Flat Gels

Key Takeaways

• The API RP 13B-1 and 13B-2 procedures for measuring gel strength use the direct-reading viscometer (Fann VG meter or equivalent): after shearing the mud sample at 600 RPM for 10 seconds to break any pre-existing gel structure, the viscometer is set to low shear (3 RPM) and then quickly stopped; the maximum dial reading observed in the first 10 seconds after stopping is the 10-second gel strength (in lb/100 ft2), and the maximum dial reading at exactly 10 minutes after stopping (without disturbing the sample) is the 10-minute gel strength; a third reading at 30 minutes (30-minute gel) is also commonly reported for wells where long static periods (surveys, wiper trips) are anticipated; the ratio of 10-minute to 10-second gel strength is the gel strength index, with values near 1.0 indicating flat gels and values greater than 2.0 to 3.0 indicating progressive gels that may create swab/surge and lost-circulation risks during tripping; typical target gel strength ranges for water-based muds are 3 to 10 lb/100 ft2 for the 10-second gel and 5 to 15 lb/100 ft2 for the 10-minute gel, with the 10-minute value no more than 2 to 3 lb/100 ft2 above the 10-second value for a well-performing flat-gel mud.
• The chemical basis of flat gels behavior in water-based drilling muds involves the balance between electrostatic edge-to-face attraction (which causes clay platelets to form a card-house structure rapidly after shear ceases) and the thixotropic strengthening mechanism (in which additional hydrogen bonds, van der Waals forces, and clay platelet overlapping develop progressively over minutes to hours after the initial gel network forms); muds with high concentrations of smectite clay (bentonite) tend to develop strong progressive gels because the high surface area and charge density of smectite platelets continue to form additional cross-links after the initial gel network is established; muds with polyanionic cellulose (PAC) or xanthan gum polymers added as fluid loss control and viscosity agents tend toward flat gels because the polymer chains provide immediate bridging between clay platelets (establishing the gel network rapidly) while also steric-stabilizing the clay surfaces against progressive strengthening through additional inter-platelet bonds; synthetic polymers such as PHPA (partially hydrolyzed polyacrylamide) and biopolymers such as xanthan gum are specifically chosen for their flat-gel rheological profiles in technically demanding well designs.
• Consequences of progressive gels (the opposite of flat gels) during drilling operations include increased equivalent circulating density (ECD) on pump startup after connections (as the high gel strength requires additional pump pressure to break circulation, temporarily increasing the bottomhole pressure), increased swab pressure on trips (as the drill string pulling through the gelled mud creates a negative pressure pulse that may reduce the bottomhole pressure below the formation pore pressure and cause a kick), increased surge pressure on connections (as the drill string is lowered into the gelled mud, creating a positive pressure pulse that may exceed the fracture pressure and cause lost circulation), and difficulty restoring circulation after lost circulation events (because the high gel strength requires high pump startup pressure that further fractures the formation); progressive gels are particularly problematic in wellbores with tight ECD windows (where the mud weight window between pore pressure and fracture pressure is narrow) and in horizontal wells where the highly gelled mud in the horizontal section is not assisted by gravity during circulation startup.
• Non-aqueous drilling fluid (NADF, oil-based mud or synthetic-based mud) gel strength characteristics differ from water-based mud because the gel network in NADF is formed by organophilic clays and polymeric viscosifiers (primarily organophilic bentonite treated with quaternary amine compounds that make the clay surface oleophilic, and polyamide waxes or fatty acid soaps that form waxy networks at low temperatures) rather than by hydrophilic smectite; NADF gel strengths typically increase more rapidly with decreasing temperature (because the waxy organophilic network strengthens as the temperature falls below the wax pour point) and less rapidly with increasing temperature than WBM gels, creating a potential progressive gel problem when NADF is static at high-temperature-high-pressure (HPHT) wellbore conditions after a connection or survey; the temperature sensitivity of NADF gel strengths requires that gel strength tests be performed at the anticipated downhole temperature (using a pressurized viscometer or a high-temperature rheology cell) rather than at the ambient surface temperature, since surface gel strength measurements may significantly underestimate the gel strength at the bottom of a deep HPHT well.
• Field management of gel strength toward the flat-gels target involves a combination of chemical treatment and mechanical practices: for water-based muds that are developing progressive gels due to excess bentonite or high-molecular-weight polymer content, treatments include deflocculants (chrome lignosulfonate, lignite, polyphosphates) that reduce inter-platelet edge-to-face attraction by altering the clay surface charge, and thinners (low-viscosity polymers) that compete with the clay surface sites for polymer chain attachment; for muds with insufficient gel strength (at risk of barite sag or cuttings settling), treatments include increased bentonite concentration, addition of xanthan gum (which builds flat, low-magnitude gels), or addition of attapulgite clay (a fibrous clay mineral that provides flat gels from fiber-to-fiber entanglement rather than face-to-face clay platelet attraction); mechanical practices that help maintain flat gels include circulating the mud immediately before any long static period (connections, surveys, core recovery) to ensure the gel is broken before the critical period begins, and maintaining high pump rates during circulation to keep the mud in a fully sheared, non-gelled state until the static period starts.