Kansas Geological Survey, Open-file Report 2004-42
by
K. David Newell and David Grisafe
Kansas Geological Survey
KGS Open File Report 2004-42
Sept. 15, 2004
Coal and dark shale samples from the Pennsylvanian section in eastern Kansas (encompassing the Forest City basin, Cherokee basin, and Bourbon arch) were selected for density measurements and ashing experiments as part of an on-going coalbed gas research project at the Kansas Geological Survey. These samples consist of both core and drill cuttings. The cuttings samples were gathered from both air- and mud-drilling rigs.
Drill cuttings are characteristically lithologically mixed, with the mixing being due to differential lifting up the well bore, minor caving of the well bore, and collection of several thin-bedded lithologies during sampling. The mixed lithological characteristics of cuttings requires processing to separate out cuttings composed of pure coal. In this study cuttings were washed of drilling mud and then dried in air for several days. After drying, the cuttings were dry sieved into five size fractions: >0.0930", >0.0661", >0.0460", >0.0331", and <0.0331". The coal (or shale) fraction was isolated by sorting the cuttings in each size fraction with aid of a dissecting microscope.
Processing of the core samples was simpler. Coal and shale cores were washed of drilling mud and dried in air for several days, or dried for two or three days in an oven at 125 °F (50 °C).
Cuttings and core density measurements were made utilizing separate techniques. Cuttings samples (4 to 5 grams) were weighed and then placed in water in a 10-cc graduated cylinder to determine the volume of the sample. The volume of the sample is determined by rise in the water level in the cylinder. Core samples were weighed and then immersed in water in a beaker filled to its brim. Placing the sample in the beaker caused the displaced water to spill from the beaker into another container. This displaced water was subsequently weighed. The weight of the water displaced by the sample is thus easily converted to volume by using 1 gram/cc for the density of the water. Measurements were repeated three times for each sample and then averaged.
Full proximate analyses were carried out for selected core samples. Proximate analysis characteristically is done with a substantial amount of sample -- at least several hundred grams. Core samples selected for proximate analysis were prepared by cutting the core in half along its vertical axis, with half being archived and half being utilized for the proximate analyses. Proximate analyses were carried out at Luman's Laboratories in Chetopa, Kansas.
A less elaborate and less expensive ashing experiment was carried out for cuttings and some core samples. This simple ashing utilizes a muffle furnace at the Kansas Geological Survey. Samples are first weighed and then subjected to a 110 °C temperature until their weight stabilizes. This first firing, which lasts about 24 hours, approximates moisture content. A second firing at 750 °C for three to four days essentially ashes the sample. Two crucibles of sample are utilized for both the 110 °C and 750 °C firings. Each crucible is filled with approximately 1.5 grams of pulverized coal (i.e., <0.0460" sieve size). Results are accepted if the difference in weight loss for each sample is less than 2%.
Results for simple ashing and proximate analyses for selected samples are presented in Table 1. [Available as an Excel file (84k) or an Acrobat PDF file (60k)] Two hundred forty-seven samples from 34 wells are presented. Cores are represented by 115 samples, with the remainder being cuttings.
A comparison of ash contents by proximate analysis and ash contents by the simple ashing technique show a general correlation (Figure 1). Scatter in this diagram is likely be due to a sampling problem in that it is difficult to get a representative sample of the coal core with just 1.5 grams of sample that is used for the simple ashing.
Figure 1. A comparison of ash contents derived by proximate analysis vs. that obtained from an abbreviated ashing experiment (see text for discussion).
Comparison of density and ash content of cuttings and core samples in a cross plot (Figure 2) shows an increase of ash content with density. The general bimodal distribution of points in this diagram reflects the lithologies studied (i.e., coal and shale), and the lack of thick stratigraphic intervals that represent transition between these two distinct lithologies. Scatter for the proximate analysis and the simple ashing appears to be similar. The similar scatter with both ashing techniques suggests the scatter may be due more to variations in lithology than sample size or technique.
Figure 2. Density vs. ash content for Pennsylvanian coals and shales in eastern Kansas.
Density vs. sample depth (Figure 3) shows the density of shale generally increases with increasing depth. This is probably due to compaction. Coals show no such trend. Overlap of some data points in this graph is due to some samples, although identified as "coal" or "shale" by stratigraphic position, should more likely be characterized respectively as organic-rich shale or high-ash coal.
Figure 3. Density and ash content as a function of subsurface depth.
Moisture vs. ash content (Figure 4) shows no trends, although in general, shales, which have high ash content, show slightly higher moisture content than coals, which have low ash content. The cause of the extraordinarily high moisture content of some samples is not known, but it may be due to incomplete drying of the sample before analysis.
Figure 4. Moisture vs. ash content for coal and shale samples.
Proximate analyses and a less-elaborate ashing procedure produce comparable results for ash content and moisture of coals and shales in Kansas. In general, the density of a coal and shale increases with increasing ash content, and this relationship can be used to approximate ash content of coals and shales by their density. High-resolution density logging can thus be utilized for a quick approximation of ash content in areas where core or cuttings samples are not available.
The density of shales increases with depth, but this same relationship is not evident for coals in eastern Kansas. No strong relationship exists between ash content and moisture content in coals or shales, but shales generally have slightly higher moisture content than coals.