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Factors That Affect Reservoir Quality in Sedimentary Rocks
Storage, distribution, and flow of liquid are regulated by the reservoir. The capacity to store and deliver hydrocarbon determines the quality of the reservoir. Deliverability is defined by permeability whereas reservoirs' size and effective porosity determine the capacity of the hydrocarbon storage. The percentage of the interconnected pores volume in the sedimentary rocks is known as porosity. The remaining space is occupied by the sedimentary rocks matrix.
The ability of liquids to be transmitted within the rocks is called permeability and is measured in Darcies. Permeability is a function of distribution, size, and shape of the pores in the rocks, the type and number of fluid present and the rate at which the fluid flows within the rock. Permeability and porosity are directly related within the clastic rocks. Reservoir heterogeneity is a term used to define a reservoir's geological complexity and its relationship to the flow of fluids that pass through it. The relationship is not always the same because it varies depending on the formation and the kind of rock. When porosity increases, permeability increases too.
Factors Affecting Reservoir Quality
Reservoir quality of sedimentary rocks is affected by the deposition environment. It also determines the initial pore connections that have been deposited and the ones that have been buried shallowly. The reservoir and geological properties of the majority of sedimentary rocks mainly depend on the interplay of the sea level, tectonics, climate, sediment supply, and biological and physical processes of deposition and sediment transport (Dandekar, 2013). Geometric arrangement is produced through the interactions of the processes on the basin scale and involves different depositional environment through the process called stratigraphic architecture. The external and internal geometry of the sedimentary rocks are controlled by the process on a smaller scale. The interpretation of depositional environments and lithofacies becomes significant for the evaluation of a reservoir on a smaller scale. This emphasizes the grain characteristics that control permeability and porosity (Dandekar, 2013).
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The characteristics of clastic rocks include the size of the grain, packing, and many matrix materials. Sedimentary rocks with the best reservoir quality have well-rounded grains that don’t have matrix material which makes them well-sorted. The sedimentary structures mostly affect the initial reservoir because they impart a pattern preferential in the reservoir (Civan, 2015). Laminations, planar bedding, and various stratification features can lead to a stratified planar flow in a situation where the barriers of permeability such as graded beds, partings of clay, and laminae, that are finely grained, are available. Slump structures produce a bad flow path reducing permeability. They can also increase permeability by producing faults that are small in size, and this, in turn, leads to a looser packing of grain.
Pore network of sedimentary rock is redefined by diagenetic. The major variation in the degree of lateral and vertical continuity and reservoir quality level within oil and gas fields is controlled by depositional factors. (Zhang, Pe-Piper & Piper, 2015). The rock property inhomogeneities may reverse the trends produced by dispositional controls influencing the reservoir properties.
Major inhomogeneities are also produced by diagenetic alteration. The processes of diagenesis can be divided into two main categories: physical and chemical (Amos, 2011a, p. 28). As well as diagenetic features of sedimentary rock, reservoir quality is also influenced by textural parameters such as grain orientation, shape, and packing of grains (Amos, 2011a, p. 16). The development of barriers of permeability or extreme permeability stratification can lead to the need to repose the location of wells or drill addition wells, inject and perforate reservoir units and emphasize the revision of the decisions relating to the suitability of the thermal recovery operations.
A complex diagenetical reservoir has major inhomogeneities that affect the fluid's production and distribution which are mostly controlled by the events of diagenetic. Diagenetic inhomogeneities are an area of more or less permeability of porosity generated through a combination of processes including cementation, dissolution, replacement, and fracturing. (Bjørlykke, 2014). A complex reservoir has diagenitic inhomogeneities neither correlated nor controlled by the factors affecting deposition. Mjor diagenetic mechanisms affecting the reservoirs' quality include dissolution and recrystallization as well as compaction and cementation. Controlling of the mechanism can be achieved by burial depth of the rock, pore fluid pressure, burial time, burial temperature, and pore fluids. (Rahman & Worden 2016).
Permeability and porosity of the rock are reduced when the rocks are compact, what causes rearrangement and makes them tight. Flow into the pore throats are allowed by the plastic deformation of the ductile grains, the pressure solution being in the form of stylolitization and grain suturing (Ritzi, Freiburg & Webb, 2016). Some sedimentary rocks experience a decrease in porosity and permeability because their ductile grains are forced to flow plastically into other pores. Fracture shattered by brittle grainsor may also collapse in the case of some fossils and porous grains. The sedimentary rocks made up of strong minerals undergo little permeability and porosity reduction during the process of compaction because of grain rotation and rearrangement (Zhou et al., 2016).
Cementation and Dissolution
Cementation is a critical factor referring to the filling of initial pore space by cement and occurring during the early or late stages in the diagenetic history of sedimentary rock. Reservoir quality is usually reduced by the precipitation of authigenic minerals. However, the original porosity can be preserved by the early formation of authigenic minerals ensuring that the sedimentary rock is protected from later degradation by cementation or compaction. The dissolution of minerals that are less chemically stable in sedimentary rocks can significantly influence the increase in permeability and porosity. Dissolution is also very important in the sedimentary rocks buried to shallow depths (Tiab & Donaldson, 2015).
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Recrystallization and Structural Deformation
The alteration of cement and grains to clays and the recrystallization of carbonates have a considerable impact on the quality of a reservoir in carbonates and sandstones. Most sedimentary grains represent the recycled material derived from the erosion and weathering of the sedimentary rocks already existing (Schacht, 2011 p. 14). The dolomitization of calcite cement or limestones in sandstones increases permeability and porosity. Clay replacement, on the other hand, increases the general porosity of the sedimentary rock. However, the pores associated with the clay minerals may have micropores that may contain irreducible water. The clay flakes may also mix with the flowing pore fluids and clog the pore throats. In most cases fracture porosity is low, and it only provides about one percent of porosity (Tiab & Donaldson, 2015).
However, there are other instances where fractures in large reservoirs may have considerable reserves. The fracture permeability may be high to the levels of tens of Darcies, and it is known to be directional in nature. On the other hand, the fractures filled by mineralization may produce a barrier which is perpendicular to the direction of the fracture. This might lead to brecciation along the fracture or the fault zones due to dissolution and shearing. Brecciation can increase permeability and porosity considerably except the areas where mineralization has already occurred (Schön, 2015).
Capillary Pressure and Wettability
Wettability affects the production amount of water. Flowing through the pore system, water occupies the central parts of the pores where the sedimentary rock is oil wet. Water-wet reservoir restricts the water flow to the system pores until the removal of the oil. The water saturations, that are irreducible, are higher than those of the oil-wet reservoirs. Capillary pressure is critical because it determines the reservoir quality in sedimentary rocks (Chris et al. 2015).
Capillary pressure of a reservoir has a great impact on the distribution and magnitude of water saturation and, therefore, the hydrocarbon volume also relates to capillary pressure. Capillary pressure is determined by the interfacial tension, the capillary radius, and the angle of contact between the solid and the water. Zones with large pores throats and pores in a reservoir have a less irreducible saturation of water, high hydrocarbon pore volume, and less capillary pressure. Sedimentary structures are formed as a result of sediment transport conditions (Amos, 2011b, p. 7). If they include variation in grain size, for example, heterolithic stratification (Amos, 2011c, p. 41), they could impact the reservoir quality by acting as barriers to the flow and, thus, reducing permeability as one of the principal components oof reservoir quality (Amos, 2011a, p. 16).
Assessing Various Factors That Influence Reservoir Quality
The factors determining the reservoir quality can be assessed using different methods. The methods are grouped into two groups according to their impact on the reservoir quality. The first method is microscopic while the second is macroscopic. Seismic data is the key determinant in both methods because it is used in detecting the quality of the reservoir. The evaluation and assessment of the data predetermines the steps which will be taken to implement the necessary procedures. Porosity lithology and fluid saturations affect the impedance of sedimentary rock (Kenny et al. 2016). The relationship is realized by analyzing the seismic estimates and the properties of the rock in the laboratory. Wireline logs are grouped into three categories depending on the information they provide. The first and the second group have many similarities because they both consist of neutron logs. The last group contains fluid saturation logs including resistivity logs. Permeability can also be determined from a combination of log responses (Kenny et al. 2016).
The spontaneous potential log has mostly been used as an indicator of the degree of permeability of a formation. The drill stem testing, also known as formation testing, is also a form of macroscopic technique that can be used to measure the reservoir quality. The test should be performed once the well has been completely conditioned and all the zones have been sealed. The procedure is carried out to allow the continuous production of fluids. The next step involves testing the fluids for any hydrocarbon content and measuring the flow and pressure rates (Zhou et al. 2016). The pressure measured for a long time determines the permeability while the flow rates and different types of fluids are used to measure the productive capability.
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The procedures performed on the obtained samples are important in the reservoir quality of the sedimentary rocks because they are more accurate. Summation of fluids or the Boyle’s Law can be used to measure and analyze the core porosities. There are two methods which can be exploited to determine the permeability of air: the steady state and the unsteady state methods. The first method is employed to determine the permeability of air while the other - to measure pressure changes and flow rates (Civan 2015). The second method is usually adopted for measuring less permeable samples (Zhou et al. 2016).
The microscopic techniques used to determine the reservoir quality include the petrographic image analysis, thin section analysis, X-ray diffraction, and the scanning electron microscopy. Thin section analysis is the most popular technique compared to the rest, and it determines the distribution and types of pores as well as the degradation by diagenesis. The SME method can also be used to assess the reservoir quality. This method facilitates a thorough examination of a sedimentary rock at high magnification with a good depth of field so that the clay minerals and pore network can be seen. The SME method is applied with the energy dispersive X-ray which provides the analysis of cement, grains, and clays (Bjørlykke, 2014).
The potential for formation is facilitated by the analysis which is very important to be implement. It is necessary because it helps in providing porosity and permeability values for various samples that are not qualified, such as the cuttings and sidewall cores. The main aim of the analytical procedure is to determine the geometrical characteristics using a certain research grading system. The functioning of the system is rather complex because it generates an image representing the material and porosity of the rock from the parts which were not damaged when the sample was obtained. The purpose of the image is to determine the perimeter and diameter which are then used to determine permeability and porosity (Civan, 2015).
The present paper revealed that porosity and permeability of a sedimentary rock determine its reservoir quality. There are many factors which can affect the reservoir quality of a sedimentary rock, but the most important are the environment of deposition because it determines the quality of the reservoirs which is the most significant aspect while assessing the rock. There are various methods which can be used to assess the factors affecting the reservoir quality in sedimentary rocks. They range in scale from the macroscopic to the microscopic ones. The primary task of geologists is to come up with better mechanisms and techniques for controlling the factors affecting the reservoir quality of sedimentary rocks so as to encourage further research in the area and offer reliable information.
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