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Mud is a common and not so commonly thought-of, critical exploration tool. While you are sorting out what you can still do after A.I. takes your job, and that drone delivers your frothy latté, consider that no computer, yet, can tell us what actually happens with mud content, extent of invasion, time to set up stable rheological properties, or even measure consistent and persistent homogeneous mud mixed fluid resistivity.

Water-cut, the standard Gulf Coast metric for producible sands, can limit success, especially in geopressured reservoirs. Determination of accurate water-cut is 100% dependent on in-situ fluids, including drilling mud, but not seismic, not mud logs or cores, magic or hope. That is, properties such as mud type, density [most often incorrectly called weight], oil content, water molecular content with dissolved solids, formation invasion, viscosity, and then and only then, resistivity. I have argued for years that water-cut is too high. The shale players prove me correct every day, but being right does not make me rich.

R_wa: I have seen mud samples taken by experienced loggers for Blue, Red and Gray from the walls of an excavated mud pit, from a bucket thrown into the mud pit, from a dryish shale shaker, and from a mud bucket collected hours earlier by a drill hand—no notes from where.  

I measured mud resistance with ohm mud-meters from all those trucks, compared to lab-quality calibrated mud-meters. None of the service devices were reliable, including that of the mud logger.  Using a reagent-grade calibration fluid, no device on the pad from over five official devices and users was accurate, all were  more than 30% off, and two more than 100%. I paid for rig time to circulate conditioning mud until mud resistivity from the discharge pipe stabilized. That took 6 hrs on a 4,500-ft well after TD in a vuggy limestone, topped with shaly sand. The work was published as a USGS Open File Report in Cheyenne, Wyo.

Often, you are forced to use diesel to slick a hole, to prevent the drillstring from getting stuck. More often, drillers want diesel, simply because it facilitates faster drilling than typical mud. There have been technologies using slick water, barite, etc. to compensate, but little testing exists to tell us what effect is created within a reservoir by those muds. You need decent logs to predict invasion, but you won’t get them with diesel. You are seriously impaired, especially if looking for shows and classic curve cross-over. A lot of decent shows were never seen, as a result.

Mud invasion counts, a lot, and it usually greatly exceeds the radius of investigation of E, neutron and density tools pushing oil and gas away from the wellbore, making oil detection spurious if not impossible. You cannot see oil that is not “there.” Analysts must rely on corrections [old charts now digitally applied, not calculated] to approximate or wish the extent of invasion. Charts and digital substitutions have absolutely no directionality. Directionality is important, especially if fracs, faults and joints are present. The Borehole Gravimeter [BHG] tool, I believe, is the best “way-beyond-the-borehole” tool going, and a wonder tool when paired with 3C-3D-VSP.

So, what do you do in geopressure? Mud weight vs. formation GOM muds? “What’s only 2 to 3 lbs. more going to do?” says my tool pusher. Over-balance guarantees that you cannot see where drilling mud ends, and natural formation begins. You also will see mud frac that sand.

“Depth to the top of overpressure in the deepwater Gulf of Mexico: Garden Banks, Green Canyon, Keathley Canyon, and Walker Ridge,” Sharon Cornelius, University of Houston; Peter A. Emmet, Brazos Valley GeoServices Inc.:

“…overpressure is defined as the point below the seafloor at which the geopressure gradient reaches 0.70 psi/ft, or a 13.5-ppg drilling mud weight (Bebout et al., 1982 and Rabinovich, 2011). The geo-pressure gradient was calculated from 150 wells, using the bottomhole mud weights converted to psi and then subtracting the hydrostatic pressure of the water column…There are 12 contributing factors to the formation of overpressure in the study area, but the only one correlating over the whole region is water depth. Disequilibrium compaction is widespread, due to high sedimentation rates during the Cenozoic; but numerically, it does not contribute greatly. Other more significant contributors are hydrocarbon generation, smectite to illite transformation, and sandstone diagenesis. Location factors include seafloor topographic highs or lows, presence of salt bodies, salt movement, proximity to faults, geothermal gradients, and thickness of clastic lithological units. https://www.gshtx.org/

You should want slightly underbalanced mud weight. Drillers only want a fast, safe hole. You contracted for a long-term well that can be completed safely. I have experienced several disappointments over drill mud weights far exceeding balance, creating significant skin damage, masking water cut where second exploration wells on the nearby similar rock produced.

The Bureau of Ocean Energy Management (BOEM) and the Bureau of Safety and Environmental Enforcement (BSEE) are proposing new rules that would discount, and even eliminate, royalties for new shallow-water wells, in an effort to encourage shallow-water drilling in the Gulf of Mexico. “High royalty rates prevent as much as $20 billion in shallow-water energy projects,” said U.S. Sen. Bill Cassidy (R-La.) This proposal plans to include reconditioning brown fields, such as workovers and step-outs. After all, new U.S. Energy Secretary Dan Brouillette said recently that the U.S. has technology no one else has. Might as well use some of it.  Looked at your old mud weight records lately for left-behind reserves?

About the Authors

William (Bill) Head

Contributing Editor

William (Bill) Head is a technologist with over 40 years of experience in U.S. and international exploration.