Basic Collector Parameters
Rocks that have the capacity to contain oil, gas and water and give them in industrial quantities during development are called collectors. Most reservoir rocks are of sedimentary origin. The collectors of oil and gas are terrigenous (sandstones, siltstones and some clay rocks), carbonate (limestones, dolomites), siliceous (radiolarites, spongolites). In rare cases, collectors may be igneous and metamorphic rocks. The nature of the void space in the rocks is determined by the texture features of the rock, the size and shape of the mineral grains, the composition of the cement, the ability of the rocks to fracture.
The main parameters of the reservoirs are porosity and permeability.
Porosity is the fraction of the void space in the total rock volume. The value of the porosity can be expressed as a percentage or fractions of a unit.
There are common, open and effective porosity. The total (total, absolute) porosity is the volume of all pores in the rock.
In industrial evaluation of oil and gas deposits, open porosity is taken into account - the volume of only those that are connected, communicate with each other.In petroleum geology, along with the concepts of general and open porosity, there is the concept of effective porosity, which is determined by the presence of such pores from which oil can be extracted during development. Subcapillary and isolated pores are considered ineffective.
Another important parameter characterizing the filtration properties of reservoir rocks is the permeability - property of the rocks to pass through liquids and gases. Permeability is expressed in fractions of a square meter. Typically, the permeability measured parallel to the bedding is higher than the permeability defined perpendicular to the bedding.
There are several types of permeability: absolute, phase (effective) and relative .
Absolute permeability - permeability measured in dry rock with dry inert gas passing through it (nitrogen, helium); often it is measured by air.
Phase (effective) permeability - the ability of a rock to pass through itself one fluid in the presence of others; for individual fluids depends on their quantitative ratio. This is especially noticeable in the development of the field. When pumping and reducing the amount of oil in the reservoir, its phase permeability gradually decreases.
Relative permeability is the ratio of the effective permeability of a given fluid to the permeability value at 100% rock saturation with the given fluid. It is continuously changing during the exploitation of the deposit, since the ratio of fluids varies. The relative permeability of the rock to any fluid increases with increasing its saturation with this fluid.
Reservoir fluids - oil, gas, water - are accumulated in the hollow space of the reservoir rock, represented by pores, caverns and cracks. According to the prevailing type of voids, reservoir rocks are divided into pore, cavernous, fissured and biopustaceous [9, 10].
Pore (granular) are mostly sandy aleuritic rocks and some differences of carbonate - oolitic, clastic limestones. The hollows of the collectors are represented by pores, their dimensions do not exceed 1 mm (Figure 89).
Fig. 89. Porcine connectors
Fractured reservoirs can be sedimentary rocks, igneous and metamorphic. Cracks determine mainly the permeability of these formations. As a fissure collector, sedimentary rocks are most often carbonaceous, but there are also sandy aleuritic and even clayey rocks, which previously could have been petroleum-producing (Figure 90).
Fig. 90. Fractured collectors
Cavernous reservoirs are most often associated with zones of leaching with the formation of voids (caverns) in the carbonate strata. Cavity sizes exceed 1 mm. Void space is also formed during metasomatic replacement of calcite with dolomite (Figure 91).
Fig. 91. Cavernous collectors
Biopoolot collectors are associated with organogenic carbonate and siliceous rocks, the voids are of intra-skeletal and inter skeletal nature (Figure 92).
According to the formation time, all types of voids can be primary, formed together with the rock, and secondary, formed already in the finished rock. Pores are more often primary, and caverns and cracks are secondary. In the carbonate rocks, there may also exist relict voids, for example, cavities of shells.
Fig. 92. Biopustot collectors
The influence of post-sedimentation processes on the change of a void space
After completion of sedimentation, the porosity of the formed sand sediment is called hypergenic sedimentation. Subsequent processes of diagenesis and catagenesis (densification, carburization, regeneration) contribute to a reduction, reduction of free pore space (Figure 93).
Fig. 93. Reduction of the hole space in sandstones due to secondary processes. Grinds
Along with the decrease in porosity of rocks at depth, processes sometimes develop that contribute to the increase of pore space: dissolution, leaching, recrystallization, cracking, metasomatism (Figure 94).
Fig. 94. Processes that promote the formation of secondary porosity in reservoir rocks. Grinds
Conservation of oil and gas accumulations in reservoir rocks is impossible if they are not blocked by impermeable to fluids (oil, gas and water) by rocks - fluids (tires, screens). The best tires are the salt-bearing strata, but are most common in this quality of clay.
The screening properties of clay depend on their composition, strength and consistency, sandiness or silty, secondary changes, fracturing. A great importance is also found in clays water and organic matter.
The most important quality of clays for forming shielding properties is plasticity - the most important quality of clays, which provides the ability to rebuild the structure under the influence of the applied load without disrupting the continuity of the clay-layered formation. It excludes mechanical failure in the breakdown of oil and gas under excessive pressure (up to a certain limit). However, as the pressure rises for a sufficiently long time, the plasticity limit can be traversed, the clay becomes brittle and fragile and loses its shielding properties.
Salts, gypsum and anhydrite are tires, although a slow but steady flow of hydrocarbon passes through their thickness. More plastic rock salt covers are better in quality than anhydrite and gypsum. As the depth increases, the plasticity of salts and sulfate rocks increases, and their shielding properties also improve.Pokryshki, belonging to the category of density, are usually formed by the thicknesses of homogeneous monolithic, crack-free finely crystalline limestones, less often dolomites, marls , argillites. Carbonate tires are characteristic for oil deposits of platform areas, for conditions of gently sloping rocks.
Regional, zonal and local tires differ in area of distribution. Regional tires have a wide area distribution, characterized by considerable power and lithological consistency. They are usually kept within individual oil and gas bearing areas. Zonal tires can withstand at least one oil and gas accumulation zone. Local tires have a limited distribution, often occupying the area of one or more deposits. They determine the safety of individual deposits and the nature of their distribution in the context of the deposit.
Carbonate tires are often associated with the same collectors, the boundaries between them have a very complex surface. For
Carbonate tires are characterized by their rapid acquisition of insulating capacity (due to the rapid lithification and crystallization of the carbonate sediment). For density tires, power, increasing the strength of the rock as a whole, is of great importance.
Density tires lose their integrity at great depths due to the appearance of cracks in the mechanical formation.
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