Summary
A reservoir carbonate core plug has been imaged in 3D across a range of
length scales using high-resolution X-ray microtomography. Data from the
original 40-mm diameter plug was obtained at the vug scale and allows the size,
shape, and spatial distribution of the disconnected vuggy porosity to be
measured. Within the imaged volume over 32,000 separate vugs are identified and
a broad vug size distribution is measured. Higher resolution images on subsets
of the plug exhibit interconnected porosity and allow one to measure
characteristic, intergranular pore size. Pore scale structure and petrophysical
properties (permeability, drainage capillary pressure, formation factor, and
NMR response) are derived directly on the highest resolution tomographic
dataset. We show that data over a range of porosity can be computed from a
single plug fragment. Data for the carbonate core is compared to results
derived from 3D images of clastic cores and strong differences noted.
Computations of permeability are compared to conventional laboratory
measurements on the same core material with good agreement. This demonstrates
the feasibility of combining digitized images with numerical calculations to
predict properties and derive cross-correlations for carbonate lithologies.
Introduction
Carbonate reservoirs contain more than 50% of the world's hydrocarbon
reserves. In carbonate rocks, the processes of sedimentation and diagenesis
produce microporous grains and a wide range of pore sizes, resulting in a
complex spatial distribution of pores and pore connectivity. A reliable
petrophysical interpretation for predicting the transport properties and
producibility of carbonates is lacking.
Much of the poor reliability in estimating carbonate properties is due to
the diverse variety of pore types observed in carbonates. Unlike sandstones,
many carbonate sediments have a bi- or tri-modal pore size distribution with
organisms playing an important role in forming the reservoirs. Carbonate rocks
are further complicated by the significant diagenesis occurring through
chemical dissolution, reprecipitation, dolomitization, fracturing, etc. For
these reasons the size and shape of any porous network is expected to be very
heterogeneous and exhibit pore sizes ranging from sub-micron to meters.
Excluding fractures, three qualitatively different contributors to porosity can
be identified: Vuggy porosity (r ≥ 100 µm), intergranular (r ≥ 5 µm) and
intragranular (r < 5 µm).1 The sizes associated with the three types of
porosity may vary across studies and are given as indicative values only. These
features distinguish the petrophysical properties and productivity of carbonate
fields from other sedimentary rocks including sandstones and shales.
© 2005. Society of Petroleum Engineers
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History
- Original manuscript received:
6 June 2004
- Revised manuscript received:
21 July 2005
- Manuscript approved:
26 July 2005
- Version of record:
15 December 2005
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