Study Uses Engineered Vesicles for Cement-Integrity Applications
In the complete paper, a novel system that uses engineered features of permeable polymeric shell walls for the controlled release of encapsulated cargo is described.
Encapsulation-based systems are of interest for the industry in applications such as chemical-additive preservation, small molecule release, particle delivery, and self-sealing materials. Many methods are used to encapsulate relevant chemical additives for the controlled release of contents such as polymeric vesicles, inorganic shells, and mesoporous materials. In the complete paper, a novel system that uses engineered features of permeable polymeric shell walls for the controlled release of encapsulated cargo is described.
Competent cement systems are designed to allow the placement of cement behind the casing at a set time by using chemical additives. The function of these chemical additives is extensive and used to control cement properties such as setting time, rheology, fluid loss, gas migration, density, and enhanced mechanical properties. Challenges arise when additives are depleted promptly or manifested prematurely before cement has been placed behind the casing. Additives are used to accelerate the set time of the cement only after being placed downhole properly during the construction of a well.
To ensure cement quality, innovative polymers based on a family of polymers known as aromatic polyamides, or aramides, are used here as a single additive in cementing. The vesicular design of the polymer offers unique delivery-system features, providing chemical reagents when needed. In this way, the polymer is designed to produce a vast array of encapsulants for controlling the release of additives downhole during cementing operations. The application of using vesicles for the development of right-angle-set cements is shown in the paper. Thereafter, the spent vesicles remain embedded as an elastomer and the robustness of the shell imparts beneficial mechanical properties to the cement sheath.
An emulsion template is used to form vesicles for the modified release of additives (Fig. 1). Interfacial polymerization consisting of two immiscible solvents—a chloroform/cyclohexane mix and water—are stirred to form an emulsion. Droplets are captured by a semipermeable polymer shell. The dispersed phase contains the accelerant calcium chloride to serve in the design of right-angle-set cements. The general procedure for polyamide synthesis is provided in the complete paper.
Cement Mixing and Curing Procedure. Cement samples are prepared using Class G cement. A cement slurry consists of six components: water, cement, dispersant, cement retarder, applied polymer at 3 wt% of cement, and antifoamer. The polyaramide vesicles are added as a single additive.
The slurry is mixed at 4,000 rev/min for 15 seconds and then increased to 12,000 rev/min for 35 seconds. The cement slurry is poured into a cement consistometer for measurement of slurry-thickening time. After the slurry is brought to the fill line in the slurry cup, the cup is assembled with a paddle and capped and a thermocouple is inserted. Cement viscosity is recorded up to 100 Bearden units of consistency (Bc) at 100 and 300°F at 3,000 psi.
Result and Discussions
Smart Vesicles and Modified Release of Chemical Additives Into Cement. Vesicles, aromatically crosslinked, are synthesized using two immiscible solvents, water and oil. After synthesizing these aromatic polyamides, the resulting shapes of the polymers are uniquely vesicular in form, which allows for the development of chemical delivery systems. With light microscopy, the vesicles are shown to be hollow. After drying, the vesicles are suspended in water and allowed to swell. The dynamic motion of the shell can be observed easily. This characteristic provides a novel and efficient tool needed to develop the next generation of chemical additives for the oil and gas field. Overall, the flexibility in chemical design for these structurally sound technologies for unconventionals may lead to a range of smart cement formulations with controlled-release additives.
Generally, cementing a well consists of pumping cement slurry from the surface down the casing so that it returns to the surface in the backflow through the annulus between the casings or the formation. Because of the hydraulic pressure from the height of the cement column, the injected slurry is also capable of preventing gas migration. When cement begins to set, it is impermeable to gas. However, there is a critical transition time between these two phases, which lasts several hours during which the slurry no longer behaves as a liquid with hydrostatic pressure or as an impermeable solid. During this transition time, gas migration occurs, which can lead to pressure buildup and loss of zonal isolation. When the cement sets, the channels become permanent. These cement challenges are costly and dangerous to production and safety.
The solution to lengthy transition times are right-angle-set cements, which are characterized by a 90° curve on the set-time graph as measured by a consistometer. In one scenario, a slurry design using a calcium salt cement accelerant is encapsulated for delayed release into the slurry. The initial rheology of the slurry is key to measurements using a cement consistometer. The combination of a dispersant and a retarder to control cement gelation was formulated as the base slurry design. The consistency of the cement after the addition of the vesicles with the encapsulated calcium salt remained less than 70 Bc.
A second polymer delivery system was formulated and measured at two different temperatures using the same slurry design. The initial consistency measurements were similar in these two experiments, but an increase in temperature from 100 to 300°F caused the cement to set significantly faster. The data show that changes in monomer concentrations in vesicle design can be used to control the permeability of the shell and the release rate of the encapsulant at a lower temperature. Efforts toward the optimization of this delivery system at 300°F are ongoing to allow for the development of the next generation of additives in cementing.
Cement Mechanical Properties From Spent Vesicles. Polyaramide vesicles serve to deliver additives and reagents into the slurry when required. Once this task is complete, the spent vesicles remain intact and do not dissolve or degrade as is commonly found in commercial products (Fig. 2). Subsequently, once embedded in hard-set cement, the polymer membrane remains intact. These spent capsules continue to enhance cement performance for long-term integrity. Embedded in the set cement, the empty polymer shells continue to impart beneficial mechanical properties to the cement sheath during the lifetime of the oil well. The upcycling of this technology after the encapsulant is spent makes this technology unique.
This technical solution serves dual functions for the controlled release of chemicals into the slurry and the enhancement of the lifespan of the well. The system exhibits a promising prospect for a variety of cement-slurry applications.
Vesicle systems are appealing for delivery of beneficial agents such as chemical additives and small molecules. The vesicles are of interest because of potential breakthrough applications that will extend the lifetime of oilwell cement. In addition, the formation of capsules or vesicles in a cost- and time-efficient manner allows for the design of high-temperature-resistant chemical additives for more-challenging environments. Polymeric additives are an effective means of designing complex systems to deliver chemical reagents under challenging conditions and to impart favorable physical properties to the slurry and set states of cement. From chemical delivery to their controlled release of chemical additives during placement of a slurry downhole, these vesicles prove useful in the preparation of cement-slurry formulations.
This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 193538, “Engineered Vesicles for the Controlled Release of Chemical Additives and Application for Enhanced Oil-Well-Cement Integrity,” by Elizabeth Q. Contreras, SPE, Kenneth D. Johnson, and Diana Rasner, SPE, Aramco, et al., prepared for the 2019 SPE International Conference on Oilfield Chemistry, Galveston, Texas, 8–9 April. The paper has not been peer reviewed.