Microresistivity, ”Rxo”
Interpretation Details
FLUSHED ZONE FORMATION RESISTIVITY, Rxo
Flushed zone formation resistivity, Rxo, can be determined, with minimal environmental
corrections, from any of the microresistivity measurements, except for the Microlog. This value is used in Archie’s equation to determine flushed zone water saturation, Sxo, or to indicate moved fluids by comparison to the undisturbed formation resistivity, Rt.
FLUSHED ZONE WATER SATURATION, Sxo
Archie's Equation:
Sxo = flushed zone water saturation Rmf = mud filtrate resistivity
Rxo = flushed zone resistivity (from the microresistivity log) φ = porosity
a = cementation factor m = cementation exponent n = saturation exponent
Archie's equation assumes that all electrical conductivity occurs in the water saturated portion of the porosity in a rock, with the rock matrix and any hydrocarbons acting as insulators. The presence of clays in the formation (a "shaly sand") creates additional formation conductivity (a lower formation resistivity than an equivalent "clean" sand). In this case, Archie's equation will predict a water saturation greater than is actually in the formation. Several "shaly sand
equations" have been developed to account for the effects of clays. The most commonly used are Simandoux, Dual Water, and Waxman-Smits.
In the flushed zone form of Archie's equation shown here, Rt is replaced by Rxo, and Rw is replaced by Rmf, with the assumption that all the original water has been replaced by drilling mud filtrate. Comparison of Sxo and Sw (using the same form of Archie's equation) gives some indication of (qualitative) permeability, and the amount of hydrocarbons which will be moved during production.
Because of the design of MicroLogs, the resistivity from the log may vary significantly from the actual resistivity of the formation. They should not be used in these calculations.
INDICATION OF PERMEABILITY
For Micrologs: The micronormal resistivity is greater than the microinverse resistivity ("positive separation"). There should also be mudcake, as shown by a decrease in the caliper reading.
For other Rxo tools: Compare the reading with the resistivity from deeper reading tools. The relationship between the readings will depend on the contrast between the formation water resistivity and the mud filtrate resistivity.
Microresistivity, ”Rxo”
FRACTURE IDENTIFICATION
Rapid curve movement, or "hashiness", may be an indicator of fractures as the tools see
conductive mud-filled fractures alternating with less conductive beds. Rough hole may cause the same response. This technique should be used only as one piece of information along with others in trying to determine the presence of fractures.
THIN BED DEFINITION
These tools will identify very thin beds. The bed definition can be used qualitatively to estimate the effect on the deeper reading tools. Bed thickness information from these tools can also be used in software which attempts to make quantitative thin bed (or laminated reservoir) corrections to other resistivity and porosity tools.
INVASION CORRECTIONS TO OTHER RESISTIVITY MEASUREMENTS Rxo tools are usually run in combination with two deeper reading tools (e.g. deep and shallow laterolog, deep and medium induction log). Using the combination of three measurements, invasion corrections may be made using "tornado charts" or equivalent algorithms.
Microresistivity, ”Rxo”
Secondary Effects
ENVIRONMENTAL EFFECTS
Mudcake corrections may need to be made if the measurements are to be used quantitatively.
Micro Laterologs provide good Rxo readings for invasion thicknesses of as little as four inches, but require mudcake corrections for mudcakes larger than 1/4 inch. On the other hand, no mud cake correction is required for the Proximity log unless mudcake thickness is over 3/4 inch or very high Rxo to mudcake resistivity (Rmc) ratios exist. However, the Proximity log has a much larger depth of investigation, and unless flushing has proceeded to 40 inches from the wellbore, one cannot be sure of getting an Rxo reading not affected by the uninvaded rock resistivity. The MSFL tool is a compromise to give reasonable Rxo readings without requiring mudcake correction except for large mudcakes
Rough hole will cause the pad to lose contact with the borehole wall. No corrections can be made to the data to correct for the effect.
INTERPRETATION EFFECTS
Clays in the formation have the same effect on these resistivity tools as on the deeper reading tools. Flushed zone forms of the various shaly sand equations can be written. Where
hydrocarbons have been flushed away from the vicinity of the wellbore, the resistivity effect may be less severe for the Rxo device than for the deeper reading tools responding to the uninvaded (or less severely invaded) rock beyond the "flushed zone". However, Sxo calculations may still be affected similarly to Sw calculations in fresh water mud systems since clay conductivity effects are more pronounced in less saline environments. Then, with fresh mud systems in saline water saturated rocks, the resistivity effect will be larger for the Rxo devices (i.e. clay conductivity component more significant).
Microresistivity, ”Rxo”
Environmental Corrections
This table indicates the corrections for the borehole and formation conditions that can be made for each logging measurement. The corrections that are applicable to the measurement are shown in bold.
CORRECTION COMMENTS borehole
mud weight bed thickness invasion mud cake borehole salinity formation salinity standoff
pressure temperature excavation propagation time attenuation lithology
Not all acquisition companies may have the correction indicated on this chart, or make corrections for all generations of the tool.
For newer logs, corrections may have been made at the time of data acquisition. Check the log header for information.
Algorithms which are equivalent to (or often better than) the chartbooks may be available from the acquisition company, or in some formation evaluation software packages.
Quality Control
Microresistivity curves should overlay deeper-reading curves in impermeable beds.
Separation with deeper-reading logs should be indicative of invasion or borehole effects.
Curves may not repeat as well as other logs due to variations in pad path and possible resulting changes in hole conditions or fracturing.
Check caliper for very thick mudcakes requiring quantitative corrections.
Shale values should be similar to those in nearby wells.
Check repeatability; curves should have the same values and character as those from previous runs or repeat sections.
Cross-check the curve character with other curves from the same logging run.