Abstract
The completion of wells using solids-laden fluids can impair the reservoir production and also damage the functioning of downhole completion tools, therefore completing wells with clear-brine fluids is the preferred alternative. Clear brines are typically halide or formate salt solutions in water, but they, too, have shortcomings. At lower temperatures or increased pressures, the salts in these fluids can crystallize causing potential well control concerns and/or costly operational disruptions. Completion of high-pressure wells, with densities above approximately 14.3 lb/gal for calcium bromide or 13.1 lb/gal for potassium formate, has historically required the use of brines containing zinc bromide or cesium formate to minimize formation damage, yet, in addition to their merits,both fluids have inherent liabilities. Zinc-based fluids, for example, are restricted and classified as priority pollutants due to their potential harmful effects on the environment, and the low pH(acidity) of zinc-based halides increases the potential for corrosion of metal components and risk to personnel safety. With cesium formate fluids, their limited production may restrict supply and lead to higher cost in high-volume deepwater applications. Moreover, when used as a packer fluid, literature (Javora 2003) suggests that formates may cause hydrogen-induced cracking (HIC), especially in the presence of carbon dioxide (CO2) that could lead to failure of production tubing.
An offshore operator required a priority-pollutant-free completion fluid for a subsea development,whose produced fluids (oil and water) are combined and processed with that from several other fields at a shared production facility. Associated produced water separated from the crude is dischargedoverboard and must be free of priority pollutants; detection of any such pollutants would requireextensive processing or, in the worst case, result in shutting down production from all the fields and the facility.
This paper describes the development and successful field applications of a novel family of completion fluids, created to address the deficits of conventional high-density clear brines. The new fluids extend the conditions for onset of crystallization to a higher density range and meet environmental concerns, as they are formulated with sustainably sourced materials. The novel high-density,non-zinc, solids-free completion fluid (HDNZ) meets the challenges and requirements of ultra-deepwater environments for fluid densities between 14.4 and 15.3 lb/gal. An overview of the extensive laboratory test data needed to develop the fluid and verify its viability as a completion brine and packer fluid is described. The paper outlines the design criteria and qualification testing performedto ensure that the technical challenges were addressed for this challenging deepwater project. The laboratory data include testing of pressurized crystallization temperatures (PCTs), stress corrosion cracking (SCC), elastomer compatibility, formation regain permeability, long-term stability, and compatibility with multiple fluid types (mud, control line, spacer, frac fluids, sour gases and chemical additives).
The discussion on fluid usage will encompass details of the plant trial to validate the performance of the fluid and case history detailing the operational implementation in the first five ultra-deepwater well completions in the GOM.
Additionally, engineering these fluids led to the development of a new method to measure brine crystallization temperature at elevated pressures, as there currently is no industry standard for such measurement in downhole conditions. The new method is accurate, repeatable, and executable in rig-site laboratories.
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