The Relationship Between Permeability and the Morphology of Diagenetic Illite in Reservoir Rocks

Abstract

The Relationship Between Permeability and the Morphology of Permeability and the Morphology of Diagenetic Illite in Reservoir Rocks Summary The permeability characteristics of two reservoir sandstones from the Magnus and West Sole fields in the U. K. North Sea are essentially similar and are strongly influenced by the occurrence of filamentous illitic clay. However, the amount of illite in the West Sole sandstone is 20 times that in the Magnus sandstone, suggesting that the distribution and form of the clay is more important than its actual amount. Scanning electron microscope (SEM) observations show that the clay mineral occurs as an open, tangled web of long, thin ribbons virtually filling the pore spaces, a morphology that is properly revealed only after critical point drying. Introduction Early work at the BP Research Center on the permeability characteristics of reservoir sandstones from the Magnus permeability characteristics of reservoir sandstones from the Magnus field in the U.K. North Sea revealed puzzling differences between results from well testing and those measured by routine core analysis. Core data always indicated a higher permeability, which, in the case of the water zone, permeability, which, in the case of the water zone, amounted to 20 to 30 times that found by well testing. Similarly, there was a very wide discrepancy between gas and brine permeabilities of sandstone plugs measured in the oven-dried state and in the wet, preserved state. A possible explanation for this anomaly was that there could possible explanation for this anomaly was that there could be dehydration and shrinkage of clay minerals within the sandstone pores. Indeed, conventional SEM observations did reveal the presence of diagenetic filamentous illite coating the pore walls, although at that stage it was thought to be unlikely that this mineral could be responsible for the large differences in permeability. Work at the Macaulay Inst., however, showed that the morphology of the illite in the Magnus sandstones, as inferred through SEM, depended crucially on the manner of drying the specimen. The in-situ morphology of the clay mineral was revealed only after critical-point drying, when it was observed as an open, tangled web of very thin, long ribbons virtually filling the pore spaces. In contrast, after air- or freeze-drying, the illitic mineral occurred in dense mats packed against the pore walls. Following these observations, it was demonstrated conclusively 1 that the "interface-sensitive" nature of the filamentous illite provided a complete explanation for the discrepancies between the well-test data and routine core permeabilities. it was further suggested that the permeabilities. it was further suggested that the indiscriminate adoption of routine core analysis procedures could lead to serious errors in the assessment of reservoir quality. Our investigation compares the permeability and clay mineralogy of Magnus sandstones with those of West Sole sandstones, also in the U.K. North Sea, to show that fluid flow characteristics cannot be predicted from the quantitative clay content of reservoir sandstones. The West Sole sandstones contain abundant filamentous illite, which amounts to about 10% of the rock, compared with about 0.5% in the Magnus sandstones. The SEM and X-ray diffraction characteristics of the separated illite from both sandstones are dissimilar, but despite these differences the fluid flow properties of the two sandstones are comparable. Permeability Measurements Permeability Measurements Core plugs were cut from preserved Magnus and West Sole cores using degassed, simulated formation water as the cutting fluid. The core plugs, 1 in. [2.54 cm] diameter and 1 in. [2.54 cm] long, were cut from the center of the drilled pieces to reduce possible mud contamination. They were stored under formation brine. Complete brine saturation was ensured before permeability measurements were made. The cores were evacuated under brine to remove any gas and a pressure of 1,000 psi [6895 kPa] was then applied for >72 hours to dissolve any remaining gas. After saturation, the initial brine permeability was measured, during which time the sample was confined in a Hassler core holder with a sealing pressure of 400 psi [2758 kPa] on the rubber sleeve. Then the samples were dried in a humidity oven. After drying, the gas permeability was measured. All the samples were then resaturated permeability was measured. All the samples were then resaturated with brine and the brine permeability redetermined. Table 1 shows that the permeability characteristics of the two sandstones are similar and that, in particular, the gas permeability measured after oven drying was 20 times higher than the initial brine permeability measured on preserved samples. The resaturated brine permeabilities are higher than the initial brine values, permeabilities are higher than the initial brine values, indicating a permanent effect caused by drying. JPT P. 2225