consistent and dependent on particle texture while that of carbonate reservoirs are highly varied and independent of particle size (Ahr 53). Shale reservoirs differ from sandstone reservoirs in that shale reservoirs can contain organic mudstones which are not present in sandstone reservoirs. They can also have open fractures which are not common in sandstone reservoirs. Conglomerate reservoirs differ from sandstone reservoirs in that conglomerate reservoirs exhibit an extra-high heterogeneity as a result of a complex pore structure. They also display rapid changes in permeability and porosity between their layers which is not the case with sandstone reservoirs (Zou 298). Advantages of Sandstone Reservoirs Porosity of sandstone reservoirs is uniform over time since it does not diminish with increasing saturation of the reservoir. Chemical processes do not affect the permeability and porosity of sandstone reservoirs. Effects and of micro and macro organisms do not impede fluid transfer through sandstone reservoirs as can be the case with carbonate reservoirs (Renpu 11). Porosity prediction is easy in sandstone reservoirs since processes such as leaching and cementation do not significantly affect them; making them relatively uniform. Sandstone reservoirs have a high capacity for fluid storage due to the fact that their surface area to volume ratio is high. They also have a great resistance to loss of porosity (Stoneley 41). Disadvantages of Sandstone Reservoirs In the case that grains in a sandstone reservoir are poorly sorted, smaller grains occupy the spaces left between the larger grains leading to a reduction in porosity. Similarly presence of clay mixed with sand grains blocks the gaps and pores thus reducing the reservoir porosity. Fluid release of sandstone reservoirs is relatively slow compared to reservoirs like a carbonate reservoirs (Stoneley 41). Conclusion Reservoirs play an important role in transport and storage of fluids beneath the earth’s surface. They are made up of different types of materials and formed by different processes. This is what gives each type of the reservoirs its unique characteristics. Common types of reservoirs all over the world are sandstones, carbonate rocks, shales, and conglomerates. Each of these is unique in chemical and physical properties and formation environment. Sandstone reservoirs are high porosity and permeability reservoirs common in many parts of the world. They exhibit relative uniformity and their characteristics are not significantly affected by chemical processes. Absence of fractures is also a distinguishing feature of these types of reservoir. Works cited Abdus, Satter, Ghulam M. Iqbal and James L. Buchwalter. Practical Enhanced Reservoir Engineering: Assisted With Simulated Software. Denver, CO: Penn Well Books, 2008. Print. Ahr, Wayne M. Geology of Carbonate Reservoirs: The Identification, Description and Characterization of Hydrocarbon Reservoirs in Carbonate Rocks. New York, NY: John Wiley & Sons, 2011. Print. Renpu, Wan. Advanced Well Completion Engineering. Houston, TX: Gulf Professional Publishing, 2011. Print. Stoneley, Robert. Introduction to Petroleum Exploration for Non-Geologists. New York, NY: Oxford University Press, 1995. Print. Zou, Caineng. Unconventional Petroleum Geology. Oxford, UK: Newnes, 2012. Print. Read More