Recovery of hydrocarbons from the reservoir is an important process in petroleum engineering and estimating permeability can aid in determining how much hydrocarbons can be produced from a reservoir. Permeability is a measure of the ease with which a formation permits a fluid to flow through it. To be permeable, a formation must have interconnected porosity (intergranular or intercrystalline porosity, interconnected vugs, or fractures).
To determine the permeability of a formation, several factors must be known: the size and shape of the formation, its fluid properties, the pressure exerted on the fluids, and the amount of fluid flow. The more pressure exerted on a fluid, the higher the flow rate. The more viscous the fluid, the more difficult it is to push through the rock.Viscosity refers to a fluids internal resistance to flow, or its internal friction. For example, it is much more difficult to push honey through a rock than it is to push air through it. Permeability is measured in darcies. Few rocks have a permeability of 1 darcy, therefore permeability is usually expressed in millidarcies or 1/1000 of a darcy.
Permeability is usually measured parallel to the bedding planes of the reservoir rock and is commonly referred to as horizontal permeability. This is generally the main path of the flowing fluids into the borehole. Vertical permeability is measured across the bedding planes and is usually less than horizontal permeability. The reason why horizontal permeability is generally higher than vertical permeability lies largely in the arrangement and packing of the rock grains during deposition and subsequent compaction. For example, flat grains may align and overlap parallel to the depositional surface, thereby increasing the horizontal permeability, see Figure 25. High vertical permeabilities are generally the result of fractures and of solution along the fractures that cut across the bedding planes. They are commonly found in carbonate rocks or other rock types with a brittle fabric and also in clastic rocks with a high content of soluble material. As seen in Figure 25, high vertical permeability may also be characteristic of uncemented or loosely packed sandstones
Basic Petroleum Geology and Log Analysis Halliburton 36 Basic Petroleum Geology 2001, Halliburton Examples of variations in permeability and porosity Some fine-grained sandstones can have large amounts of interconnected porosity;
however, the individual pores may be quite small. As a result, the pore throats connecting individual pores may be quite restricted and tortuous; therefore, the permeabilities of such fine-grained formations may be quite low. Shales and clays which contain very fine-grained particles often exhibit very high porosities. However, because the pores and pore throats within these formations are so small, most shales and clays exhibit virtually no permeability. Some limestones may contain very little porosity, or isolated vuggy porosity that is
not interconnected. These types of formations will exhibit very little permeability. However, if the formation is naturally fractured (or even hydraulically fractured), permeability will be higher because the isolated pores are interconnected by the fractures. POROSITY IS NOT DEPENDENT ON GRAIN SIZE PERMEABILITY IS DEPENDENT ON GRAIN SIZE
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