Learning module title

Effective porosityEffective porosity implies the ratio of the total volume of interconnected voids Vp to the bulk volume Vb of the rockEffective porosity is the percentage of interconnected void space with respect to the bulk volume.

Formula:

= effective porosity VP = total volume of interconnected voids Vb = bulk volume

BackNext1 General Aspects2 Idealised Models3 Measurements of PorosityDevelopersReferencesTopic OverviewTitlepage5Effective porosity is typically less than total porosity.Effective porosity is the interconnected pore volume or void space in a rock that contributes to fluid flow or permeability in a reservoir.It excludes isolated pores and pore volume occupied by water absorbed on clay minerals or other grains.Effective porosity depend on several factors like rock type, heterogeneity of grain sizes and their packing, cementation, weathering, leaching, type of clay, its content and hydration, etc.

Genetically the following types of porosity can be distinguished:Rock media having both fracture and intergranular pores are called double-porous or fracture-porousmedia.

Intergranular porosityFracture porosityMicro- porosityVugular porosityIntragranular porosity

BackNext1 General Aspects2 Idealised Models3 Measurements of PorosityDevelopersReferencesTopic OverviewTitlepageIntergranular porosity

Unfilled interparticle porosity (in Oolite). Porosity is black.BackNext1 General Aspects2 Idealised Models3 Measurements of PorosityDevelopersReferencesTopic OverviewTitlepage7Fracture porosityFracture porosity is a form of secondary porosity generated by tectonic fracturing of the rockSuch porosity can develop in any rock, allowing the development of productive reservoir in rocks such as granites and gneisses

A strongly fractured chalk from an area of only mild deformation. Such fractures are commonly late diagenetic, and postdate most other diagenetic features in the rock.NextBack1 General Aspects2 Idealised Models3 Measurements of PorosityDevelopersReferencesTopic OverviewTitlepage8A strongly fractured chalk from an area of only mild deformation. Such fractures are commonly lte diagenetic, and postdate most other diagenetic features in the rock.Absolute porosityTotal or absolute porosity is the total void space in the rock whether or not it contributes to fluid flow

Formula:

Vb = bulk volume VPa = total void volume voids a = absolute porosity

BackNext1 General Aspects2 Idealised Models3 Measurements of PorosityDevelopersReferencesTopic OverviewTitlepagePotential Error in Porosity EstimationExperimental dataInvolve a degree of uncertainty related to the possible measurement errors

The measurement of porosity is normally a function of Vp, Vm and/or Vb

BackNext1 General Aspects2 Idealised Models3 Measurements of PorosityDevelopersReferencesTopic OverviewTitlepage

Estimation of porosity accounting to this model:Regular Rhombohedral-Packed Spheres

BackNext1 General Aspects2 Idealised Models3 Measurements of PorosityDevelopersReferencesTopic OverviewTitlepage21Idealised porous medium represented by regular system of rhombohedral-packed spheres.Nomenclature in section 3.2Nomenclature :

p Pressurep1 Initial pressurep2 Final pressuren Total numbers of molesR Universal ideal gas constantT TemperatureV VolumeVref Reference volumeVS Volume of the sample chamberVg Grain volume

BackNext1 General Aspects2 Idealised Models3 Measurements of PorosityDevelopersReferencesTopic OverviewTitlepageNomenclature in section 3.3Nomenclature :

Vb Bulk volumem1 mass of the sample in a cup of waterm2 mass of the sample in air water density

BackNext1 General Aspects2 Idealised Models3 Measurements of PorosityDevelopersReferencesTopic OverviewTitlepage

Estimation of porosity accounting to this model:

Parallel Cylindrical Pores

BackNext1 General Aspects2 Idealised Models3 Measurements of PorosityDevelopersReferencesTopic OverviewTitlepage18Idealised porous medium represented by a system of parallel cylindrical pores (pipes).

It is rather obvious that rocks do not have pores like this and this model gives a unrealistically high porosity value.

This model may though, be used in some situations where fluid flow under simplified conditions is modelled.As we can se the porosity is independent of radius.

Estimation of porosity accounting to this model:Regular Orthorhombic-Packed Spheres

BackNext1 General Aspects2 Idealised Models3 Measurements of PorosityDevelopersReferencesTopic OverviewTitlepage20Idealised porous medium represented by a regular system of orthorhombic-paced spheres.Consolidated From the point of view of pores susceptibility to mechanical changes, one should distinguish between consolidated and unconsolidated porous mediaConsolidated porous media pertain to sediments that have been compacted and cemented to the degree that they become coherent, relatively solid rockA typical consequences of consolidation include an increase in density and acoustic velocity, and a decrease in porosity

Sandstone with quartz cement and secondary porosity

BackNext1 General Aspects2 Idealised Models3 Measurements of PorosityDevelopersReferencesTopic OverviewTitlepageGeneral aspectsOne may distinguish between two types of porosity, namely absolute and effectiveAbsolute and effective porosity are distinguished by their access capabilities to reservoir fluidsArt-micrograph of sandstone with oilBackNext

Void spacescontributesto absoluteporosityPermeablespacescontributesto effectiveporosity1 General Aspects2 Idealised Models3 Measurements of PorosityDevelopersReferencesTopic OverviewTitlepage3Porosity is the best known physical characteristic of an oil reservoir. It determines the volume of oil and gas present, and all recovery computations must be based on knowledge of its value. The porosity constitute the part of the total porous rock volume which is not occupied by rock grains or fine mud rock, acting as cement between grain particlesPorosity is the ratio of void volume to the bulk volume (grains plus void space). This void space consists of pore space between grains or crystals, in addition to crack space. A pore is a discrete void within a rock, which can contain air, water, hydrocarbons or other fluids. In a body of rock, the percentage of pore space is the porosity.In sedimentary rocks, the amount of pore space depends on the degree of compaction of the sediment (with compaction generally increasing with depth of burial), on the packing arrangement and shape of grains, on the amount of cementation, and on the degree of sorting. Typical cements are siliceous, calcareous or carbonate, or iron-bearing minerals. Porosity determines the storage capacity of the sand and is generally expressed on a percentage basis or as a fraction or a decimal. In oil reservoir, the porosity represent the percentage of the total space/pore volume or void space or the volume within the rock that is available for occupancy by either liquids or gases/that can contain fluid.One may distinguish two types of porosity, namely, absolute and effective.

Porosity Estimation from Geophysical Well LogsPorosity can be estimated from:

Formation resistivity factorMicroresistivity logNeutron-gamma logDensity (gamma-gamma) logAcoustic (sonic) logBackNext1 General Aspects2 Idealised Models3 Measurements of PorosityDevelopersReferencesTopic OverviewTitlepage44Sediment porosities can be determined from numerous borehole log measurements (reviewed by Serra, 1984). At Sites 994, 995, and 997 we have attempted to use data from the lithodensity (HLDT), neutron porosity (CNT-G), and electrical resistivity (DITE) logs to calculate sediment porosities. Core-derived physical property data, including porosities (Shipboard Scientific Party, 1996a, 1996b, 1996c), have been used to both calibrate and evaluate the log-derived sediment porosities.

ReferencesFigures taken with permission from the authors ofReservoarteknikk1: A.B. Zolotukhin and J.-R. Ursin

Figures also taken with permission from Ola Ketil SiqvelandBackNext1 General Aspects2 Idealised Models3 Measurements of PorosityDevelopersReferencesTopic OverviewTitlepageSection 3.2: Grain-Volume Measurements Based on Boyle`s LawHelium gas is often used due to its following properties:

The small size of helium molecules makes the gas rapidly penetrate small poresHelium is an inert gas that will not be absorbed on the rock surface and thus yield erroneous results

Alternatives: N2 and CO2

BackNext1 General Aspects2 Idealised Models3 Measurements of PorosityDevelopersReferencesTopic OverviewTitlepageSection 3.3: Bulk-Volume MeasurementsFluids normally used:

Water which can easily be evaporated afterwards.

Mercury which normally not enters the pore space in a core sample due to its non-wetting capability and its large interfacial energy against air.

A very accurate measurement, with a uncertainty of 0,2%.BackNext1 General Aspects2 Idealised Models3 Measurements of PorosityDevelopersReferencesTopic OverviewTitlepageSection 3.3: Bulk-Volume MeasurementsExample: Uncertainty analysis in measuring the bulk volume using Archimedes` principle.The core is measured in two steps:Weighing the sample in a cup of water; m1 (Assuming 100% water saturation) Then weighting the sample in air as it is removed from the cup; m2The bulk volume is:

Differentiating the equation above gives us:

BackNext1 General Aspects2 Idealised Models3 Measurements of PorosityDevelopersReferencesTopic OverviewTitlepageSection 3.3: Bulk-Volume MeasurementsIf the density measurement as well as the two mass-measurements above, is considered to be independent measurements, the relative uncertainty in the bulk volume is:

It may also be written as:

If the uncertainty in determined the water density is estimated to 0,1% and the weighting accuracy is equal to 0,1g , we find a relative uncertainty in the bulk volume of approximately 0,5%.

BackNext1 General Aspects2 Idealised Models3 Measurements of PorosityDevelopersReferencesTopic OverviewTitlepageSection 3.4: Pore-Volume MeasurementsCalculations of the pore volume

It is important to notice that the Hassler core holder has to be coupled to a volume of known reference, Vref.

BackNext1 General Aspects2 Idealised Models3 Measurements of PorosityDevelopersReferencesTopic OverviewTitlepageNomenclature in section 3.4Nomenclature :

p0 pressurep1 pressurep2 pressureVp Pore volumeVref Reference volumen Total numbers of molesR Universal ideal gas constantT Temperature

BackNext1 General Aspects2 Idealised Models3 Measurements of PorosityDevelopersReferencesTopic OverviewTitlepageSection 3.5: Fluid-Summation MethodThe laboratory procedure provides the following information:First sub sample gives the rock`s weight, WS1 , and the volumes of oil, Vo1 , and water, VW1 , are recorded.

Second sub sample gives the volume of gas, Vg2 , and the rock`s bulk volume, Vb2.

Fraction of the gas-bulk volume:

Also:

BackNext1 General Aspects2 Idealised Models3 Measurements of PorosityDevelopersReferencesTopic OverviewTitlepageSection 3.5: Fluid-Summation MethodThe formation oil- and water factor are calculated as follow:

The sum of the fluid-volume factor then gives the porosity value:

BackNext1 General Aspects2 Idealised Models3 Measurements of PorosityDevelopersReferencesTopic OverviewTitlepageSection 3.5: Fluid-Summation MethodExample: Use of pycnometer in matrix volume calculation.In order to define the matrix volume, Vm , of a core sample, the following measuring steps are carried out:The pycnometer cell is fully saturated with mercury.The pycnometer piston is withdrawn and a gas (air) volume of V0 is measured.The core sample is placed in the cell, and the cell volume is sealed. The equilibrium condition inside the cell is written:Mercury is injected into the cell and a new gas volume, V1 , and pressure, is measured.New equilibrium is reached and we write:Finally; the matrix volume is found as follows:

BackNext1 General Aspects2 Idealised Models3 Measurements of PorosityDevelopersReferencesTopic OverviewTitlepageNomenclature in section 3.5Nomenclature :Fg,o,w Fraction of the gas, oil, water-bulk volumeSg,o,w Saturation of gas, oil and waterVg1 Gas volume of the first subsampleVb1 Bulk volume of the first subsampleVg2 Gas volume of the second subsampleVb2 Bulk volume of the second subsample PorosityVo1 Volume of oil in the first subsampleVw1 Volume of water in the first subsampleWS1 Weight of the rock in the first subsampleWS2 Weight of the rock in the second subsample Apparent bulk density of the fluid-saturated rock sample

BackNext1 General Aspects2 Idealised Models3 Measurements of PorosityDevelopersReferencesTopic OverviewTitlepagePotential Error in Porosity Estimation

BackNextIf the porosity is defined asThe equation can be differentiatedThe potential error of prosity measurement is then1 General Aspects2 Idealised Models3 Measurements of PorosityDevelopersReferencesTopic OverviewTitlepage46Lyd side 56-57 i boka