Core samples were recovered from the 2, 900 meter B gas bearing bed of HigashiNiigata NS-13 well. Purpose of this paper is to verify the log analysis results derived from various porosity tools by some special core analysis data.
Recently, much progress of porosity tools was achieved. Combination of any tools may be useful for evaluating the effective porosity and clay (shale) content of reservoir rock.
In this study, core porosities were measured after the method proposed by Associate Prof. Hirakawa in 1, 959. Porosities under the reservoir conditions were properly obtained from this method. Then, the porosities of this reservoir rock range from 12 to 15 percents. On the other hand, cross-plot of Sonic-Formation Density shows the effective porosities of about 10 percents. Thus, it is shown that the correlation of data obtained from both log interpretation and special core analysis methods was established fair well.
On the contrary, core porosities measured by the routine core analysis procedure were extremly high. Dehydration and drying of the core samples at considerable high temperatures should give a fracturing effect on the shale fraction contained in the reservoir rock. Insofar as the (very) shaly formations found in Japanese fields frequently, porosity values derived from the routine analysis procedure are not the representatives of porosity in situ.
Clay and/or shale contents in the reservoir rock were analysed in the laboratory using the X-ray diffraction and dry sieve analysis methods. According to these methods, clay and/or shale contents are considered to be in the range from 10 to 20 percents. Cross-plots of Sonic-Density and DensityNeutron, however, show the abnormally high contents of shale fraction (p percents) ranging from 63 to 76 percents. Megascopically, core samples collected from 2, 900m B gas bearing bed seem relatively to be clean as the Japanese reservoirs. It appears that above mentioned shale contents obtained from cross-plots may be overestimated. Lack of compaction above the sediments and/or high content of residual gas in the reservoir around the bore hole would give an erroneous effects on the measured values by various logging tools. Studies on the relation between the lithologically measured shale fractions and p percents values derived from logs should be advanced drastically based on the laboratory experiments and field examples.
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