The
Relationship between Rank and Elastic Properties of Coal
Morcote, Anyela1,
This paper summarizes the bulk and
dynamic elastic properties of five dry rock coal samples and powder from
bituminous coal. The measurements correspond to one sapropelic cannel coal and
four humic coals. The humic coals cover different ranks, from bituminous to
anthracite.
The sapropelic coal presents the lowest
dry density which is 1.1 g/cc. Dry density in humic coals increases with coal
rank, ranging from 1.30 to 1.67 g/cc, the highest density corresponding to the
highest coal rank (anthracite). Porosity of coal rock samples ranges from 1.11
to 4.9%, whereas porosity of the powder coal sample is 20%.
Velocities were measured at increasing
confining pressure up to 40 MPa, in both loading and unloading cycles. Core
plugs were taken both parallel (90°) and perpendicular (0°) to bedding planes.
Coal velocities, and therefore bulk modulus, depend on both the applied
confining pressure and coal type and rank. Sapropelic coal presents the lowest
velocities of all samples and the smallest dry bulk modulus while velocities
and bulk modulus of humic coals increase with coal rank. The dependence of velocities
on confining pressure is greater at pressures up to 5 MPa, the closing pressure
for most of the cleats or fractures.
We obtained a linear empirical
relationship of Vp-Vs that covers a relatively wide range of confining
pressures and coal ranks.
The data show that coals have significant
intrinsic anisotropy at high confining pressures. This intrinsic anisotropy may
be due to very fine layering and preferred orientation of coal minerals.
AAPG Search and Discover Article #90063©2007 AAPG Annual Convention, Long Beach, California