Mixed metal hydroxide (MMH) drilling fluid technology dates back nearly thirty years (Burba III et al. 1988), but its adoption remains limited by the expertise required to deploy the system and the sensitivity of the system to contaminants. In the Permian Basin (specifically the Delaware Basin), a robust MMH formulation mitigates losses and enables control of mud weight through a challenging salt layer of 2,000 to 5,000 feet, eliminating a casing interval. This success has been repeated on over 200+ wells with continuous optimization.
MMH systems provide a unique rheological profile as the net positively charged MMH crystals form a complex with negatively charged bentonite, resulting in near instantaneous gel strength at low shear while providing typical flow properties and pump pressures at high shear. MMH applications range from improved suspension during milling operations to minimizing losses in highly fractured formations. Historically, MMH properties break in the presence of anionic materials, such as coal, deflocculants, anionic polymers as well as at elevated salinity. The robust MMH system discussed offers greater stability and lower risk of failure associated with a variety of contaminants.
The robust MMH system was deployed to minimize washout in the salt as the highly thixotropic fluid is near static under the low shear conditions near the wellbore. Without the turbulence of conventional fluids, minimal salt incorporation results in near-gauge wellbores and stable fluid density. The original gel system in turbulent flow provides the required properties for drilling, but salt incorporation from the formation causes not only washouts but escalating density.
Conventional systems require dilution to maintain density as the salt formation blends with the drilling fluid and elevates density. The MMH system minimizes salt incorporation and allows greater flexibility with increased density as mud density induced losses were less frequent due to its high viscosity at low shear. It is estimated the MMH system performs at +0.2-0.4 lbm/gal higher mud weight than a conventional system before losses occur as the properties of MMH systems inherently limit fluid invasion through fractures. The greater operational window provides more flexibility to insure well integrity.
Overall, a combination of proper fluid selection and execution with continuous operational improvement yields great benefits, lowering overall drilling costs 40% and drilling days by 50%. A 50% reduction in dilution volumes and easier fluid management benefits were complemented by lower waste volumes and the elimination of earthen pits.