Presentation on theme: "LABORATORY DETERMINATION OF POROSITY"— Presentation transcript:
1LABORATORY DETERMINATION OF POROSITY RESERVOIR PETROPHYSICSLABORATORY DETERMINATION OF POROSITY
2POROSITY DEFINITIONPorosity: The fraction of the bulk volume of a rock that is porous.Porosity is a static property – it can be measured in the absence of flowDetermining effective porosity requires fluid flow to determine if pores are interconnected = Porosity, expressed as fractionVb = Bulk volume of reservoir rock, ft3Vm = Matrix volume, ft3Vp = Pore volume, ft3Vb = Vm + Vp
3ROCK MATRIX AND PORE SPACE Matrix - non pore space; the grains of sandstone, limestone, dolomite, and/or shalePore space - filled with fluids: water, oil, and/or gas
4MEASUREMENT OF POROSITY Core samples (measure two of: Vb, Vp, or Vm)Openhole wireline logsSamples of the rock are obtained from drill cuttings or by coring. The drill cuttings are usually tiny and irregularly shaped, which limits our ability to use them.See sketches from previous lecture.Core samples are either obtained using the drilling rig with a special coring bit and barrel (whole core) or using a logging tool (side-wall coring). Whole cores are often 4 to 5 inches in diameter and are usually obtained in 30 or 60-foot segments. They are generally preferred for technical evaluation but they are also more expensive. Side-wall cores can be obtained by making an additional logging run with a special logging device. A geologist usually studies the initial logs and picks intervals where he thinks that he needs a formation sample. Side-wall cores are of less use to the engineer because they are often irregularly shaped and partially damaged (often fractured) from the side-wall coring process.Open-hole porosity measurement logs (density, neutron, and sonic) are routinely used to estimate formation porosity.
5LABORATORY DETERMINATION OF POROSITY Most methods use small samples (core plugs)multiple samples must be analyzed to get statistically representative resultssampling technique is importantoften all samples are taken from “sweet spots” skewing analysisTo determine porosity, measure 2 of 3volumetric parameters:Bulk volume, VbMatrix volume, Vm (also called grain volume)Pore volume, Vp
6Volume is an extensive property Fraction of volume consisting ofpores or voidsFraction of volume consistingof matrix
7MATRIX DENSITIES (m) OF TYPICAL PURE COMPONENTS OF RESERVOIR ROCK These values are important for core and log analysis. Commit them to memory.Unfortunately, few rocks consist of pure components but exist as a mixture of numerous minerals of varying sizes and compositions. Therefore, one can rarely assume the matrix density and get an accurate estimate of matrix volume.
8LABORATORY METHODS OF POROSITY DETERMINATION Bulk volume determinationsDirect calculationFluid displacement methodsGravimetricVolumetric – mercury pycnometer ( a precisely calibrated bottle)
9BULK VOLUME BY DIRECT MEASUREMENT Applicable for regularly shaped cores or core plugsCalculate from core dimensionsFor example; volume of right circular cylinderVb = Bulk volumed = DiameterL = LengthMost core analyses are conducted on core plugs that are cut from the whole core and are right circular cylinders. Special saws are used to cut the core plugs and their faces, so they are ready for special core analyses.Irregularly shaped cores require a different measurement.consistent units, usually cm
10LABORATORY METHODS OF POROSITY DETERMINATION Bulk volume determinationsDirect calculationFluid displacement methodsGravimetric (Archimedes) methodsVolumetric – in pycnometer
11ARCHIMEDES METHOD Wsat - Wdry fluid Vp = Wdry - Wsub Vm = Vb = =Wsat - WsubVb =Archimedes principle: buoyant force is equal to the weight of the fluid displaced.Self Study: Review difference between mass and weight.Self Study: A boat containing a person and a large rock floats in a swimming pool. The level of the water is marked on the side of the pool.Then, the person throws the rock out of the boat, and it sinks to the bottom of the pool. Does the water level in the pool rise, fall or stay the same?
12EXAMPLE 1 Bulk Volume Calculated by Displacement A core sample coated with paraffin immersed in a container of liquid displaced 10.9 cm3 of the liquid. The weight of the dry core sample was 20.0 g, while the weight of the dry sample coated with paraffin was 20.9 g. Assume the density of the solid paraffin is 0.9 g/cm3.Calculate the bulk volume of the sample.
13SOLUTION - Example 1 Weight of paraffin coating, Wparaffin = Weight of dry core sample coated with paraffin - Weight of dry core sample Wparaffin = g = 20.0 g = 0.9 gVolume of paraffin coating = Weight of paraffin / density of paraffinVparaffin = 0.9 g / 0.9 g/cm3 = 1.0 cm3Bulk volume of core sample = (Bulk volume of core coated with paraffin) – (volume of paraffin)Vb = 10.9 cm3 – 1.0 cm3 = 9.9 cm3(V = m/ρ)
14LABORATORY METHODS OF POROSITY DETERMINATION To determine porosity, measure 2 of 3basic parameters:Bulk volumeMatrix volumeAssume matrix (grain) densityDisplacement methodBoyles LawPore volume(Vm)(Vb)(Vp)
15LABORATORY METHODS OF POROSITY DETERMINATION Matrix (Vm)Assume rock density based on lithology and measure dry massDisplacement methodsvolumetricgravimetric (see previous description)Boyle’s Law:Ideal Gas Law: R = pV/nT; valid at low pressures and high temperatures (e.g. lab conditions).R is the Universal Gas Constant. The value and units of R are determined by specification of the units of p,V, n, and T.For example, R = (psia*ft^3)/(lbmol*deg.R), andR=8.314 (Pa*m^3)/(mol*K)Boyle’s Law simply says pV is constant if nT is constant.
16MATRIX VOLUME FROM MATRIX DENSITY Known or assumed matrix density
17APPLICABILITY AND ACCURACY OF MATRIX MEASUREMENT TECHNIQUES Known or assumed matrix densityAccurate only if matrix density is known and not assumedCore samples are often mixtures of several components with varying matrix densities, so density must be measured
18LABORATORY METHODS OF POROSITY DETERMINATION To determine porosity, measure 2 of 3basic parameters:Bulk volume2. Matrix volumeAssumed matrix (grain) densityDisplacement methodBoyles Law3. Pore volume(Vm)(Vb)(Vp)
19MATRIX VOLUME FROM DISPLACEMENT METHOD Reduce sample to particle sizeMeasure matrix volume of particles byVolumetric methodArchimedes method (gravimetric measurement)Volumetric - submerge particles into a liquid and observe change in liquid volume.Archimedes (gravimetric) - measure change in weight of particles submerged in liquid.
20EXAMPLE 2 SOLUTION Calculating the Matrix Volume and Porosity of a Core Sample Using the Displacement MethodBulk Volume, Vb = 9.9 cm3Matrix Volume, Vma = 7.7 cm3Porosity,It is total porosity.
21SOLUTION - Example 2 Calculate the Porosity of a Core Sample Using the Displacement Method and Matrix VolumeThe core sample from Example 1 was stripped of the paraffin coat, crushed to grain size, and immersed in a container with liquid. The volume of liquid displaced by the grains was 7.7 cm3.Calculate the matrix volume and the core porosity. Is this effective porosity or total porosity? (It is total porosity)Bulk Volume, Vb = 9.9 cm3Matrix Volume, Vma = 7.7 cm3The core sample from Example 1 was stripped of the paraffin coat, crushed to grain size, and immersed in a container with liquid. The volume of liquid displaced by the grains was 7.7 cm3.Calculate the matrix volume and the core porosity. Is this effective porosity or total porosity?=9.9 cm3 – 7.7 cm39.9 cm3= 0.22
22LABORATORY METHODS OF POROSITY DETERMINATION To determine porosity, measure 2 of 3basic parameters:Bulk volume2. Matrix volumeAssumed matrix (grain) densityDisplacement methodBoyles Law (Gas Expansion)3. Pore volume(Vm)(Vb)(Vp)
23MATRIX VOLUME FROM GAS EXPANSION METHOD Involves compression of gas into poresUses Boyle’s lawp1 = Pressure at initial conditions, psiap2 = Pressure at final conditions, psiaV1 = Initial volumeV2 = Final volumeConsistent units
24GAS EXPANSION METHOD TO CALCULATE Vma Initial conditions, with volumes of 2 cells knownPlace core in second cell, evacuate gas (air) from second cellOpen valve
25GAS EXPANSION METHOD TO CALCULATE Vma Initial conditionsP1CoreV1With V1 known, record p1p1 = pressure at initial conditionsV1 = Volume of cell 1Place cleaned, dried core sample in cell 2Evacuate cell 2Open valveValveclosedEvacuateCell 2Cell 1
26GAS EXPANSION METHOD TO CALCULATE Vma Final conditionsP1P2CoreWith V1 known, record p1p1 = pressure at initial conditionsV1 = Volume of cell 1Place cleaned, dried core sample in cell 2Evacuate cell 2Open valveValveopenCell 1Cell 2
27GAS EXPANSION METHOD TO CALCULATE Vma Vf = Volume of Cell 1 + Volume of Cell 2 - Matrix Volume of CoreVt = Volume of Cell 1 + Volume of Cell 2Vm = Vt - VfThis method assumes that the core becomes saturated with the gas. Incomplete saturation would lead to an overestimate of the matrix volume.
28APPLICABILITY AND ACCURACY OF MATRIX MEASUREMENT TECHNIQUES Displacement method - Very accurate when core sample is crushed without destroying individual matrix grainsGas expansion method - Very accurate, especially for samples with low porositiesNeither method requires a prior knowledge of core properties
29LABORATORY METHODS OF POROSITY DETERMINATION To determine porosity, measure 2 of 3basic parameters:Bulk volumeMatrix volumePore volume(Vb)(Vm)(Vp)
31PORE VOLUME FROM SATURATION METHOD Measures the difference between the weight of a core sample saturated with a single fluid and the dry weight of the corePore volume,Vp = Pore volume, cm3Wsa = Weight of core saturated with fluid, gwdry = Weight of dry core, gf = Density of saturated fluid, gm/cmMethod follows Archimedes Principle: A body wholly or partly immersed in a fluid is buoyed up with a force equal to the weight of the fluid displaced by the body.
32EXAMPLE 3 Archimedes Method of Calculating Porosity a Core Sample Using the gravimetric method with the following data, calculate the pore and bulk volumes and the porosity. Is this porosity total or effective?Dry weight of sample, Wdry = gWeight of sample saturated with water, Wsat = gDensity of water (f ) = 1.0 g/cm3Weight of saturated sample submerged in water, Wsub = gUsing the gravimetric method with the following data, calculate the pore and bulk volumes and the porosity. Is this porosity total or effective?Dry weight of sample, Wdry = gWeight of sample saturated with water, Wsat = gDensity of water, f = 1.0 g/cm3Weight of saturated sample immersed in water, Wsat,I = g
34Applicability and Accuracy of Pore Volume Measurement Techniques Saturation (Archimedes) methodAccurate in better quality rocks if effective pore spaces can be completely saturatedIn poorer quality rocks, difficult to completely saturate sampleSaturating fluid may react with minerals in the core (e.g., swelling clays)This method is more difficult to apply to core samples that require a jacket or rubber sleeve such as an unconsolidated sandstone. The jacket creates experimental problems, reducing its accuracy.This method cannot be used for determining porosity under confining stress, whereas the gas expansion (Boyle’s law) method can be conducted at multiple values of confining stress.
36PORE VOLUME FROM GAS EXPANSION METHOD Initial conditionsP1V1CoreThe core plug is placed in a Hassler sleeve, making the volume of Cell 2 equal to the bulk volume.This method is a continuation of the measurement of the matrix volume and uses Boyle’s law. The experiment is set up differently to measure pore volume.Boyle’s law:Initial cell conditions: measure V1 in Cell 1Put core in Hassler sleeve, evacuateValveclosedCell 1Cell 2
37PORE VOLUME FROM GAS EXPANSION METHOD Final conditionsP1P2CoreValveopenCell 1Cell 2
38PORE VOLUME FROM GAS EXPANSION METHOD Very accurate for both high-quality (high ) and low-quality (low ) core samplesShould use low-molecular-weight inert gases (e.g., helium)Measures effective (connected) pore volumeThe gas expansion method (Boyle’s law method) is probably the preferred method for measurement of core porosity, except for samples that are not perfect right cylinders or ones with large surface vugs or chips. The method is preferable for poorly consolidated samples that require a rubber sleeve or jacket.One advantage of the method is that it is accurate and reasonably fast. Another important feature is that the measurement can be made at confining pressures approximating reservoir stress conditions.
39SUMMARY To determine porosity, measure 2 of 3 basic parameters: Bulk volumeMatrix volumePore volume
40CORES Allow direct measurement of reservoir properties Used to correlate indirect measurements, such as wireline/LWD logsUsed to test compatibility of injection fluidsUsed to predict borehole stabilityUsed to estimate probability of formation failure and sand productionCores from the reservoir allow direct measurements of important reservoir properties. It is important to gather cores from a representative part of the reservoir, as reservoir properties vary horizontally and vertically. If a reservoir is known to be highly heterogeneous, many core samples will be required to describe the reservoir accurately.