Properties of reservoir rocks Porosity Permeability Fluid saturation Mahboob Ahmed
Porosity Ratio of the volume of space to the total volume of a rock. Porosity of a rock is a measure of its ability to hold a fluid. Porosity is expressed as a percentage of the total rock which is taken up by pore space. a sandstone may have 8% porosity. This means 92 percent is solid rock and 8 percent is open space containing oil, gas, or water Total porosity the ratio of the entire pore space in a rock to its bulk volume. Effective porosity percent of bulk volume occupied by interconnected pores spaces. A total porosity less the fraction of the pore space occupied by shale or clay In very clean sands, total porosity is equal to effective porosity.
Primary porosity porosity of the rock that formed at the time of its deposition. primary porosity of a sediment or rock consists of the spaces between the grains. Primary porosity decrease due compaction and packing of grains. Intergranular pores of clastics or carbonates. Primary porosity less than one percent in crystalline rocks like granite. more than 55% in some soils. Secondary porosity develops after deposition of the rock. Vugular spaces in carbonate rocks created by the chemical process of leaching. fracture spaces formed due to stress distortion in reservoirs rocks.
Porosity in sandstone• Sandstone usually has regular grains; and is referred to as a grainstone.• Porosity Determined mainly by the packing and mixing of grains.• Fractures may be present.
Porosity in Sandstone The porosity of a sandstone depends on the packing arrangement of its grains.
SANDSTONES POROSITY TYPESIntergranular (Primary)Interstitial Void Space Between Framework Grains.MicroporesSmall Pores Mainly Between detrital Framework Grains or Cement.DissolutionPartial or Complete Dissolution of or Authigenic Grains (Can Also Occur Within Grains)FracturesBreakage Due to Earth Stresses.
CARBONATES POROSITY TYPES Interparticle porosity Each grain is separated, giving a similar pore space arrangement as sandstone. Intergranular porosity Pore space is created inside the individual grains which are interconnected. Intercrystalline porosity Produced by spaces between carbonate crystals. Mouldic porosity Pores created by the dissolution of shells, etc.
CARBONATES POROSITY TYPES Fractured porosity Pore spacing created by the cracking of the rock fabric. Channel porosity Similar to fracture porosity but larger. Vuggy porosity Created by the dissolution of fragments, but unconnected.
Permeability The rate of flow of a liquid through a formation depends on: – The pressure drop. – The viscosity of the fluid. – The permeability. Permeability measures the capacity and ability of the formation to transmit fluids. it controls the directional movement and the flow rate of the reservoir fluids in the formation. The unit of measurement is the Darcy. Reservoir permeability is usually quoted in millidarcies, (md).
DARCY LAWK = permeability, in Darcies.L = length of the section of rock, in centimetres.Q = flow rate in centimetres / sec.P1, P2 = pressures in bars.A = surface area, in cm2.μ = viscocity in centipoise.
types permeabilityAbsolute PermeabilityWhen the medium is completely saturated with one fluid, then the permeabilitymeasurement is often referred to as specific or absolute permeabilityEffective PermeabilityWhen the rock pore spaces contain more than one fluid, then the permeability to aparticular fluid is called the effective permeability. Effective permeability is a measureof the fluid conductance capacity of a porous medium to a particular fluid when themedium is saturated with more than one fluidRelative PermeabilityDefined as the ratio of the effective permeability to a fluid at a given saturation to theeffective permeability to that fluid at 100% saturation.
PERMEABILITY AND ROCKS In formations with large grains, the permeability is high and the flow rate larger.The permeability in the horizontal direction is controlled by the large grains.
In a rock with small grains the permeability is less and the flow lower. The permeability in the vertical direction is controlled by the small grains Grain size has no bearing on porosity, but has a large effect on permeability.
CLASTIC RESERVOIRS Permeability• Determined mainly by grain size and packing, connectivity and shale content.
CARBONATE RESERVOIRSPermeabilityDetermined by depositionand post deposition events,fractures.
FLUIDS IN A RESERVOIR A reservoir normally contains either water or hydrocarbon or a mixture. The hydrocarbon may be in the form of oil or gas. The specific hydrocarbon produced depends on the reservoir pressure and temperature. The formation water may be fresh or salty. The amount and type of fluid produced depends on the initial reservoir pressure, rock properties and the drive mechanism. Initially, pore space filled 100% with water Connate water saturation remains in hydrocarbon zone.
Critical oil saturation, Soc For the oil phase to flow, the saturation of the oil must exceed a certain value which is termed critical oil saturation. At this particular saturation, the oil remains in the pores and, for all practical purposes, will not flow. Movable oil saturation, Som Movable oil saturation Som is another saturation of interest and is defined as the fraction of pore volume occupied by movable oil as expressed by the following equation:Som = 1 - Swc - SocwhereSwc = connate water saturationSoc = critical oil saturation
Critical gas saturation, SgcAs the reservoir pressure declines below the bubble-point pressure, gasevolves from the oil phase and consequently the saturation of the gasincreases as the reservoir pressure declines. The gas phase remainsimmobile until its saturation exceeds a certain saturation, called criticalgas saturation, above which gas begins to move. Critical water saturation, SwcThe critical water saturation, connate water saturation, andirreducible water saturation are extensively used interchangeablyto define the maximum water saturation at which the water phasewill remain immobile.
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