Petrophysical log interpretation of tight gas sands: Final report, 1985-1986
In tight-gas sands, clay minerals are detrimental to the accurate evaluation of porosity, permeability, and water saturation derived from well logs. In this study, the effects of individual clay minerals on these petrophysical properties are examined. Sixty-five samples were randomly chosen from the cored interval, 9374 to 9811 feet in depth in Amoco's Joe S. Kinsey No. 1 well of the dirgin Field in Rusk County, Texas. Samples include both shaly and sandy units of tight, delta-front sandstones of the Cotton Valley Group. Mineralogy of these samples was determined by x-ray analysis. Petrophysical properties were determined for each sample depth. The relationships between clay-mineral weight percentages and petrophysical properties were determined by analysis of simple correlation coefficients and canonical variable weights. Results show that water-sensitive clay minerals (illite and mixed-layer illite/smectite) are more effective in altering petrophysical properties derived from well-log responses than non-water-sensitive clay minerals (kaolinite and chlorite). This occurs because water-sensitive clay minerals have more bound water, larger surface areas, and higher cation exchange capacities. Porosity is affected by clay mineralogy. Log-derived porosities are increased by the presence of water associated with water-sensitive clay minerals. Bound water and water in micropores of water sensitive clay minerals increase the concentration of hydrogen ions, decrease bulk density, and increase acoustic transit time. Permeability is probably reduced preferentially by some clay minerals. However, the inaccuracy of laboratory measurements of permeability prevents these relationships from being revealed. Water saturation calculations are affected by clay minerals. Archie's water saturation is high because it does not account for the resistivity-reducing effects of water-sensitive clay minerals. 21 refs., 11 figs., 24 tabs.
- Research Organization:
- Alabama Univ., University (USA). School of Mines and Energy Development
- OSTI ID:
- 5896571
- Report Number(s):
- NP-8900091; ON: TI88900091
- Country of Publication:
- United States
- Language:
- English
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