Hydroxyethyl Starch

Key Takeaways

• Fluid loss control mechanism of hydroxyethyl starch in drilling fluids relies on the swelling and gelation of HES molecules in the presence of water, which causes HES to accumulate in the filter cake (the layer of solids deposited on the wellbore wall when filtrate passes into the formation) and create a low-permeability barrier to further filtrate invasion: the HES polymer molecules in the mud filtrate partition preferentially into the filter cake (because the high solid concentration in the cake excludes the polymer and concentrates it at the cake surface), where they swell and interconnect to form a gel layer that dramatically reduces the filter cake permeability; the filtration control performance of HES is measured by the API fluid loss test (measuring the filtrate volume collected from a mud sample in 30 minutes under 100 psi pressure differential through a filter paper, with the API fluid loss target for most drilling fluid systems being less than 10 mL per 30 minutes) and the HPHT (high-pressure, high-temperature) fluid loss test (measuring filtrate at up to 500 psi and 300 degrees F to simulate downhole conditions); HES provides effective API fluid loss control at concentrations of 2-6 pounds per barrel of mud, and the concentration can be adjusted to achieve the target fluid loss specification with minimal impact on other drilling fluid properties (rheology, density); HES degrades at temperatures above approximately 250-300 degrees F (120-150 degrees C), losing its filtration control performance in high-temperature wellbores and requiring replacement by high-temperature-stable filtration control agents such as sulfonated asphalt, sulfonated polymers, or graphite.
• Salt tolerance of hydroxyethyl starch compared to unmodified starch makes HES the preferred starch derivative for saline drilling fluid formulations used in deepwater drilling (seawater muds), in high-salinity formations (salt domes, evaporite sequences), and in potassium chloride polymer muds used for inhibiting clay swelling: unmodified corn starch or potato starch has poor solubility in seawater or concentrated salt brines (salt causes precipitation of the starch polymer chains by disrupting the hydration shell that keeps the polymer in solution), and even if the starch is dissolved in freshwater and then salt is added, it tends to flocculate and lose its filtration control performance; the hydroxyethyl groups added to HES increase the polymer's hydrophilicity (water affinity) by providing additional water-binding sites, maintaining the polymer in solution at salt concentrations up to NaCl saturation (approximately 26% NaCl by weight) and in mixed-salt brines including KCl, CaCl2, and NaBr that are used as drill-in fluids and packer fluids; the salt tolerance of HES also makes it compatible with cement slurries used in primary cementing operations, where HES can be added to the cement slurry as a fluid loss additive to prevent filtrate from the cement from invading the formation during cement placement and creating a dehydrated cement cake that is difficult to displace; the compatibility of HES with both drilling fluids and cement slurries has made it a versatile polymer additive used throughout the well construction process.
• Biocide interaction with hydroxyethyl starch is required when HES is used in water-based muds at surface temperatures (above approximately 60-80 degrees F) where bacterial populations can grow rapidly in the mud system: despite the improved enzyme resistance of HES compared to native starch, bacterial populations in the mud pit can produce extracellular amylases that slowly degrade HES, reducing its molecular weight and diminishing its filtration control performance over days to weeks in a system without biocide treatment; the standard biocides used to protect starch-based drilling fluids include paraformaldehyde (PFA), glutaraldehyde, and proprietary quaternary ammonium compounds that kill the bacteria before they produce sufficient enzyme to degrade the HES; the biocide concentration must be maintained above the minimum inhibitory concentration (MIC) for the bacterial populations present in the mud, which varies depending on the mud composition, temperature, and the specific bacteria contaminating the system (SRB — sulfate-reducing bacteria — are of particular concern in sour formations where the produced H2S can also contribute to biological starch degradation); in high-temperature wellbores (above 200 degrees F bottomhole temperature), the thermal deactivation of bacterial enzymes typically protects the HES without biocide, but the surface mud system remains at risk from surface bacterial contamination and still requires biocide protection.
• Hydroxyethyl starch versus polyanionic cellulose (PAC) comparison is a routine decision in drilling fluid design, as both are polysaccharide-based filtration control agents with different performance profiles and cost characteristics: PAC (polyanionic cellulose, derived from wood pulp or cotton cellulose and chemically modified with carboxymethyl groups) provides superior thermal stability to HES (remaining effective to approximately 300-350 degrees F versus approximately 250-300 degrees F for HES), better filtration control at lower concentrations in freshwater muds (0.5-2 ppb PAC versus 2-6 ppb HES for comparable filtration control), and better salt tolerance in highly concentrated brines; HES is typically less expensive than PAC and provides adequate performance for moderate-temperature, moderate-salinity applications where the lower cost of HES makes it preferable on a cost-per-unit-filtration-control basis; in deepwater drilling with seawater-based muds at bottomhole temperatures below 200 degrees F, HES is frequently the preferred choice; in high-temperature high-pressure (HTHP) wells with bottomhole temperatures above 250 degrees F, PAC or sulfonated polymer systems replace HES because HES has degraded to an ineffective low-molecular-weight residue at those temperatures and its filtration control performance is unreliable; the decision between HES and PAC also considers the regulatory status of each additive in the drilling location, as both are approved for use in North Sea, Gulf of Mexico, and most international offshore locations but may have different discharge restrictions depending on the jurisdiction's assessment of biodegradability and toxicity.
• Formation damage potential of hydroxyethyl starch filtrate must be evaluated when drilling through the productive reservoir interval, because HES polymer molecules that invade the formation with the filtrate can adsorb on the reservoir mineral surfaces or plug pore throats, reducing near-wellbore permeability: the molecular size of HES (molecular weight typically 200,000-500,000 daltons for commercial drilling fluid grades) is large enough to be filtered out by the pore throats of tight formations (permeability below 0.1 millidarcy) but small enough to invade the pore network of higher-permeability formations (permeability above 1 millidarcy) with the filtrate; in carbonate reservoirs, the HES filtrate is generally non-damaging because HES does not interact strongly with carbonate mineral surfaces; in clay-rich sandstone reservoirs, the HES polymer may cause clay particle bridging and reduced permeability if the polymer molecular weight is in the range that can bridge between clay particles on opposing pore walls; return permeability tests (core flood tests that measure the permeability before and after mud filtrate invasion, expressed as the ratio of post-invasion to pre-invasion permeability) are used during drilling fluid design to quantify the formation damage potential of HES-containing drill-in fluids in the specific reservoir rock, with target return permeability above 80-90% for reservoir sections where formation damage would impair production economics.