While polylactide (PLA) polymers, a bio-based and degradable thermoplastic, have been used for more than a decade as a diverter to enhance fracture stimulation, it has only been in the last few years that materials science and best practices have begun to reduce the guesswork in diverter application. It is now possible to achieve predictable oil and gas production enhancements in well after well using PLA diverters.
“The heterogeneity that exists across the Eagle Ford Shale laterals makes it a good candidate for PLA diversion,” said Bill Kreimeier, manager of completions at Lonestar Resources. “We’ve been pumping diverter for over two years on our wells and have documented a 15% to 20% enhancement in cluster efficiency from using PLA diversion.”
“If an operator is not seeing positive results with a PLA diverter, there is something amiss,” added Brad Todd, owner of Completion Science. “Our advice is for operators and their service providers to work with an experienced adviser on a plan for increasing effectiveness. It’s a rare occurrence when best practices do not help increase production.”
Degradable polymer for diverters
In Blair, Neb., starch milled from field corn, grown solely for industrial use, is hydrolyzed and fermented into L-lactic acid, the building block of PLA. Then, at the NatureWorks 150,000-metric-ton processing facility in Blair, the L-lactic acid is converted into lactide monomer and then polymerized into PLA resin, marketed under the brand name Ingeo.
Ingeo PLA resins are produced in various grades optimized for a wide range of consumer and industrial applications. Using the toolbox of lactide monomers available, NatureWorks can tune the rate of degradation of PLA grades to provide the desired downhole performance over the range of 49 C to 149 C (120 F to 300 F).
PLA grades can be tailored to the desired degradation rate for the temperature of the well and are commercially available with different degradation rates. For optimum results, the correct PLA grade must be selected based primarily on downhole temperature and usage time. Because it is thermoplastic, PLA can be converted into various sized pellets, powder, flake and other potential shapes that can be mixed and matched for various fracture configurations and can be delivered to the desired downhole location with little or no agglomeration or mass loss. As such, they can deliver more stable forms of solid acid equivalents, as required in acid fracturing, where the controlled release of acid can be achieved as PLA hydrolyzes.
With proper grade selection, any residuals will be a hard, friable, nonsticky powder that does not adversely impact the permeability of the formation. Testing has shown 100% regain to oil for the DH1000 low-temperature grade. Once the PLA material has performed the desired downhole function (e.g., diversion), it hydrolyzes to generate oligomers of progressively lower molecular weight, which eventually become soluble in the aqueous hydrolysis medium with no scaling observed. The final product of hydrolysis is lactic acid, which is a natural product and generally recognized as safe by the U.S. government.
Diverters and the path of least resistance
If rock formations were uniform in composition and stress levels, every perforation would respond to high hydraulic pressures to produce fractures that receive proppant throughout their length creating maximum stimulation. However, the reality is that the lack of uniformity leads to some fracture networks expanding and reducing the pressure for unequal stimulation within a stage. In other words, fracturing fluids containing proppant will follow the path of least resistance, leaving some fractures overstimulated and some understimulated.
The concept behind diversion relies on temporarily blocking the high flow and low-pressure network. Blockage increases pressure in the stage, allowing all clusters within a stage to be fractured and receive fracturing fluid containing proppant, thereby developing robust fracture networks with a high stimulated reservoir volume.
Diversion has led to increasing the number of clusters per stage, which saves time and money as fewer frac plugs are set and need to be removed before the well goes to production. Diverters are being used to increase near-wellbore cluster efficiency and to limit connection between parent and child wells because of far-field stimulation.
PLA diverters are being used to complete wells with casing and wellbore issues, particularly when the well casing is deformed or parted, which limits where frac plugs can be set. PLA diverters can be used in sections where casing was unable to be run to bottom, leaving an openhole section where frac plugs are not an option. Refracturing older wells is another application making use of diverters.
PLA diverter versus pumping sand
To achieve the highest stimulation using PLA diverters, it is vital that the diverter supplier, service provider and operator adhere to diversion best practices. For example, service providers and operators should utilize the correct grade of PLA diverter to achieve 50% mass loss within a useful time frame at a given downhole temperature. Aiming for the fastest degradation is not always the best decision. For example, when refracturing a well, the diverter needs to remain in place until the entire well is stimulated. Every well and need is different, so it is important to review the degradation information with the PLA diverter supplier. Figure 1 shows degradation rates versus well temperatures of various tuned PLA grades.
Following the identification of the appropriate PLA grade, the ideal mix of shapes and sizes of the diverter should be determined. The reason for considering a mix of pellets, powder or flake rather than a uniform formulation is that the various shapes will bridge a fracture and then fill in tightly around the bridge. For example, a mixture may contain 10 parts large particles (8/16, 12/20 mesh), four parts medium particles (20/40, 30/50 mesh) and two parts fine particles (4/70, 100 mesh) (Figure 2).
Todd recommends using larger pellets near the wellbore where the fractures will be wider and then using progressively smaller sized pellets and powders farther from the wellbore.
“The first question we ask a client is whether the application will be near-wellbore or far-field,” Todd said. “And if it’s near-wellbore, then we ask what the largest particle is that the pumps can accommodate. From there, the sizing takes care of itself. For far-field, normally the solution is in the smaller particle distribution range.”
It is important to know or determine the hardness of the rock formation because the harder rock is more likely to use less diverter than softer rock. Service providers and operators can use this information to gauge the amount of diverter to initially pump.
The use of diverters is more about application, feedback and adjustment than it is simply about application. To get the most stimulation from diversion, take feedback readings after pumping and adjust accordingly. While there is a slowdown in pumping when using a near-wellbore diverter, the uptick in production should compensate for that.
“We work with an excellent engineer who really understands the science of fracking,” Todd said. “He routinely does step-rate testing to calculate the number of open perforation holes. This takes time. But he is meticulous on location about pumping procedures and making sure he teases every bit of information from a well to help him continue to improve design and maximize stimulation.”
“PLA diverters properly designed will enhance stimulation efficiency and reduce completion costs,” Kreimeier added. “If there is the attitude that fracking is a cookie-cutter process, then companies are leaving money in the ground.”
Both Todd and Kreimeier agree that one pill size does not fit all and that using the same PLA grade and particle mix interwell is not optimum.
“PLA diverters are relatively new,” said Charles Busceme, marketing manager at A. Schulman. “In our estimation, greater attention to the process and expertise in delivery will lead to improved results. And, as the industry progresses, the supplier community is laying the foundation for new solutions.”
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