Lost circulation is one of the main causes of nonproductive time during drilling and impacts the success of cementing operations. Losses into the reservoir not only impact drilling, they potentially impact the reservoir, due to influx of quantities of drilling fluids that are potentially damaging, or will influence the production rate. Existing solutions are based mainly on particulates, which often are added to drilling fluids to plug fractures or to build up filtercake to cure fluid losses. When particulates are applied for curing losses in reservoir sections, it is desirable that the plugging materials maintain stability for sufficient time to allow well completion but eventually self-degrade to leave undamaged formation for future hydrocarbon production. The main challenges are the design of the lost-circulation material to cure losses into fractures of various widths and to provide plug stability and cleanup within a desired time frame over a broad bottomhole temperature range.Fibers have shown good fracture-plugging behavior. Parameters affecting fiber performance include, but are not limited to, fluid viscosity, fiber concentration, fiber geometry, flow rate, effect of the wall, and fracture width. To effectively apply fibers as lost-circulation applications, a novel, fiber-laden fluid was designed for easy preparation on surface, allowing compatibility with bottomhole assemblies (BHAs). The decrease in velocity inside the fracture enables fibers to bridge and then plug the fracture, thus regaining circulation. The fibers are specially designed to degrade in an adjustable time frame sufficient to ensure plug stability until the well is completed. With time, the plug undergoes further degradation, leading to nondamaged formation for production.This novel degradable solution has been successfully proven during field trials in various drilling scenarios ranging from severe to total losses with effective and efficient loss mitigation, without issues on placement through BHA and bit nozzles, and mitigating further reservoir damage.
Drilling fluid removal by spacer fluids is an important step in ensuring proper cementing and therefore adequate zonal isolation, especially when non-aqueous fluids (NAF) are used. In addition to the fluid mechanics aspect of displacement, a tensioactive package containing surfactant(s) and solvent(s) is typically added to the spacer base fluid to clean all surfaces, including the casing. However, current surfactants and solvents on the market have limited application in terms of temperature and type of drilling fluids. API and ISO standards recommend practices for evaluating the suitability of a spacer. The viscosity of NAF, spacer, and cement mixtures at various fixed ratios determines their rheological compatibility. The reverse-emulsion test or spacer-surfactant screening test (SSST) determines the percentage of spacer for inverting the NAF emulsion. Other non-standardized tests such as bottle tests or grid tests determine the efficiency of a surfactant/solvent aqueous solution to remove NAF from metallic or glass surfaces. But all these tests suffer from lack of reproducibility and limited automation. Improved alternative laboratory procedures have already been proposed. They include measuring rheology during SSST, which clearly shows the positive impact of early emulsion inversion on viscosity of NAF/spacer mixtures. In addition, proper preparation of metal surfaces clearly improves repeatability of the cleaning test, which now is performed with weighted spacer under temperature and pressure. Using these improved experimental methods, we performed more than 3,000 different tests on more than 200 tensioactive blends. Statistical analysis helped in selecting the optimum chemistry as a function of the conditions (type of base oil, salinity, and temperature). This allowed developing both guidelines and a tensioactive package comprising a limited number of chemicals (surfactants and solvents), from which the field user would select the ones to combine for the application. Only a few confirmation tests would be necessary at the location for planning a given cement operation. An advantage of developing a package of chemicals is ease of implementation at field level since it requires having only a few chemicals in inventory to cover all situations and limited local laboratory testing. The new methods have been successfully applied in several wells in which different types of nonaqueous fluids were used at various temperatures.
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