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Carbonate Petrophysical Parameters Derived from 3d Images *

 

Mark Knackstedt1, Mahyar Madadi1, Christoph Arns1, Gregor Baechle2, Gregor Eberli2, and Ralf Weger2

 

Search and Discovery Article (2009)

Posted March 20, 2009

 

*Adapted from extended abstract prepared for presentation at AAPG Annual Convention, San Antonio, Texas, April 20-23, 2008

 

1Australian National University, Canberra, Australia ([email protected] )

2Comparative Sedimentology Lab, University of Miami, Miami, FL. ([email protected] ) 

 

Summary

 

Carbonate rocks are extremely diverse and their Previous HitporeNext Hit spaces complex and heterogeneous. Large uncertainties in the petrophysical properties of carbonates are due to wide variations in Previous HitporeNext Hit type, Previous HitporeNext Hit shape and interconnectivity. Petrophysical properties such as acoustic velocity, permeability, and resistivity are directly correlated to the amount and type of porosity, the dominant feature size, and the interconnectivity of different porosity types. Accurately measuring these attributes requires the quantitative 3D analysis of the Previous HitporeNext Hit structure of carbonates. In this article we describe the imaging and analysis of two types of carbonate core: a set of vuggy, recrystallized dolostones and a set of oomoldic limestones. The structure and topology of the Previous HitporeNext Hit space is accurately determined via micro-CT analysis and the porosity consistent with experimental data. Acoustic velocity-porosity, Previous HitporeNext Hit connectivity, and porosity permeability relationships are derived directly on the image data via numerical simulation and compared with measured data on the same rock. Acoustic velocity:porosity trends are good. Previous HitPoreNext Hit structural properties (Previous HitporeNext Hit size, aspect ratios, Previous HitporeNext Hit and throat shape and connectivity) are determined. The correlations between Previous HitporeNext Hit geometry and topology and elastic and flow properties can be directly probed in a systematic manner. Three dimensional imaging and analysis of carbonate core material can provide a basis for more accurate petrophysical modeling and improve carbonate reservoir characterization.

 

 

 

 

uSummary

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uIntroduction

uResults

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uSummary

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uIntroduction

uResults

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uConclusion

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uSummary

uFigures

uIntroduction

uResults

uPorosity

uConclusion

uReferences

 

 

 

 

 

 

 

 

 

 

 

 

uSummary

uFigures

uIntroduction

uResults

uPorosity

uConclusion

uReferences

 

 

 

 

 

 

 

 

 

 

 

uSummary

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uIntroduction

uResults

uPorosity

uConclusion

uReferences

 

 

Figure Captions

fig01

Figure 1. 2D slices within 3D tomographic images of (upper row) three recrystallized dolostones and (lower row) three oolitic grainstones.

fig02

Figure 2. Comparison of moduli measured for (a) recrystallized dolostones and (b) oolitic grainstones. The results on the full core are shown in yellow and the comparable data for the micro-CT image is shown in orange. Data on the full set of rock types are shown in blue.

fig03

Figure 3. (a) Previous HitPoreNext Hit partitioning of sample in upper left of Figure1 shown by different colors. (b) Left; focus on a subset of the Previous HitporeNext Hit network in a sub-sample of the entire volume. Right: Previous HitPoreNext Hit network of most of the volume of sample; over 250,000 pores and throats are visible. The thickness of the Previous HitporeNext Hit connection is a direct measurement of the Previous HitporeNext Hit throat.

fig04

Figure 4. Previous HitPoreNext Hit partitioning of sample in lower left of Figure1 shown by different colors; (a) shows the partitioning of all pores, while (b) shows the macroscopically connected pores only; a small subset of the Previous HitporeNext Hit space is connected; (c) shows the network based analysis of all pores while (d) shows the connected porosity.

Introduction

 

Many studies have demonstrated the importance of the Previous HitporeNext Hit structure in carbonates on petrophysical properties (e.g., Anselmetti and Eberli, 1993, Wang, 1997; Saleh and Castagna, 2004; Kumar and Han, 2005; Rossebø et al., 2005). Traditional Previous HitporeNext Hit type classifications describe the Previous HitporeNext Hit structures but fail to quantify the Previous HitporeNext Hit system for correlations to the rock’s physical properties. In order to quantitatively describe 2-D Previous HitporeNext Hit size, Previous HitporeNext Hit surface roughness, aspect ratio, and Previous HitporeNext Hit network complexity in carbonates, a digital image analysis (DIA) methodology was developed that produces repeatable quantitative Previous HitporeNext Hit shape parameters (Weger, 2006). Each of these quantitative parameters describes a certain aspect of the Previous HitporeNext Hit shape. When these parameters are compared to acoustic data, the two DIA parameters that capture the Previous HitporeNext Hit complexity and the Previous HitporeNext Hit size plus the amount of microporosity prove to be the most influential for the acoustic behavior of the samples. Each of these parameters explains about 60% of the variations in velocity at similar porosity (Baechle et al., 2004; Weger et al., 2005, Weger 2006). These 2D studies have added much to the understanding of the influence of the Previous HitporeNext Hit structure on acoustic properties, yet their 2D nature is a limiting factor for a comprehensive mathematical treatment of Previous HitporeNext Hit shapes in simulations of acoustic properties.

 

There is now an opportunity to image and characterise the Previous HitporeNext Hit structure of cores in 3D. This is based on coupling high resolution x-ray micro-tomography and high end computational software methods, including visualizing core material at the Previous HitporeNext Hit scale in 3D, measuring structural properties, and directly predicting physical properties directly from digitised 3D images (Arns, 2004; Arns, 2005) . This allows one to extend previous 2D studies to the 3D Previous HitporeNext Hit scale structure. In this paper we describe the 3D study of a set of carbonate samples including Previous HitporeNext Hit partitioning and simulation of the elastic properties directly on image data. Predictions from digital images are directly compared to measured data.

 

Results

 

The investigated samples are three recrystallized dolostones and three oolitic grainstones with moldic pores and variable amounts of microporosity. These samples were considered as they are end members in regards to Previous HitporeNext Hit structure. In all samples the 2-D Previous HitporeNext Hit structure was analyzed using the DIA methodology. In addition, porosity, permeability and the elastic properties were measured on the samples. 3D micro-CT scans (2.8 micron per voxel) were obtained on sub-samples of 5.5 mm in diameter drilled out of the original plug. Slices from images are shown in Figure 1. Microporosity is estimated by analyzing the attenuation of the x-ray signal, and its structure is correlated to analysis of SEM images at higher (down to 100 nm) resolution.

 

In the recrystallized dolostone samples, the separation between pores and grains (crystals) is easily achieved. In the moldic limestones two Previous HitporeNext Hit size populations are detected; large, partly leached, round pores and small pores (1-2 micron). In these samples mercury injection indicates presence of micro- or disconnected porosity (Figure 1 ). The volume of the microporosity in these three samples is 0.3% of 14.1%, 8.3% of 24.8%, and 10.4% of 19.5% total porosity. The porosity is calculated directly from the image data. In all samples the image porosity is comparable to the measured porosity.

 

From the 3D images the elastic properties of the samples are calculated using an elastic simulation (FEM) that takes into account both the solid matrix and the microporosity (Arns, 2002). Voxels in the solid matrix are assigned values of the elastic moduli for calcite (K,G = 63.7, 31.2 GPa) and the elastic properties in the microporous regions are based on the effective medium theory for sintered granular like structure in which the bulk (K) and shear modulus (G) are a function of porosity. Comparison of the simulated and measured sonic velocities are given in Figure 2 . For both the re-crystallized dolostones and moldic limestones, the calculated sonic velocity:porosity (Vp:porosity) does compare very well with the measured velocity.

 

Macro/Microporosity and Macropore Phase Connectivity

 

The three dolostone samples exhibit 100% connected porosity; in Figure 3 we show an example of the Previous HitporeNext Hit partitioning within a slice of one dolostone sample and a 3D network structure for the same sample. The 3D volume allows displaying the Previous HitporeNext Hit connectivity. In the sample a number of Previous HitporeNext Hit structural parameters can be measured; for example, the average Previous HitporeNext Hit connectivity is measured as 15.7, meaning that a Previous HitporeNext Hit is connected to 15 other pores. The maximum Previous HitporeNext Hit connectivity is 83. Connectivity, Previous HitporeNext Hit size information and Previous HitporeNext Hit shape can be directly used to calculate permeability and multiphase flow properties (Arns, 2005).

 

The oolitic limestone samples exhibit both resolvable macroscopic porosity and significant microporosity. Tests for connectivity of the resolved macroporous phase in the oolitic Previous HitsystemsNext Hit show that only one system is macroscopically connected at image resolution (see Figure 4 ). The other two samples exhibit disconnected macropores at the resolution of the image.

 

Conclusion

 

The results of the 3D imaging and analysis study show that the structure and petrophysical properties of carbonate core can be quantified in 3D. Specific conclusions include:

 

1. High-resolution CT scans with a resolution of 2.5 microns provide a 3D quantitative structure of carbonate samples. Image porosity correlates well with measured plug porosity.

2. Calculated sonic velocities from FEM simulation based on information from high-resolution CT scans compare well to measured sonic velocities.

3. Recrystallized dolostones exhibit well connected porosity which can be directly quantified in 3D. The Previous HitporeNext Hit space topology and Previous HitporeNext Hit size information is directly measurable. Moldic rocks exhibit significant proportions of microporosity (porosity below image resolution of ~5 microns). Only one of three moldic samples exhibits connected porosity in 3D.

4. Permeability:porosity crossplots for the same rock type will be undertaken. Correlation of permeability to key geometrical attributes (e.g., Previous HitporeNext Hit connectivity and Previous HitporeNext Hit to throat aspect ratios) and to acoustic data will be undertaken.

 

References

 

Anselmetti, F.S., and Eberli, G.P. 1993, Controls on sonic velocity in carbonates: Pure and Applied Geophysics, v. 141, p. 287-323.

 

Arns, C.H., Bauget, F., Limaye, A., Sakellariou, A., Senden, T.J., Sheppard, A.P., Sok, R.M., Pinczewski,W.V., Bakke, S., Berge, L.I., Oren, P.E., and Knackstedt, M.A., 2005, Previous HitPoreNext Hit scale characterization of carbonates using micro x-ray ct: SPE Journal, p. 475-484.

 

Arns, C.H., Knackstedt, M.A., Pinczewski, W.V., and Garboczi, E.G., 2002, Computation of linear elastic properties from microtomographic images: Methodology and agreement between theory and experiment: Geophysics, v. 67(5), p. 1396-1405.

 

Arns, C., Bauget, F., Ghous, A., Sakellariou, A., Senden, T.J., Sheppard, A.P., Sok, R.M., Pinczewski, W.V., Kelly, J., and Knackstedt, M.A., 2005, Digital core laboratory: Petrophysical analysis from 3D imaging of reservoir core fragments, Petrophysics, v. 46(4), p. 260-277.

 

Baechle, G.T., Weger, R., Massaferro, J.L., and Eberli, G.P., 2004, The role of macroporosity and microporosity in constraining uncertainties and in relating velocity to permeability in carbonate rocks: Ann. Internat. Mtg. Soc. Expl. Geophys.; Expanded Abstracts, Denver Colorado.

 

Kumar, M., and Han, D.-h., 2005, Previous HitPoreNext Hit shape effect on elastic properties of carbonate rocks: SEG Technical Program Expanded Abstracts, p. 1477-1480.

 

Rossebø, Ø.H., Brevik, I., Ahmadi, G.R., and Adam, L., 2005, Modeling of acoustic properties in carbonate rocks: SEG Technical Program Expanded Abstracts, p. 1505-1508.

 

Wang, Z., 1997, Seismic properties of carbonate rocks: Geophysical Development Series, v. 6, p. 29-52.

 

Weger R.J., 2006, Quantitative Previous HitporeTop/rock type parameters in carbonates and their relationship to velocity deviations: Ph.D. dissertation thesis, University of Miami, RSMAS, Miami, USA, 232p.

 

Weger, R.J., Baechle, G.T., Masaferro, J.L., and Eberli, G.P., 2004, Effects of porestructure on sonic velocity in carbonates: SEG Expanded Abstracts, v. 23, p. 1774.

 

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