Shale Analysis Using Rotary Sidewall Samples

Mihai A. Vasilache
SCAL, Inc., Midland, TX

New laboratory techniques and associated testing equipment were developed to measure the following shale properties on one or two rotary sidewall samples:
Desorption isotherms, lost and residual gas measurements
Tight rock analysis
Sorption Isotherms
Micro fracture porosity and permeability at confining stress
Dynamic rock mechanics
Geochemistry (TOC, rock evaluation)
X-ray diffraction, SEM, Thin section description
Acid solubility
Capillary suction time (CST) and completion fluid optimization
Mercury Injection Capillary pressure and pore size distribution

Unlike coal, shale does not have a natural fracture system (cleat). The small grain size of the shale (mud and clay range) will generate a very small matrix permeability (nano Darcy). This very low matrix permeability, combined with the fast recovery offered by the rotary sidewall coring tool, makes these samples good candidates for collecting quality desorption data.
The small sample size makes it possible to get desorption much faster. A six month’s full diameter desorption project can be performed in days with sidewall samples. A fast desorption prevents the errors associated with hydrogen generation by anaerobic bacterial growth in the full diameter desorption canisters.

After analyzing a large number of wells, it was found that the total gas content in shale is variable:

For this reason the averaging technique currently used, where 10-20 sidewall samples from various depths are sealed inside the same desorption canister, can turn an excellent prospect into a mediocre one.

The technology can accurately find the “sweet gas zone” before horizontal drilling begins. This technique is cost effective and provides major savings when compared with the cost of a full diameter core project.

AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas