Widely used throughout the drilling industry, barite (barium sulfate) performs the useful function of weighting drilling fluid to control formation pressure, contributing to drilling efficiency and safe operations. Unfortunately, barite-weighted mud can cause formation damage that reaches a meter or more into the reservoir. Additionally, barite solids left in the well can leave an impermeable filter cake on the wellbore wall and can plug screens and slotted liners used in open hole completions. Barite solids can also clog perforation tunnels in wells with perforated liners. Barite presents a stubborn problem: it cannot be dissolved in water or acid and its bridging properties make it very effective at plugging pore throats in the formation.
Barium sulfate issues are not limited to new wells. Scales, including barium sulfate, strontium sulfate, magnesium and calcium carbonate, can form in producing wells when injected or connate water is incompatible with the reservoir’s chemistry. These scales are as difficult to remove as field-grade barite.
In the mid-1990s, Well Flow International developed and introduced treatment chemicals, High Density Converters (HDC), that use chelation to dissolve barite deposited by drilling mud and to remove acid- insoluble materials in well screens and tubing. Since then, the company has gained experience throughout the world with this barite-dissolving chemistry and has introduced several versions of HDC with different compositions and viscosities to address mud solids, scales, or both, in a variety of well conditions. HDC treatments have achieved consistently favorable results in these applications.
HDC is a proprietary blend of salts and acids with a pH higher than 12.5. It is not highly corrosive and has similar corrosion behavior to seawater. HDC does not damage the formation, as proven by independent laboratory regained permeability tests. HDC is a straight chemical, spotted “neat” without water or additives, then left to soak for several hours to remove barite or scale. It can be bullheaded or placed with coiled tubing. HDC is effective at cleaning up perforated liners, wire wrap screens in open hole, and expandable screens clogged with barite left by drilling mud or scale. It also can remove scale from production tubing and completion equipment, such as electrical submersible pumps.
Chelation is a chemical reaction in which a chelating agent forms multiple bonds with metal ions to remove the metal (in this case barium, strontium, magnesium or calcium) from larger molecules. HDC is extremely effective at dissolving barium sulfate, especially in comparison to HCl acid treatments. In laboratory tests, an 18% HCl treatment dissolves a negligible amount of barite, just 2 to 3 grams per liter of treating fluid. In contrast, when HDC was first introduced in the 1990s, it dissolved 80 – 85 grams of barium sulfate and scales for every liter of treatment chemical. Since then, Well Flow International has developed improved HDC formulations. HDC-III dissolves over 300 grams/liter, and when used with a Koplus XL pre-treatment, it can remove up to 380 grams/liter.
HDC treatments, performed in two stages, first cleans barite and scale from completion components and then penetrates the perforation tunnels to reach up to one meter into the formation. HDC remains chemically active for up to 48 hours, compared to 6-12 hours for acid treatments, so that when the second treatment stage is applied it forces the first stage chemicals, which are still 30% active, into the formation behind the completion to achieve further cleaning.
HDC treatment often is a practical alternative to acid stimulation jobs, which require mobilization of expensive equipment, large crews, and significant volumes of hazardous fluids. Acid requires special handling and, because acid treatment chemicals degrade, jobs have to be performed quickly so that scheduling can be problematic. After acid treatments, thousands of gallons of waste fluid must be handled and disposed of safely. In addition, acid jobs on wells with barite contamination can create even more damage, further impairing production.
None of these problems apply to HDC treatments. HDC requires no special handling and is as safe to use as synthetic oil base mud. It is shipped in 55-gallon drums or small containers and is pumped “neat” with no additives, in small volumes, typically 10 to 20 barrels per treatment. Pumping equipment needed for HDC treatments may already be on the platform or at the land well’s location. No blenders or large crews of personnel are needed. A further benefit of HDC is that it can be stored on location for up to two years, enabling flexible scheduling of treatments.
HDC has been used in the UK North Sea to remove severe oil base mud damage from extended reach wells, increasing production by 51%. Also in the North Sea, HDC removed scale and barite from well screens and electrical submersible pumps in horizontal wells, more than doubling production. In Thailand, a HDC treatment removed barite solids that blocked perforations in an injector well to enable injection of 8,500 bpd. In Malaysia, HDC was applied to successfully restore production in a series of high temperature gas wells. A well in Nigeria went from producing no oil to flowing at 3,000 bopd after HDC treatment. In some novel applications, HDC also has been used to free fish and deferentially stuck pipe by dissolving the barite that had held the tubulars in place. In all these cases, HDC has been a practical solution for dealing with barite problems.