Drag-reducing agents (DRA), or drag-reducing polymers (DRPs), are additives in pipelines that reduce turbulence in a pipe. The primary purpose of using traditional friction reducers in stimulation treatments is to overcome the tubular drag while pumping at high flow rates. Hydraulic fracturing is the main technology used to produce hydrocarbon from extremely low permeability rock. Hydraulic fracturing has been successfully implemented since its inception in 1947. To achieve commercial production from ultra-low permeability formation, development in fracturing fluids is the key factor for stimulation operations. Although a variety of fracturing fluids have been used to increase the productivity of unconventional reservoirs, fluids like low viscous (slickwater) and high viscous (crosslinkers/linear gel) 2 SPE-195191-MS still have some problems in terms of lower capability to carry proppant, creating formation damage, high-cost operation, and environmental concerns. In recent years, the oil industry has adopted high viscosity friction reducers (HVFRs) in fracturing fluids due to several operational and economic reasons.
1.2 Historical background
Forrest and Grierson were the first to report a reduction in energy loss in the turbulent pipe flow of wood pulp fibre suspensions in water. This first report of drag reduction was unnoticed. Later, Mysels found that the skin friction for gasoline in pipe flow was significantly reduced by adding an aluminium disoap (an anionic surfactant). This was the first drag reduction result that was recognised. While doing polymer degradation research, Toms observed that adding a long-chain polymer (polymethyl methacrylate) in monochlorobenzene dramatically reduced the turbulent skin friction drag by up to 80%. At the First International Rheological Congress, Toms reported these results, identifying polymers as effective drag reducers. He observed that at a constant pressure gradient, the flow rate could be increased by adding the polymer. Therefore, it is usually referred to as the “Toms Effect.” Later, Savins first used the term “Drag Reduction.” The first commercial application for high polymer drags reduction was used in the 48-inch diameter 800-mile-long Alaska pipeline carrying crude oil from the North slope in Alaska to Valdez in the south of Alaska. Due to the very slow recovery (days) after high shear through a pump, aluminium soaps were not practical as a drag reducer. Later, drag reduction in aqueous systems using surfactant additives were found to be effective and
1.3 Basic Purpose (why they are needed)
A friction reducer is one of the main components of slickwater fracture fluids and other stimulation fluids. The primary function of friction reducers is changing turbulent flow to laminar flow, reducing frictional loss during pumping fracturing fluids up to 80%. High viscosity friction reducers (HVFRs) have recently gained popularity as drilling and hydraulic fracturing fluids because the HVFRs exhibit numerous advantages. such as the following: • Providing pipe friction reduction during fracturing treatments • High regain conductivities compared to linear and crosslinked gels • Lower operational cost ◦ Use less water compared to conventional slickwater treatments, ◦ Consume fewer chemicals by 33-48% ◦ Require less equipment on location, ◦ Require less number of tanks trucks in the field • Placing better proppant volumes as previous fluids hybrid and crosslinked • Creating more fracture complexity using a less viscous fluid system • Achieve better hydrocarbon production results compare to or greater than other fluids. • Improves the flexibility to design treatments that balance technical, economic, and operational goals • Reduces freshwater, proppant, and equipment requirements compared with conventional fluid systems • Minimizes environmental footprint with a selection of engineered additives • Simplifies operations by reducing screen out risks
2 Types of Drag Reducers
2.1 Polymeric drug reducers
Polymers are effective drag reducers owing to their ability to suppress the formation of turbulent eddies at low concentrations. Toms indicated that DR could be achieved using polymer additives and concluded that different polymers could serve as effective drag reducers. The polymer was first commercially used in 1979 as a drag reducer in a 48-inch diameter and 800-mile-long Trans Alaska crude oil pipeline from North to South Alaska. Since then, polymer additives have been acknowledged for their DR ability. DR of up to 80% can be attained by adding only a few parts per million of polymer. It was concluded that polymers must exceed a minimum chain size for effective DR to occur. Drag reducing polymers are normally long-chain, high-molecular-weight (ranging from 1 to 10 million) polymers such as polyethene oxide (PEO) and polyacrylamide (PAA). They also exhibit non-Newtonian and viscoelastic behaviours. Polymer additives are the most studied among all drag-reducing methods because, at a low quantity, a significantly large DR can be attained. Parameters influencing the performances of drag-reducing polymers include flow rate, injection point, channel size, geometry, surface roughness, molecular weight, chain flexibility, structure, concentration, solvent, salt content, pH, and temperature. Generally, DR increases with increasing polymer concentration, molecular weight, Reynolds number, and flow rate.
2.2 Solid particle suspensions as drag reducers
The drag reduction caused by the addition of suspended solid matter has been the subject of study since the beginning of the last century. The study was initiated because turbid streams of water were found to flow faster than clear ones. One of the first comprehensive experimental works was undertaken in the 1940s. Despite this early start, more than ten years before investigators began a systematic study of solid suspension drag reduction. Suspensions may be of two different types, namely: (a) Granular or nearly spherical particles (b) Fibers. To date, the effects of solids suspended in flowing liquids have not been investigated as thoroughly as those of polymer additives, but, from the industrial point of view, suspensions of solids have the edge over that of polymers for the following two reasons: 1. The solid matter can be added and removed from the flowing liquid with great ease. 2. They suffer no mechanical degradation in most cases except in a few, as noted by some workers
2.3 Biological additives as drag reducers
The drag-reducing ability of biological additives came to light as serendipity. When models of prospective new ships were tested in towing tanks to determine their drag and propulsion characteristics, Hoyt was the first to notice the drastic variation in the turbulent flow properties of water in those tanks. He explained that the long-chain polysaccharides produced by living organisms during growth were the cause of this drag reduction. The drag reduction measurements initiated the study of the polysaccharides of several fresh glasses of water and marine algae. The effect of pH and temperature was studied on the bacterial production of drag-reducing polysaccharides. It was concluded that the bacteria could be cold-loving or heat-loving, but they are often more effective in alkaline media. They also show long-term stability and are thus superior to poly (ethylene oxide), which was used initially in towing tanks. Seawater slime and other freshwater biological growths have also been effective drag reducers.
2.4 Surfactants as drag reducers
“Surfactant” is a very convenient contraction of the term “surface-active agent.” It connotes an organic molecule or an unformulated compound having surface-active properties. Three surfactants, namely, anionic, cationic, and nonionic, have been investigated for drag reduction.
3 Mechanism of Action
slip at wall theory: In 1948, Oldroyd was the first to suggest that polymer molecules, during flow, affected the region near the wall most strongly. His slip-at-the-wall theory has remained a mere engineering correlation method rather than a fundamental answer to drag reduction mechanism. It will remain until an improved method for measuring velocity profiles near the wall is found.
Sheer thinning wall theory: Toms proposed the idea of a shear-thinning wall layer with an extremely low viscosity, which resulted in lower friction coefficients for the drag-reducing solutions than for pure solvents
viscoelastic theory: It suggested that drag reduction could be due to two effects of viscoelasticity. 1)The instantaneous shear stress at the wall being smaller for viscoelastic fluid than for the corresponding Newtonian fluid 2) The renewal of the elements of liquid along the wall taking place more slowly in the case of viscoelastic fluid than in the case of a Newtonian fluid
Drag reducing additives are important in oil drilling applications and the maintenance of pumping equipment in pipelines
5 The global market of drag reducers
Here are some companies dealing with drag reducers
Kemira KemFlow friction reducers
Shandong Raitte Chemical Co., Ltd. China, Guangxi
6 Future growth in the DRA industry
Recent research focuses on the use of polymer additives as drag-reducing agents, the general formulations of the additives, major issues involving the use of drag-reducing polymers, and its potential applications. However, despite the extensive works of drag reduction polymer, there are still no models that accurately explain the mechanism of drag reduction. More studies needed to be done to understand the phenomenon better. Therefore, future research areas and potential approaches are proposed for future work.