Estimating Reservoir Rock Strength Directly from Observation of Sedimentary Characteristics
Twenty-nine core plug samples of a wide range of Uniaxial Compressive Strength (UCS) values were investigated using a suite of techniques including visual inspection at micro-scale on hand specimens and on thin sections, microscopic examination, Scanning Electron Microscopic (SEM) analysis, laser particle analysis and X-ray Diffraction analysis to characterize their compositions, texture and sedimentary properties. An intimate intrinsic link is noticed between sedimentary features observed at meso-scale and that at the micro-scale, both of which can be directly correlated to rock strength. It has been demonstrated that one can empirically estimate the rock strength (UCS range) from an inspection of core plug samples. Specifically it has revealed that:
of low UCS values (e.g. 0-5 MPa) are usually weakly cemented, poorly sorted,
primarily clay-matrix supported, heterogeneous sandy shale, shaley sandstone
and siltstone with little or no authogenic cementation with framework grains
not in direct contact.
(2) Samples with high UCS values investigated (e.g.
>50 MPa) are mostly well cemented, well sorted framework grain supported
sandstone with minor clay content, low to moderate visual porosity and moderate authogenic quartz overgrowth and calcite authogenic cementation with framework
grains in concave-convex and intergrown contact nature.
(3) Samples with intermediate UCS values have sedimentary features in between (1) and (2).
A Rock Property Index parameter which combines a number of sedimentary parameters is found to be useful in qualifying/ranking the rock strength (UCS) of the twenty-nine core samples investigated. A set of criteria of sedimentary characterization is proposed for empirically estimating the rock strength of core samples including the lithological composition, heterogeneity, cementation, quartz and clay content, grain contact nature, grain size distribution and visual porosity. This method may also be applied to describe cutting samples at rig-site to provide timely information of the strength of the formation rocks while drilling.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California