Successful Breaker Optimization for Polyacrylamide Friction Reducers Used in Slickwater Fracturing
- Paul Scott Carman (BJ Services Company) | Kay Cawiezel
- Document ID
- Society of Petroleum Engineers
- SPE Hydraulic Fracturing Technology Conference, 29-31 January, College Station, Texas, U.S.A.
- Publication Date
- Document Type
- Conference Paper
- 2007. Society of Petroleum Engineers
- 5.8.1 Tight Gas, 4.3.4 Scale, 4.1.5 Processing Equipment, 3.2.3 Hydraulic Fracturing Design, Implementation and Optimisation, 5.8.2 Shale Gas, 1.8 Formation Damage, 2.4.3 Sand/Solids Control, 5.4.10 Microbial Methods, 2.5.2 Fracturing Materials (Fluids, Proppant), 4.1.2 Separation and Treating, 4.3.1 Hydrates
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To pump a high-rate fracture stimulation with fresh water or brines, a friction reducer must be employed. Most friction reducers used in slickwater fracture stimulation are high-molecular-weight polyacrylamide emulsions. Since these friction reducers are typically pumped at low concentrations (0.5 to 2 gpt), the industry belief has been that these polymers are causing minimal damage to the formation. There has also been a perception that the polyacrylamide polymer is difficult to degrade. However, fracture stimulations in areas such as the Barnett Shale use very large quantities of water with several stages per well which introduces large volumes of friction reducer into the formation. Because these large treatments are now widely used, several production companies have recently expressed concerns about the possible fracture and formation damage caused by these friction reducers. For this reason, a study was undertaken to find suitable breakers to effectively reduce the size (or molecular weight) of the polyacrylamide polymer chains and, therefore, reduce fracture and formation damage. Several conventional oxidizers were chosen and screened to determine if they could effectively reduce the viscosity of the polyacrylamide polymer. The molecular weight degradation of the polymer was then measured using a molecular weight cut-off (MWCO) filtration technique to determine the size and percentage of polymer fragments. Surprisingly, most of the breakers tested showed some reduction in viscosity and in molecular weight. Additional laboratory testing was then conducted to ensure that the addition of breaker to the friction reducer did not adversely affect polymer hydration or friction-reduction performance. This study reports on the results of the testing and shows which breakers were most effective in reducing the molecular weight of the polyacrylamide polymers used as slickwater friction reducers.
In 1997 with the success of slickwater fracturing in the Cotton Valley Sand in East Texas, this fracturing technique was further developed and successfully used in other tight gas sand reservoirs such as the Barnett Shale. These treatments typically utilized fresh water containing friction reducer as the primary fracturing fluid. However, these early treatments with low viscosity fluids had proppant transport and more importantly, proppant placement issues. Because earlier jobs used relatively small volumes of friction reducers, there was little concern about polymer damage. Development of the technology continued and eventually resulted in fluid and proppant volumes being optimized for the required treatment performance. The treatments became larger and treatment rates increased. Fluid volumes typically used today in these treatments range from 1.0 to 2.5 million gallons of water. The concentration of chemical additives including the friction reducer remains at about the same concentrations in these fluids, but the cumulative volume pumped now is much larger. The large volumes of friction reducers now pumped have become an industry concern due to possible fracture and formation damage caused by the polymer. Because the friction reducers primarily used for slickwater fracturing are polyacrylamides which are synthetic polymers, the perception was that they would be difficult to break.
Today, service companies pump friction reducers as liquid emulsions, which make them easy to pump and meter. These emulsions are also easy to disperse and hydrate into water. There are several kinds of polyacrylamides (anionic, cationic and nonionic) used for different conditions and compatibilities. These can be used in a variety of water types from fresh to produced brines. These polyacrylamides have varying chain lengths or molecular weights. The size or molecular weight of these polymers gives them the friction-reducing properties. With too short a chain length, the polyacrylamides will not provide enough friction reduction. Polyacrylamides with long polymer chain lengths can be broken with exposure to high shear and again provide inadequate friction reduction.
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