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“HYDROPHYSICS”: THE PETROPHYSICS OF DRILLING FLUIDS AND THEI


Drilling fluids are carefully formulated to enable safe and efficient drilling of subsurface formations and, within reservoir sections, to minimize formation damage. Although the effects of these fluids on log data might be a consideration, these effects are generally accepted as a necessary consequence of achieving the most cost-effective and productive wellbores.
The response on log data due to drilling fluid chemicals is generally considered to be minimal, as tool-specific corrections are applied to account for the fluid within the borehole.
These corrections, however, adjust responses to “true” measurements of the bulk formation properties that include invaded fluid. It is still necessary to account for the properties of the invading mud chemicals during the petrophysical interpretation. This was not a problem when the drilling fluids were simple water-based or invert-oil emulsions. With the introduction of more complex drilling fluid chemistry, accounting for the invading fluid properties becomes more important. Base brines such as formates and bromides, which have significantly different log responses than other aqueous fluids, might require complex modeling to retain interpretation accuracy.
The formulation of drilling fluids for drilling purposes has been widely documented; however the effects that many common, and not so common, drilling fluid components have on petrophysical measurements are not well documented. Even the physical properties of common make-up brine and standard base oils may affect basic log data responses but have been widely ignored. Other components such as precipitating salts and paramagnetic particles may also influence some of the more complex logging tools.
This paper is intended as an overview of the primary effects of drilling fluid chemicals on petrophysical log data. References are provided to assist in additional research on individual mud systems.
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