Proceedings of the 11th SEGJ International Symposium, 2013
Seismic Rock Physics of the South Sumatra Basin Coal, Indonesia
Yuda Faisal Yushendri(1), Artini Sukotjo(1), Rusalida Raguwanti(1), Djedi Setyo Widarto(1) and
Bagus Endar B. Nurhandoko(2)
(1) Upstream Technology Center, PT. Pertamina (Persero), Indonesia (yuda.yushendri@pertamina.com), (2) Institut
Teknologi Bandung, Indonesia.
ABSTRACT
We have collected some coal samples in different ranks
from Muara Enim, South Sumatra Basin, Indonesia.
Laboratory ultrasonic velocity measurements of gas
and brine saturated coals samples in different rank
present that dynamic elastic properties of coal depend
on coal rank, overburden pressure and pore pressure. In
this paper, we investigate the relationships between
coal rank and pressure with dynamic elastic properties
of coals. We also present the effect of various coal rank
and pore pressure to the AVO response.
Coal elastic properties are strongly affected by
pressure. Coal velocities increases as overburden
pressure increases, while velocities decreases as pore
pressure increases. The increasing of pore pressure
reflects in the increasing of AVO intercepts and
gradients, while the increasing of coal rank tends to
decrease AVO intercept and gradients.
We determine the sensitivity of elastic properties
to variations of pore fluid for all coal samples. P-
impedance, S-impedance, and MuRho are good fluid
discriminator for lignite, bituminuous and anthracite
coal samples whileas they can be used as facies
discriminator for all coal samples. Another Lamé
parameter, LambdaRho, is only sensitive to
discriminate fluid in lignite and bituminuous coal
samples. Poisson’s Ratio and VP/VS Ratio are only
capable to separate the pore fluid in lignite coal
sample.
The AVO modeling example offered in this paper
shows that AVO may be used as an aid for pressure
delineation in the CBM reservoirs, to delineate the
area with high fracture density or high pore pressure.
KEY WORDS: CBM, elastic properties, coal rank,
pressure, AVO response, South Sumatera Basin
INTRODUCTION
Coal is a carbonaceous rock formed from the remains
of plants that lived and died million years ago by
diagenesis process. The chemical and physical
structure of coal changes as the pressure and
temperature increases through time. The coal rank is
depend on the degree of maturation during the coal
diagenesis. The coal rank are classified into lignite,
sub-bituminous, bituminous, semianthracite, and
anthracite; the latter has the highest rank than the
former. During the rock formation, methane gas are
generated from organic matter and stored within the
coal.
In recent decades, methane stored in coal beds has
become an important source of energy in some
countries, with a great potential for future
development. Previous authors have conducted study
of elastic properties of coal. Yu et al. (1993) presented
the increasing of acoustic velocities in dry and water
saturated coal samples as confining pressure increased
from 2 to 40 MPa. Yao and Han (2008) analized the
effects of pressure, temperature, and saturation on
ultrasonic velocities in coal samples. Morcote et al.
(2010) observed ultrasonic velocities increases as the
confining pressure increased from 0 to 40 MPa. Dry
bulk and dry shear moduli increase with increasing
coal rank whereas the VP/VS ratio decreases with
increasing coal rank. Dirgantara et al. (2011) have
measured seven coal samples and reported that
compressional and shear wave velocities increase as
coal rank increases.
AVO is a tool to detect the presence of
hydrocarbon for gas-sand reservoir. The Poisson’s ratio
contrast at shale/sand interface will cause the variation
of amplitude with offset. Poisson’s ratio contrast at a
shale/gas-sands interface is caused by gas saturation,
while in the coal seam it caused by the density increase
of the cleats and fractures (Gregory, 1976). Thus, AVO
is not a direct method to detect the presence of CBM in
coal seams. However, AVO still can be used in CBM
reservoir characterization to delineate methane rich
sweet spots within coal seams (Peng, et. al., 2006).
This paper presents the rock physics analysis of
coal samples from Muara Enim, Indonesia, with
different coal ranks, to study the effect of pore
pressure, overburden pressure and fluid saturation in
seismic elastic properties and AVO responses.
METHODOLOGY
Seismic rock physics measurement of coal core
samples were done in the laboratory. There are four
different coal samples measured in different coal rank
as summarized in Table 1. The samples were collected
from surface outcrop. The objective of this physical
modeling is to find the relationship between seismic
elastic properties and the reservoir parameters.
