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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] )
Carbonate
rocks are extremely diverse and their pore spaces complex and heterogeneous.
Large uncertainties in the petrophysical properties of carbonates are due
to wide variations in pore type, pore 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
pore 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 pore space is accurately determined via micro-CT
analysis
and the
porosity consistent with experimental
data
. Acoustic
velocity
-porosity,
pore 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. Pore
structural properties (pore size, aspect ratios, pore and throat shape
and connectivity) are determined. The correlations between pore 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.
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Many
studies have demonstrated the importance of the pore 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 pore type classifications
describe the pore structures but fail to quantify the pore system
for correlations to the rock’s physical properties. In order to
quantitatively describe 2-D pore size, pore surface roughness,
aspect ratio, and pore network complexity in carbonates, a digital
image
There
is now an opportunity to image and characterise the pore 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 pore 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 pore scale structure.
In this paper we describe the 3D study of a set of carbonate samples
including pore partitioning and simulation of the elastic properties
directly on image
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 pore structure. In all samples the 2-D pore 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
In
the recrystallized dolostone samples, the separation between pores
and grains (crystals) is easily achieved. In the moldic limestones
two pore 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
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
Macro/Microporosity and Macropore Phase Connectivity
The three dolostone samples exhibit 100% connected porosity; in Figure 3 we show an example of the pore partitioning within a slice of one dolostone sample and a 3D network structure for the same sample. The 3D volume allows displaying the pore connectivity. In the sample a number of pore structural parameters can be measured; for example, the average pore connectivity is measured as 15.7, meaning that a pore is connected to 15 other pores. The maximum pore connectivity is 83. Connectivity, pore size information and pore 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 systems 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
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 pore space topology and pore 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., pore connectivity
and pore to throat aspect ratios) and to acoustic References
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