4.2.2 Changing the Type of Polymer Ions to Improve Salt Resistance

In the on-site construction of drag reducing water, it has been found that the drag reducing effect of drag reducing agents not only decreases in the neutral energy of saline water systems, but also decreases due to the addition of other additives. This phenomenon is closely related to the properties of polymers. For example, adding AMPS monomer as a drag reducing agent not only has good drag reducing effect, but also has certain salt resistance, which can be used in acid fracturing operations.

In addition, the ion strength and ion type of polymers also have a significant impact on the performance of drag reducing agents. For example, when cationic clay stabilizers are mixed with anionic polymer drag reducing agents, cross-linking reactions may occur between them, causing changes in the molecular structure of the drag reducing agent polymer and resulting in precipitation.

In 2011, through comparative research on the drag reduction performance of several cationic and anionic drag reducing agents with the same relative molecular weight in single brine system, mixed brine system, and on-site backflow fluid, Javad Paktinat found that all drag reducers exhibit good drag reduction performance in clear water systems, while conventional cationic drag reducers FR-A and anionic drag reducers FR-B exhibit poor performance in saline water systems. Anionic drag reducers FR-C exhibit good drag reduction performance in a certain amount of monovalent ionic saline water systems, while anionic drag reducers FR-D exhibit good drag reduction performance in saline water systems containing a large amount of monovalent ions and an appropriate amount of divalent ions, and the phase transition speed is fast.

In 2009, FRC and FR-D were applied on-site in the drag reducing water fracturing operation of the Montaney shale reservoir in Canada. The fracturing fluid backflow fluid (mixed saline solution) was mixed with a clean water system in a 1:1 ratio as the drag reducing water base fluid, both showed good drag reducing performance, and FR-D had better drag reducing effect. Numerous research results have shown that anionic polyacrylamide drag reducing agents have good salt resistance.

4.3 Harmless Drag Reducing Agent

Although the polymer content in the drag reducing water system is relatively low, in recent years, with the large-scale application of multi-stage fracturing and synchronous fracturing technology in horizontal wells, the use of fracturing fluid has become increasingly large, and the accumulation of polymers in the formation has led to increasingly serious damage to the reservoir. The drag reducing agents currently used, including copolymers, are mostly polymers with C-C as the main chain, which are difficult to degrade by conventional biological enzyme methods, pyrolysis, and other methods. Therefore, chemical methods are often used for the degradation of polyacrylamide.

In 2007, P.S. Carman studied persulfate, inorganic peroxides, organic peroxides, and other oxidants as destructive agents and compared and analyzed their degradation effects on polyacrylamide. The results showed that potassium persulfate had the best degradation effect at 82℃. However, due to the need for higher temperatures, coupling reactions may occur during the reaction process, forming a cross-linking system, leading to an increase in relative molecular weight and causing more serious reservoir damage.

In 2011, Hong Sun conducted research on harmless drag reducing agents and believed that improvements could be made in below two aspects:

①.To develop more efficient drag reducing agents that can quickly hydrate and reduce the incubation period before complete hydration, as the fracturing fluid only takes 3 minutes from the surface to the perforation site.

②.To develop a drag reducing agent that can automatically degrade, which has high drag reducing performance during the pumping process, and can automatically degrade at the bottom of the well with less residue. 

Hong Sun has developed a new type of easily degradable reverse polymer drag reducing agent based on this. And this new type of drag reducing agent is more sensitive to oxidative destructive agents, and the polymer main chain is easily oxidized and degraded. Moreover, the amount of residue after gel breaking is small, which has little damage to the reservoir.

4.4 Disperse Polymer Drag Reducing Agent

At present, W/O reverse polymer drag reducing agents are the most commonly used drag reducing agents. However, the use of surfactants in these emulsion liquid systems greatly limits its application range, and the phase transition speed of polymers in emulsions is easily affected by external water quality conditions. Although many new salt resistant drag reducing agents have been developed, their salt resistance property is not strong, and their performance in high salt content and mineralization producing water deteriorates. The scope of use is narrow, and optimization needs to be based on water quality conditions. Therefore, it is hypothesized whether drag reducing agents are made into other states, such as powder or slurry, so that they may have a relatively wide usage range and effective range.

In 2014, Kristen M. Tucker studied the drag reduction performance of drag reducing agents in different states in clean water, 5% KCI, seawater, and fracturing fluid reverse drainage. He found that some W/O reverse emulsion drag reducing agents have a narrow range of use; Powder drag reducing agents have a wide range of applications but are difficult to dissolve; Slurry based drag reducing agents have good drag reducing effects in various water qualities, especially suitable for systems with poor water quality.

Jia Zhou, Marcus Baltazar, and others conducted a comparative study in 2014 on the drag reduction performance of conventional drag reducing agents FR-M, FR-N, and new dispersed polymer drag reducing agents (FRPW) in high salt content (100000-300000 mg/L) and high salinity (15000-90000 mg/L) oil reservoir produced water. Among them, FR-M is a commercially available drag reducing agent, and FR-N is a drag reducing agent improved by adding AMPS monomer. The results showed that the three drag reducing agents exhibited good dissolution and dispersion abilities in both clear water and weak salt water systems, but only the new dispersed polymer drag reducing agent (FRPW) was able to quickly hydrate and disperse in high salt content and high mineralization output water, demonstrating good drag reducing performance.

The main reason why the performance of dispersed polymers is superior to other types of drag reducing agents is due to the dispersion mechanism rather than the influence of molecular changes. The polymer in conventional W/O reverse polymer drag reducing agents requires phase inversion to take effect, while dispersed polymers do not require this step, so they can quickly hydrate and disperse in any aqueous phase. At the same time, making the drag reducing agent into a slurry form can simplify construction, reduce the types of additives, reduce costs, and the effective content of the polymer is also higher than that in the emulsion, which can reduce the pumped volume.

5. Research Status of Drag Reducing Agents in China

At present, there are few drag reducing water systems used for hydraulic fracturing in China, and there are even fewer reports on the research of drag reducing agents. Most drag reducing agents rely on foreign imports, which is very unfavorable for the development of drag reducing water fracturing technology in China. It is necessary to learn from foreign experience and develop a drag reducing water system suitable for the characteristics of reservoirs in China as soon as possible.

In 2013, Liu Tongyi prepared a water-soluble drag reducing agent for drag reducing hydraulic fracturing fluids using the reverse microemulsion method, and evaluated its drag reducing ability and sand suspension ability in a clear water system. The results indicate that it has a certain drag reduction and suspended sand capacity. In 2014, he developed a new type of drag reducing agent with low viscosity and high elasticity to address the shortcomings of conventional drag reducing agents such as low sand carrying capacity, high relative molecular weight, and severe reservoir damage. This drag reducing agent has advantages such as good sand carrying, low friction, and low damage, and its performance is superior to conventional drag reducing agents. It has been applied in coalbed methane wells in southern Yanchuan.

6. Prospect

The drag reducing hydraulic fracturing fluid system is a new fracturing fluid system developed for shale reservoir reconstruction, which has been widely studied and applied abroad. In recent years, with the increasing efforts in shale gas extraction in China, the drag reducing hydraulic fracturing fluid system will inevitably be promoted. At present, a large amount of research has been conducted abroad on the core treatment agent of drag reducing water systems—drag reducing agents, and significant progress has been made.

W/O reverse polymer drag reducing agent is the most commonly used drag reducing agent for clean water fracturing of shale reservoirs. It has the advantages of good drag reducing effect, easy backflow, and minimal damage to the reservoir, and is favored by major oil companies. Later, with the development of multi-stage fracturing and synchronous fracturing technology for shale reservoir horizontal wells, the amount of fracturing fluid used was increasing, and harmless salt resistant drag reducing agents that could be used for producing water from oil reservoirs and fracturing fluid backflow became a research focus. The research and application of this new drag reducing agent can not only solve the problem of reverse drainage treatment, but also alleviate the shortage of freshwater resources.

At present, salt resistant drag reducing agents mainly improve their salt resistance and compatibility with various additives by introducing surfactants into traditional W/O reverse polymer drag reducing agents or modifying polymer molecules. However, the presence of surfactants in drag reducing agents greatly limits their scope of use, and they are still not suitable for high salt content and high salinity produced water. Therefore, the research on dispersed polymer drag reducing agents has been put on the agenda in the past two years. Due to the different dispersion mechanisms of dispersed polymer drag reducers, there is no need for phase transition time in the aqueous phase. Therefore, the drag reduction performance in high salt content and high mineralization output water is superior to other types of drag reducers, with a wider range of applications, and will become another hot research field of drag reducers.