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Water saturation refers to the pores in a rock that are occupied by water. Water saturation in rocks is defined by the level of porosity which is the amount of pores that a rock has to accommodate water. Different rocks have different degrees of porosity that define the amount of water that each can hold. There are also other different factors that jointly contribute to the differences in porosity levels of rocks. Both chemical and physical properties as well as the ambient conditions around a rock determine the amount of water that it will absorb. There are numerous effects of water saturation within rocks since it determines the rate and type of weathering process that a rock exhibits. It also affects the various human economic activities taking place in a particular region. This document discusses the various concepts related to water saturations. Some of the areas covered in the document include factors that influence water saturation, properties of a rock that determine the saturation degree and water saturation effects.
Water is regarded as a chemical impurity when suspended in oils. Saturations that are found from assessment of the bearing rocks display various problems associated with water saturation in the oilfields. Some of these problems include additive reduction, oil oxidation, and corrosion as well as reduced quality of lubrication (Parrett, 2011). Water saturation in oilfields can therefore be a cause of major degradations of oil quality. The levels of saturation should be monitored to prevent degradation of the oil. The best thing to do is to prevent build up of water by monitoring its content even before it reaches saturation levels.
Water exists in oil in various phases, free, emulsified or dissolved states, depending on the level of saturation. First, water can be dissolved in the oil bearing rocks. The oil can have scattered water molecules that are hard to note unless through use of various screening processes to determine the levels of water in it. As a result, the oil can have considerable amounts of water in it without a possible indication of it (Parrett, 2011). The oil becomes saturated once the level of water dissolved in it exceeds the threshold value. Further increase in the amount of water in the emulsion leads to its separation into two sections, that of water and the other for emulsified oil.
Use of Karl Fischer’s test helps to determine the percentage per volume of water that oil can hold. However, this test does not differentiate between the three states of water in oil. Since most of the damage is caused by water that is in free and emulsified states, a more elaborate screening approach is employed. The new technology known as Pickering’s approach intends to establish the relationship between water saturation measurements using Karl Fischer’s approach and how to integrate it with alarming systems (Parrett, 2011). Pickering approach aims at testing oil for any possible alarming conditions. It aims at quick identification of the high levels of water to allow counteractive action to be taken before the situation can deteriorate.
Oil belongs to the hydrocarbon group of organic chemicals; therefore, it bears the characteristics of hydrocarbons in relation to water. During an experiment to find out the level of saturation of water in oil, if the concentration of a sample of oil in terms of water saturation matches the expected value, it can be used to represent the status of crude oil in the reservoir. Such an experiment would help to estimate the saturation levels of water in an oilfield. Then, preventive measures can be taken to contain the problem before it persists (Parrett, 2011).
Factors that influence water saturation
Under this category, there are quite a hefty number of reasons as to why different types of rocks will have different amounts of water in their pores. It has been noted that sometimes rocks within the same geographical area will have different water saturation rates and similarly, some in different geographical areas will share similar saturation levels. Some of these areas are covered below (Collins, 1975).
Lithology of a rock refers to its physical characteristics in terms of texture, grain size, color, composition and microscopy among others. Depending on the different types of rock textures, different rocks will have different degrees of water saturation. This follows the fact that the texture of a rock explains the connection between sole grains and the other huge particles making up the rock (Reddy, 2007). The larger the particles of a rock are the higher the rate of porosity and consequently the rate of water accumulation. Large particles allow passage of water more easily unlike with the rocks that have lesser texture.
Another factor is considering the amount or quantity of water that can flow into or outside the rock due to the influence of both time and pressure. This characteristic of rocks is influenced by the size of the pores between rock particles. In case a rock has tiny pores, water cannot easily infiltrate meaning that the rock cannot absorb water (Donaldson & Tiab, 2004). On the contrary, if the pores are large, water can easily infiltrate making the rock qualify to be absorptive. When there is run off over an impermeable rock there is less infiltration to the ground which leads to a high volume of water flow (Donaldson & Tiab, 2004). Among other reasons associated with saturation dissolving of chemical elements which are likely catalyze the weathering process.
The amount of water within a rock is defined by the amount of pores it has. Fused rocks like granite, Shale, limestone and sandstone have more complicated types of pores unlike with the alluvial sediments. This can be divided into two main parts namely the connected and unconnected porosity. The degree of connected porosity is effectively measured with the amount of water that flows into the rock. On the other hand, fluids cannot access the unconnected pores. There are different types of porosities with the primary porosity being the main system in a rock and consequently enhancing the other subsequent types. Secondary porosity is the other type which enhances the overall passage of water within the rock like fracture porosity, vuggy, open, dual, macro and micro porosity (Reddy, 2007)
Availability of water
Rocks can be permeable to allow water infiltration, but there might be less or no water to fill the pores. Water availability is therefore a crucial determinant of the amount of water saturation that will take place in a rock. Areas that receive high precipitation are likely to have rocks that are highly saturated but only if they are permeable (Donaldson & Tiab, 2004). Those that are impermeable are likely to remain dry. Other rocks, mostly those at the river bed or close to the oceans are mostly impermeable hence are less saturated despite the fact that they are always next to a source of water.
A normalized equation of water saturaton is as shown below
Where, Sw is the water saturation
Swi is the minimal water saturation
Sorw is the minimal oil saturation