MAGNETIC SUSCEPTIBILITY OF DRILL CUTTINGS IN A NORTH SEA OIL WELL: A RAPID, NONDESTRUCTIVE MEANS OF CHARACTERIZING LITHOLOGY

摘要

Magnetic susceptibility measurements provide a non-destructive method to rapidly characterize drill cuttings at the wellsite or laboratory. Our previous studies on core plugs, slabbed core and whole core have demonstrated that magnetic measurements can identify lithological variations, estimate mineral content, and correlate with key petrophysical properties (such as permeability) and with downhole gamma ray data. In the present study volume magnetic susceptibility measurements were undertaken on drill cuttings from a North Sea oil well using a portable low field magnetic susceptibility sensor. The values were then converted to mass magnetic susceptibility by dividing by the bulk density of each sample. The results clearly indicated the main lithological zonations in the well, and provided estimates of basic mineral type (diamagnetic versus paramagnetic or ferrimagnetic) significantly quicker and cheaper than undertaking XRD measurements. The magnetic results also showed a correlation with the downhole LWD (logging while drilling) gamma ray profile. Interestingly, the correlation was the opposite way round to that observed in most other reservoirs we have studied. However, this provided additional mineralogical information for the well in the present study. Normally a low gamma ray signal (e.g., in a clean sandstone interval) would give a low or negative magnetic susceptibility due to diamagnetic quartz, whereas a high gamma ray signal (e.g., in shale) would give a higher magnetic susceptibility signal due to paramagnetic clays etc. In the present study many of the low gamma ray sandstone intervals exhibited a higher magnetic susceptibility signal, which indicated that there are additional paramagnetic and/or ferrimagnetic minerals present in those intervals in addition to the main diamagnetic matrix mineral (quartz). These additional higher magnetic susceptibility minerals can, for example, be due to a strongly paramagnetic mineral such as siderite, small amounts of a ferrimagnetic mineral such as magnetite or the canted antiferromagnetic mineral hematite. These minerals can affect the permeability, and may explain why productivity has been lower in this well.