New Cement Spacer Chemistry Enhances Removal of Nonaqueous Drilling Fluid
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.
Enhanced Testing, Optimized Chemistry
Over the past 5 years, Schlumberger has improved both laboratory methods and equipment to evaluate spacer efficiency and select optimum spacer chemistries for specific downhole conditions. There are three fluid zones between the NAF drilling fluid and the cement slurry (Fig. 1 above). Zone 1 contains a mixture of NAF and spacer fluid. Zone 2 contains the spacer and surfactant. Zone 3 contains a mixture of spacer and cement. Specific laboratory tests are applied in each zone to assess different spacer properties.
In Zone 1, for example, the spacer surfactant selection test (SSST) determines the percentage of surfactant required for the aqueous spacer to efficiently invert the NAF emulsion. Enhanced laboratory procedures include the measurement of rheology during the SSST and the correlation of rheology behavior with emulsion inversion. Testing indicates that early emulsion has a positive effect on the viscosity of the NAF/spacer mixture.
In both Zones 1 and 2, the API’s recommended static sedimentation and rheology tests are run to assess spacer stability and surface water wettability during the removal of NAF-based drilling fluid. In Zone 2, two additional tests assess the capacity of the spacer to remove mud from well surfaces. One is a modified cleaning test for temperatures of up to 85°C. The other is a specially developed cleaning test for temperatures above 85°C.
Traditional cleaning tests use either a grid or a rotor to determine how much NAF has been removed from a surface. A sandblasted rotor was found to provide superior test conditions and improved repeatability, and a new method of measuring cleaning efficiency was developed. Normally, a metallic rotor is dipped in NAF and cleaned with an aqueous solution containing only a surfactant for 10 minutes, the typical amount of time that a spacer has to remove NAF cakes and films while pumping cement. By weighing the remaining NAF on the rotor with an analytical balance, the cleaning efficiency is determined.
Because existing laboratory equipment allows testing only at atmospheric pressure, temperature is limited to 85°C. A new apparatus was developed to heat fluids to 180°C under sufficient pressure to avoid boiling. Using a modified dynamic filter press with the rotor containing the NAF and an external reservoir with a piston to pump the weighted spacer, a new “combined test” accurately measures cleaning efficiency while displacing drilling fluid.
In the traditional test, the aqueous solution lacks both the viscosifying agent and the weighting agent found in a fully formulated spacer. Adding these agents yields erroneous results. To address the problem, a chemical titration method has been added to measure the remaining NAF on the rotor by using an ion tracer present only in the NAF, not in the spacer.
In Zone 3, standard API compatibility tests are performed with a 10% level of spacer contamination to evaluate the effect of the aqueous solution and surfactant on cement thickening time, rheology, and compressive strength. The test is run twice to distinguish the effect of the spacer alone from that of the spacer with surfactant.
Once improved, reproducible laboratory methods were available and more than 3,000 tests were performed on more than 200 blends of surfactants and solvents. Selecting the optimum spacer chemistry as a function of wellbore conditions—base oil, salinity, and temperature—was achieved by using a statistical experimental design method known as response surface methodology.
This yielded a new spacer comprising a certain number of surfactants and solvents, recently commercialized as CemPrime engineered chemistry spacer. The spacer products in this line are engineered blends, and the chemistry’s design rules enable engineers to select the most efficient spacer formulation for any type of drilling fluid under specific well conditions (Fig. 2). Not only does the engineered chemistry spacer remove NAF more effectively than traditional spacers, but the contamination of the cement slurry has a negligible effect on the thickening time and the development of compressive strength.
Thailand Case Study
Mubadala Petroleum operates concessions in the Gulf of Thailand, including the Jasmine field, which has six platforms and a floating, production, storage, and offloading vessel. Oil production comes from numerous sandstone reservoirs with bottomhole temperatures ranging from 65°C to 105°C. The producing section is drilled with an 80/20 low-toxicity, oil-based mud and cemented with extended Class G cement, both formulated with seawater.
More than 100 wells have been drilled using a conventionally weighted spacer with surfactant for mud removal and zonal isolation. In recent years, the quality of production-section primary cement jobs has been evaluated as having poor zonal isolation, with the spacer effectiveness considered as one of several potential causes. In certain cases, the presence of NAF residue is suspected to have affected cement placement and set properties. Various chemicals have been added to thin the mud, reduce interfacial tension, and mitigate incompatibility with cement slurries.
Although no service quality issues or nonproductive time for remedial work occurred, the operator sought to assess and improve spacer performance before drilling operations in a nearby field, where zonal isolation is considered a critical success factor. Specifically, the challenge is to isolate three main pay sands spaced over 500 ft in total vertical depth, allowing each to be produced independently by means of a multizone completion.
Mubadala chose to compare the efficiency of the new spacer with that of the current spacer. Drilling fluid samples were sent to a regional laboratory, where all the tests above were conducted. A selection diagram similar to Fig. 2 determined the optimum spacer formulation.
Three key parameters were controlled during testing: the type of NAF base oil, the typical bottomhole temperature, and the potential seawater salt concentrations from the Gulf of Thailand used to prepare the spacer. By investigating how seawater compositions affect the KCl salinity used to build the selection diagram, two acceptable spacer formulations were identified. Cleaning tests determined the best one of 2 gal/bbl of Surfactant 1 plus 2 gal/bbl of Solvent 2.
This solution provided a twofold increase in cleaning efficiency. After 10 minutes, Mubadala’s current formulation had removed 30% of the NAF, while the engineered spacer removed 60%. The emulsion inversion was also twice as efficient. Where cement was contaminated with 10% spacer, the previous formulation modified its compressive strength by 40% over a 24-hour period, while the engineered spacer had essentially no effect.
Based on these and other comparisons, Mubadala decided to field test the new spacer while cementing a 7-in. casing string in the 8½-in. producing section of the Jasmine D-28 well. The job was performed with bottom and top plugs. Zonal isolation was confirmed by cement bond logging and ultrasonic imaging. The TOC was located approximately 200 ft above the previous casing shoe, as planned. The use of the engineered spacer had zero effect on job execution. As a result, the new formulation is being adopted for additional cement jobs in the Jasmine field, as well as the upcoming field development program.
Because extensive optimization testing under differing conditions was conducted during product development, only a few laboratory tests will be necessary to confirm the engineered spacer chemistry for each new location in future cement jobs.
New Cement Spacer Chemistry Enhances Removal of Nonaqueous Drilling Fluid
Jason McClure, SPE, Mubadala Petroleum; and Ines Khalfallah, SPE, Salim Taoutaou, SPE, Jorge Andres Vargas Bermea, SPE, and Slaheddine Kefi, SPE, Schlumberger
01 October 2014
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