1. Preface

Shale gas is a high-quality, efficient, and clean low-carbon energy source that has received increasing attention from countries around the world in recent years, and is also a key focus of attention in China's 12th Five Year Plan. Shale reservoirs have the characteristics of low porosity and permeability, making exploration and development difficult. Most tight shale gas reservoirs require volume transformation to achieve ideal production. In countries such as the United States and Canada where shale gas extraction technology is relatively mature, the main method used is the drag reducing hydraulic fracturing fluid system. This system has low polymer content, easy reverse flow, low cost, and minimal damage to reservoirs. It has been widely promoted and applied in multiple regions, and has achieved significant economic benefits.

In recent years, with the discovery of multiple large shale gas reservoirs in China and the country's emphasis on shale gas extraction, shale gas has become another popular energy source after oil and natural gas. Therefore, in the future, China's shale gas extraction operations will inevitably increase, and the demand for fracturing technology and fluids will gradually increase. The high-performance drag reducing water fracturing fluid system that has been widely used abroad will also be increasingly valued. Drag reducing agents are the most important additives in drag reducing water systems, and their performance determines the application range of the entire system. However, China mainly studies polymer drag reducing agents used in petroleum pipeline transportation systems, and there are few reports on drag reducing agents used in hydraulic fracturing.

This article combines the research status of drag reducing agents for hydraulic fracturing at home and abroad, focusing on the research progress of drag reducing agents abroad, aiming to improve China's overall understanding of drag reducing hydraulic fracturing fluids, and has certain guiding significance for the research of drag reducing agents in China.

2. Drag Reducing Agents

In 1947, Toms accidentally discovered the drag reduction effect of polymers while studying the solution of polymer under high shear stress, adding a small amount of (25 μg/g) of polymer can achieve a high drag reduction rate (70%~80%). Later, polymers were widely used as drag reducing agents in crude oil transportation pipelines, which can greatly reduce frictional resistance and improve crude oil transportation speed. In 1950, drag reducing agent was used in the fracturing fluid system for reservoir reconstruction, which was called drag reducing hydraulic fracturing fluid, but was soon replaced by polymer gel system. In the late 1990s, due to the rapid development of unconventional oil and gas reservoir extraction, drag reducing water was once again applied to reservoir fracturing and development, especially in shale reservoirs. In 1997, Mitchell Energy Company first applied drag reducing water to the fracturing operation of Barnett shale gas, which increased the final recovery rate of shale gas by 20% and reduced the fracturing cost by 65% compared to large-scale hydraulic fracturing.

The main components of drag reducing hydraulic fracturing fluid are a large amount of water and proppant, and a small amount of surfactant, drag reducing agent, clay stabilizer, etc. Its polymer content is small, so the drag reducing water system has a lower viscosity than the conventional cross-linked gel system, and the ability to carry proppant is poor. To overcome this disadvantage, the pumping speed of drag reducing water is usually high, which can cause turbulence and lead to high energy consumption. Therefore, drag reducing agents need to be added, which can reduce the huge energy loss caused by friction between the friction pipeline and water or saline solution at higher pump speeds. The commonly used drag reducing agents abroad are generally polyacrylamide based polymers, which have advantages such as high drag reduction performance, low concentration of use, and cost-effective. They have been widely applied in the fracturing and reconstruction of shale reservoirs. Numerous studies have shown that the drag reduction effect of polymers mainly includes two aspects:

①.The properties of polymer molecules, including relative molecular weight, monomer properties, charge intensity, and type;

②. The phase transition speed in environments such as low temperature, high salinity, and acid fracturing.

It is generally believed that the drag reduction performance of drag reducing agents is directly proportional to their relative molecular weight, and the ion type (cationic, anionic, non ionic) of the polymer determines its physical properties and applicability. In recent years, in order to adapt to various complex geological conditions, many new drag reducing agents have been developed by changing the structure and ion type of polymers, and have achieved good application results.

3. Mechanism of Action of Drag Reducing Agents

The drag reduction mechanism of drag reducing agents is relatively complex. Since the 1950s, a large amount of research has been conducted on the drag reduction mechanism of drag reducing agents, proposing effective slip hypothesis, eddy current energy dissipation/generation inhibition hypothesis, viscoelasticity hypothesis, etc. At present, the widely accepted theories include turbulence suppression theory and viscoelastic theory.

①.Turbulence Suppression Theory

The flow patterns of fluids in pipelines can be divided into turbulent and laminar flow. When the fluid flow rate is high, it is easy to form turbulence and generate a large number of vortices. Due to the momentum transfer between turbulent vortices and the pipe wall, as well as the momentum transfer between vortices of different sizes, energy consumption is caused, which is the cause of fluid resistance. When a high polymer drag reducing agent is added to a pipeline, the drag reducing agent relies on its own viscoelasticity to naturally stretch the long chain of molecules along the flow, affecting the movement of fluid microelements, offsetting some of the forces acting on the fluid microelements in the radial direction, and changing their size and direction to convert some of the radial forces into axial forces in the clockwise direction, thereby reducing eddy current energy dissipation and reducing flow resistance. However, there is no eddy current energy dissipation in laminar flow, so drag reducing agents cannot take effect in laminar flow.

②. Viscoelastic Theory

The polymers in drag reducing agents are linear polymer solutions with viscoelasticity. These polymer solutions interact with turbulent vortices, absorbing some of the eddy energy and storing it in the form of elastic properties, reducing eddy energy dissipation. The drag reducing agent forms an elastic bottom layer inside the pipeline, reducing the friction between the fluid and the pipe wall, and achieving the effect of drag reduction.

4. Research Progress in Drag Reducing Agents Abroad

4.1 W/O Inverse Polymer Drag Reducing Agent

Polymer drag reducing agents can generally be divided into dry powder polymer drag reducing agents (DPAMs), W/O inverse polymer drag reducing agents (EPAMs), and polymer solutions (SPAMs) due to their different physical states, with a wide range of relative molecular weight distribution (1×106~3×107), mainly limited by the dispersion state of the polymer. Among them, the W/O inverse polymer drag reducing agent has the highest relative molecular weight of the polymer and the best drag reducing performance, making it the most commonly used drag reducing agent in drag reducing hydraulic fracturing fluid systems. W/O inverse polymer drag reducing agent is a low oil-water equilibrium emulsion system formed by using water-soluble polymer solution as the inner phase and petroleum alkane oil phase as the outer phase.

When it is added to the clean water fracturing fluid, due to the increase in water phase volume, the high polymer will be quickly released from the reverse emulsion liquid system and quickly hydrated, playing a drag reducing role in the fracturing fluid injection process. W/O reverse polymer drag reducing agents have been widely used due to their good drag reducing effect, but their temperature resistance is poor, and their phase transition speed, which is the rate at which polymers are released from emulsion liquid systems and dissolved in water, is easily affected by external conditions. Therefore, it is necessary to select suitable drag reducing agents based on water quality conditions. Harsha Kolla developed a new type of low-temperature resistant emulsion drag reducing agent in 2012. The system still has a fast phase transition speed at -35 ℃, and can be completely dissolved in a water and salt water system within 60 seconds, achieving good drag reducing effect. Compared with conventional dry powder polymer drag reducing agents and inverse emulsion drag reducing agents, it has better shear stability, temperature resistance, short phase transition time, and easy degradation.

4.2 Salt Resistant Drag Reducing Agent

In recent years, the rapid development of drag reducing water fracturing technology has led to the widespread application of multi-level horizontal well fracturing and synchronous fracturing technology in shale reservoirs, resulting in a significant increase in the required volume of drag reducing water. However, the use of clean water is also increasing, especially for offshore oil fields and areas lacking water resources, where water sources have become a major problem. At the same time, the treatment of a large amount of produced water has also increased the burden on major enterprises, so many oil fields are considering recycling the produced water from oil reservoirs and fracturing fluid backflow. After general chemical and mechanical treatment processes, the soluble salts in the water produced by oil fields are difficult to remove, and conventional drag reducing agents can only be used in clean water systems, which do not perform well in saline water systems. This makes the demand for salt resistant chemical additives increasingly urgent.

4.2.1 Adding Surfactants to Improve Phase Transition Speed

The performance of conventional W/O reverse polymer drag reducing agents will decrease in saline systems, which is not only related to the performance of the polymer, but also to its phase transition speed. In a saltwater system, the presence of a large amount of dissolved salt can inhibit the rate of polymer release from W/O emulsion, delaying its drag reduction effect.

In 2009, C.W.Aften developed a W/O reverse polymer drag reducing agent that can be applied to clean water and 2% KCI solution. By adding a high HLB value surfactant (non ionic) as a phase converter in the system, the polymer can be quickly released. Usually, this type of surfactant does not immediately have a phase transition effect when in contact with a drag reducing agent. Only when it comes into contact with a large amount of aqueous fracturing fluid, it will have a phase transition effect, so it can be pre added to the drag reducing agent. This new type of drag reducing agent has better performance than conventional drag reducing agents of the same specification (with the same relative molecular weight and dosage), with a dosage of 0.5μg/g, the drag reduction rate in the clear water system increased by 29%, and in the 2% KCI system, the drag reduction rate increased by 119%. Due to its excellent drag reduction rate and phase transition speed, when conveying the same volume of fracturing fluid at a certain speed, it can reduce energy consumption or increase its conveying speed at the same power.