Technology Update An efficient removal of drilling fluid is essential to successful cementing operations. When a cement slurry comes into contact with mud residue, the cement may not set properly or adhere to the casing and formation, thereby preventing the isolation of permeable zones under different pressure regimes. This can cause stimulation out of zone, production of unwanted fluids because of communication between zones, loss of hydrocarbons into lower-pressure formations, corrosion of casing, and blowouts. In recent years, the industry has focused considerable attention on enhancing cementing practices to ensure well integrity and zonal isolation. To clean all well surfaces after the placement of casing, an intermediate water-based fluid, or spacer, is pumped between the drilling fluid and cement slurry. Typically, a package of surfactants or solvents is added to the spacer to improve cleaning and displacement efficiency. However, over the past decade, spacer formulations have not evolved as rapidly as the chemistry of drilling fluids. Every year, operators drill deeper, more complex, and higher-temperature wells under a growing range of downhole conditions. To boost performance, drilling fluids incorporate a greater variety of nonaqueous fluids (NAFs) based on synthetic and natural oils—mineral, paraffinic, and olefinic oils—with much longer carbon chains than traditional diesel-based muds. As such, NAFs are much more difficult to clean. Although spacers are intended to separate drilling fluid from cement, a small amount of spacer fluid—from 5% to 10%—typically contaminates a portion of the cement slurry. Many spacer chemistries affect cement properties, such as thickening time, rheology, and compressive strength, often precisely where zonal isolation is most essential. If the cement has not fully hardened by the time a cement bond log is run, it may not show up clearly and the top of cement (TOC) may be uncertain. Therefore, it is vital to ensure that spacers remove the maximum amount of mud while having a minimal effect on cement properties. Since NAF compositions vary widely, there are no universal spacer formulations. Most are designed and tested on a case-by-case basis using procedures that differ from one operator or location to another. Existing American Petroleum Institute (API) recommended tests for evaluating the suitability of a particular spacer have been found nonrepeatable. In addition, no test protocols exist for well conditions above 85°C, despite the large number of cement jobs in wells with bottomhole temperatures of up to 150°C. Along with salinity and the type of drilling fluid, temperature is one of the most critical factors influencing cement integrity. Thus, enhanced test procedures and spacer formulations are needed.
Production liner cementing operations are critical due to the small cement slurry volume, the requirement to achieve good mud removal in narrow annuli and, in most cases, the need to pull the running tool out of cement at the end of the operation. The risk is increased when the operations are conducted in deepwater environment, because there is increased complexity in predicting the actual bottomhole circulating temperature due to the long water layer acting as a heat exchanger for the wellbore fluids. In deepwater environment the cement slurry suffers different heating and cooling stages, such as being heated up when batch mixed, cooled down when passing through the riser-seawater section and heated up again when sent downhole. To avoid any problem after retrieving the running tool and landing string, a synergy between the operator and service company brought an improvement to laboratory testing procedures. Non-API laboratory tests for cementing liners were developed in Brazil using temperature simulators reproducing the planned operations and performing sensitivity analysis for additives concentration, fresh water content variation, slurry density variations, contamination effects and bottomhole circulating temperature changes. These practices allow the operator to evaluate cement slurry behavior under a number of conditions that could otherwise cause major job failure or even loss of a well. This document describes in detail the procedures that significantly decrease the risks associated with liner cement jobs and the high costs related to failures in deepwater operations. Since the implementation of these measures, the operator has not experienced any liner cement job failure related to cement slurry design.
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