Diverting agents in the oil and gas industry: A comprehensive analysis of their origins, types, and applications: Scientific Paper Review and Summary

     When I used to work drilling wells in commingled reservoirs that tapped multiple zones in vertical wells, it was common to use retrievable bridge plugs to isolate the different gas-producing formations. Retrievable bridge plugs are one kind of diverting agent, in this case, a mechanical diverting agent. While I am a geologist, I developed a significant understanding of petroleum engineering in the course of my work.  

     A March 2024 paper in the journal Petroleum Research - Diverting agents in the oil and gas industry: A comprehensive analysis of their origins, types, and applications - examined all the different types of diversion used in the oil and gas industry.

     Well stimulation methods include hydraulic fracturing, acidizing, and acid fracturing. Hydraulic fracturing involves pumping large volumes of water at high pressure to induce fracturing in the target rock and large volumes of sand or other proppants with the water to flow into the fractures in order to hold them open after the pressure falls. Acidizing usually involves adding a 15% solution of hydrochloric acid (HCl) in order to dissolve carbonate components of the rock. Acid fracturing is also used in carbonates (limestone and dolomite) and uses both HCl and pressure to break the rock and dissolve it to increase permeability. Sometimes, when it is believed that the original treatments were not as effective as they could have been, the rock is refractured and/or re-acidized.

     The paper summarizes the goals and challenges of well stimulation with diversion of fluid flow as follows:

“The primary challenge of modern-day completion design in unconventional wells is equal distribution of fluid and proppant into all perforation clusters of a single frac stage. A secondary challenge is stimulating the frac stage for as long as possible without sacrificing uniform frac distribution and production performance (Murphree et al., 2020).”

     The paper defines and explains diverting agents as follows:

“Diverting agents are bridging materials that are often used in completions to temporarily seal off perforation clusters, tunnels, and near-wellbore fractures and redirect fluid and proppant elsewhere. In theory, the diverters will block off the areas of flow that are taking the most fluid and proppant and redistribute the frac slurry into areas that have been under-stimulated or not stimulated at all (Gao et al., 2019; Murphree et al., 2020). New technology developed based on degradable diverters with outstanding non-damaging plugging efficiency has opened new opportunities for protecting existing fractures, plugging them accordingly, and fracturing new zones initially bypassed because of inefficient zone coverage (Fragachan et al., 2015).”

     The graphic below recounts the early history of diverting agent development for oil and gas extraction.

     The graphic below shows the different types of diverting agents, both mechanical and chemical, that are used in the oil & gas industry.

     Mechanical diversion involves the complete shut-off of a part of the reservoir to direct all the stimulation fluid to a subsection of the open zone.

     Opposed cup packers are used for acidizing highly permeable formations. Fluid is pumped down the string and out of a port between the two cups, which are typically spaced 6-12 inches apart. The cups expand and make a seal against the casing due to pressure so that the fluid can be forced through the perforations and into the formation. Pressures and flow rates that are too high can cause leakage or cause the tool to separate from the casing.

     Ball sealers are of two types: sinkers and floaters. Sinkers are most commonly used in limestones for fracturing or acidizing ops. They do not generally work in sandstones due to the high pumping rate requirements. Floaters, or neutral-density ball sealers, match the density of the balls to the density of the fluid to achieve maximum fluid diversion through the perforations.

     A retrievable bridge plug blocks and seals.

“An establishing framework, anchor structure, sealer framework, and other components make up most of this device. A unique self-locked technology allows for effective dual-side stress distribution while avoiding the requirement for surface cementing and a dependable seal.”

These can easily be unlocked and retrieved from the hole. They are dependable, secure, and inexpensive, and have many functions and applications.

     Sliding sleeves are used in wells with tubing installed. They allow for equalization across the tubing and casing ring. They are made of stainless steel and nickel alloys. They may have primary and secondary sealing. They can be used to kill wells.

     Chemical diverters are now commonly used in horizontal wells. The key to their effective use is their degradability following their use. The paper explains them:

“Chemical diversion is a method used to help achieve better stimulation efficiency and proper fluid placement in vertical and horizontal wells (Sahu et al., 2019). For zonal isolation, chemical particles can be used as a temporary diverting agent. As shown in Fig. 4, these particles fill the perforation tunnel and enhance frictional resistance. Degradable fibers hold the particles together while they are pushed downhole and maintain them afloat as they move through tubes. The diverter aids in the formation of a composite fracture network and expands the area of contact between the fracture and the reservoir.”

“The key parameters that determine successful diversion include the density and size of the particle and proppant, the density and viscosity of the carrier fluid, buoyancy, and the effect of gravity (Spurr et al., 2016).”

A key advantage of chemical diverters is that when they are done doing their diversion, they can dissolve into the well fluid and not have to be retrieved out of the hole, which saves time and money.

“Chemical diverters are classified into four categories: coarse aggregate that disintegrates, dissolves, or melts in warm oil or water; liquid elements that degrade, dissolve, or melt in warm oil or water; and fluid molecules that degrade, dissolve, or melt in warm oil or water. Foams, and viscoelastic surfactants (VESs) are all examples of polymer gels.”

     Solid particle diverters include solid particles of different shapes and sizes. Rock salt (NaCl) is commonly used for solid particle diversion when acidizing carbonate reservoirs. Oil-soluble resins, benzoic acid flakes, and wax beads are used in acidizing sandstone reservoirs in high-temperature wells. Solid additives such as polyesters, polyacetals, polycarbonates, polymelamines, polyvinyl acetates, and polyvinyl chlorides can be used in lower temperature wells and gas wells.   

      Polymer gel diverters have higher viscosity than normal acidizing solutions, which allows them to penetrate low-permeability intervals first. They can be used in two ways: first, as a pre-flush to change the injection pressure into the formation through the perforations. This allows lower viscosity acid solutions to penetrate through damaged zones during the main flush. The second use of polymer gels is to gel the acid solution itself. The use of gelled acid delays the reaction rate, allowing better acid penetration, and the high viscosity of the gel reduces leakage.

     Foam diverters are used in acidizing operations to direct the acid into damaged or low-permeability rock. They can aid diversion and penetration. Water-soluble surfactants and gases such as nitrogen, carbon monoxide, carbon dioxide, or natural gas may be used to make the foam.

     Viscoelastic surfactants (VESs) are used similarly to foam diverters. They cost much more than polymer-gelled acids, but they basically disappear into the fluid following treatment.

     Fiber diverters combine a Viscoelastic Diverting Agent (VDA) solution with fibers. They essentially combine particle and viscosity-based deviation approaches.

     The next section of the paper delves into more detail. First mechanical diversion is explained by explaining the mechanisms behind the use of frac plugs, expandable liners, swellable packer assemblies, sand plugs, ball-activated sliding sleeves, perforation balls, and perforation pods.

     Frac plugs are used to isolate the previously fractured well intervals from the injected fluid during the next pumping stage. These are widely used in “plug-and-perf” operations. In hydraulic fracturing and refracs, they are used in combination with chemical particulate diverters. Other kinds of packers, like swellable packers and straddling packers, may be employed. Ball sealers and other devices may be employed for zonal isolation.

     Chemical diversion agents are of many types. The chemistry and fluid dynamics of the fluids is very important in selection. Polylactic acid (PLA) is a common chemical diversion agent.

“The main goal is for the diverting agents to dissolve easily in produced or administered fluids yet be somewhat insoluble in treatment fluids. Because lactic acid is an ecologically friendly substance that dissolves when exposed to water and heat, polylactic acid is frequently employed in diversion applications.”

Temperature, pH, the fluid with which the PLA interacts, and the PLA's autocatalytic behavior aid the degradation of the agent.

“Table 1 depicts the differences between chemical and mechanical diverters. The differences clearly indicate the overall advantages and superiority of chemical over mechanical diverters. Chemical methods are economical because they eliminate the need for a workover rig and mechanical isolation tools to provide temporary physical barriers. Because of their significant economic advantages, they are preferred over mechanical diverters.”

     The paper goes on to explore different fracturing and acidizing techniques in different rock types, at different temperatures, depths, and pressures, and by type of well (vertical or horizontal).

     Hydraulic proppant fracturing (HPF) is preferred in moderately to highly permeable strata and particularly in permeable sandstones. It results in conductive but short cracks. Water fracturing involves less proppant use. Hybrid fracturing involves the use of cross-linked polymers, linear gels, and sticky water solutions. This results in longer cracks and more propping of them. Carbonate rocks benefit the most from acidizing and cracking, with propped fracturing being very useful. Shale fracturing involves the use of slick water to avoid plug-offs, and modern methods, such as zipper fracturing, utilize simultaneous fracturing of multiple wells to direct the fluids better. Other methods used to fracture shale include cavitation hydrovibration fracturing, hydrajet fracturing, and exothermic hydraulic fracturing.

     Diversion can have varying functions, whether used in hydraulic fracturing or in acidizing. In both, the goal is to stimulate the most rock, including less permeable rock, and to make sure all perforation clusters are treated.

“The universal method for repeated Multistage Fracturing (MSF) is a mechanical, degradable flow diverter. Generally, the diverter used for MSF is a two-component powder consisting of 7 and 100 mesh particles that can isolate the required open area in the wellbore. The diverter has an extensive scope of application: it can be used for MSF in several vertical well zones, for MSF in wells with slotted liner completions, and for repeated MSF in wells with sliding sleeves both in a blind version and with the use of a coiled tube (CT) packer.”

“During matrix acidizing or acid fracturing, near-wellbore diversion is commonly employed to optimize reservoir coverage and minimize hours spent on zonal isolation. This method of diversion provides high efficiency, which is especially critical in offshore operations. In this method, degradable solid particles are dynamically placed in the wormholes, fractures, or perforations to divert the treatment fluid to under-stimulated zones. During downhole operation, the diverter must maintain mechanical strength and integrity before degrading at high temperatures in the presence of stimulation fluids (Vidma et al., 2021).”

“Diversion is exceedingly challenging in acid fracturing compared with matrix acidizing because of higher pressures, faster pumping rates, and larger amounts of acid employed.”

     The paper goes on to present case studies, including acid stimulation of wells in the Malay Basin using a surfactant-based diverter, hydraulic fracturing of a Well in the Cauvery Basin using a unique chemical diverter, matrix stimulation of a Saudi Arabian carbonate reservoir using a self-degradable fiber-laden diverter, and stimulation of wells in the Gulf of Mexico using a sandstone acidizing diverter system.

     Future research areas are considered:

“Fracture transmission, proppant transportation, fracture conductivity, and cracked well efficiency forecasting are each areas of present hydraulic stimulation research that have their own drawbacks. Therefore, combining these investigations is crucial for the development of tight reservoirs, which presents a significant problem for petroleum engineers.”

     The paper’s conclusion summarizes the study of diversion. It also touts the advantages of chemical diverting agents.

“Isolating pre-existing perforations and diverting fluid flow are key aspects of well treatment. Certain specialized particles or mechanical objects, generally known as diverting agents, diverting materials, or diverters, are used to aid these processes. Diverting agents are chemical or mechanical materials or particles used to divert the flow of the fluid in a well to optimize oil production. Two categories of diverters that are generally used are mechanical (e.g., packers, retrievable bridge plugs, sliding sleeves, and ball sealers) and chemical diverters (e.g., solid particle diverters, polymer gel diverters, and foam- and fiber-based diverters). Selection of the fracturing technique and diverter type for different classes of formations depends on type of wells, rock properties of the potential reservoir, depth, thickness, pressure, and temperature of the reservoir, as well as well construction type.”

“With time and application, it has been found that the chemical diverters have advantages and superiority over the mechanical diverters. Chemical methods are more commercial than the mechanical diverter techniques as they eliminate the requirement of workover rig and mechanical isolations. Chemical diverters can also be used for near as well as far field application along with open hole completion which is not the case in mechanical diverters. Mechanical diverters require frequent mechanical interventions without continuous pumping, leading to the additional treatment time which is not same for the chemical diverters.”

     This paper is best read by petroleum engineers who are knowledgeable about these techniques and how they work. Others, even a geologist like me with some understanding of petroleum engineering and diversion, will be challenged to understand everything. One thing is clear: successful diversion means successful zonal isolation, and this leads to optimizing well completion operations so that the most rock is stimulated, often leading to optimized well production.  

 

     

 

References:

 

Diverting agents in the oil and gas industry: A comprehensive analysis of their origins, types, and applications. Namrata Bist, Abhishek Nair, Kriti Yadav, and Anirbid Sircar. Petroleum Research. Volume 9, Issue 1, March 2024, Pages 72-84. Diverting agents in the oil and gas industry: A comprehensive analysis of their origins, types, and applications - ScienceDirect