After an oil well is drilled, casing is cemented in place and specific zones are perforated; companies use hydraulic fracturing to assist in recovery of the oil and gas downhole. Hydraulic fracturing, also known as fracking/ frac’ing, is a means of pumping fluids at extremely high pressures in order to fracture the rock downhole and optimize the radial drainage area so that the oil or gas can flow from the low permeability rock formations into the fractures created by the pumping and flow into the drilled wellbore so that they can be brought back to the surface.
Viscosity plays an important role in this system for two main reasons. First, you must have enough viscosity to suspend your proppant for the time it takes to travel through the wellbore and into the fracture where the proppant can be evenly dispersed throughout the fracture. Also, viscosity is directly proportional to the fracture width achieved in the fracture. The higher the viscosity, the wider your fracture will be due to higher pressure and improved filter cake build up resulting in lower leak-off rates.
There are a few different types of polymers commonly used in frac’ing; guar, guar derivatives and polyacrylamides/polysaccharides. While each is used for specific purposes or based on different applications, they are all fundamentally similar in nature. When hydrated, each molecule will begin to unravel and expand forming long chain polymers with a mannose (or similar sugar-like) backbone. At this point, the fluid will be between 8-30 cP depending on the loading (amount) of gel which was hydrated. Borate or metal ions can then be used to connect multiple polymer strands at the hydroxyl groups forming heavy, 3-D structures through a process known as crosslinking. Various chemicals are then used to break the fluids down to a viscosity close to that of the base gel or below.
Some types of fluids used in frac’ing:
- Slickwater (FR Water): Combines either a base gel or friction reducer, biocide, clay control, surfactant and breaker to produce a low viscosity system which creates a fluid system with a viscosity from 2-30 cP. Due to the low viscosity, the system will have to be pumped at a much higher rate to achieve the necessary proppant delivery characteristics. These fluids may not produce as much width as a higher viscosity crosslinked system, but they are very useful in creating fracture complexity in the near wellbore area. They will help connect various microfractures and secondary fracture networks to the main fracture which can improve fracture length and help reduce near wellbore tortuosity leading to less near wellbore screenouts.
- Crosslinked Gel: Combines gel, buffer, crosslinker (metal ion or borate), surfactant, biocide, clay control and breaker. These systems can be 100’s to 1,000’s of centipoise. The higher viscosity will allow for increased width in the fracture and lower leakoff, but the main function of the crosslinked gel is to aid in proppant transport and delivery into and down the fracture length. Delivering your proppant in a more viscous system will allow for a better proppant pack and higher concentrations of proppant to be delivered.
Viscosity of the systems are tested at various points both pre-job and during frac’ing jobs. Water from the frac’ing site will be brought in to the lab at the field camp to do pre-job testing. This is done to determine the appropriate loading of dry gel or friction reducer that will be added to the water to achieve the desired viscosity based on the job design. Testing will also confirm that the system can be crosslinked and the loading of each chemical necessary to the crosslinking can be determined. Maybe the most important lab test that is done for frac’ing systems is the break test where the appropriate loading of chemical breaker is determined as well as the time it takes to break the system down to a viscosity that can be recovered. If fluids are not broken properly, the regained permeability of your proppant pack will be reduced significantly. While some residue will always be left no matter how well the system is broken, and improperly broken system can reduce the regained permeability by up to 80%. With this testing, you know the appropriate gel loading and viscosity as well as the appropriate breaker loading and time to achieve proper viscosity after breaking. During the frac’ing job, you will take spot measurements of the linear gel to test the viscosity to make sure that you are achieving the appropriate viscosity, pull crosslinked samples to get a visual check for crosslinking and run a break test with a clean crosslinked sample.
The advantages of knowing your viscosity have been defined above. If you are not measuring your viscosity, you will not know if you are running the appropriate gel loading. Too little gel; you may not achieve the appropriate friction reduction necessary to pump the system downhole, you will not achieve the appropriate fracture width and you risk a screenout due to improper proppant transport. Too much gel; you may overpressure if your pump rate is too high, you will not have enough breaker to properly break the system with a higher gel loading leading to poor recovery rates, and you will increase job costs significantly since the base gels can be quite expensive and it is easy to run an excessive amount over design if you are not checking viscosity throughout the job. So it is always best to know your viscosity. This will lead to better jobs and improved production each and every time!
RheoSense VROC® viscometers are the perfect tool for measuring viscosity in hydraulic fracturing systems. VROC technology is easy to use, allowing operators to quickly and easily run as many spot check on frac fluids as you need to, removing any question of whether dry gels are loading correctly. The use of microfluidics and MEMS technology provides the sensitivity and ability to measure small differences in fluids, allowing lab personnel to test friction reducers prior to jobs to determine if they are still properly hydrated. With high accuracy and repeatability you can be certain that your fluid is being run as designed. With VROC technology you can ensure you keep your chemical costs on jobs down, and also keep water usage at the appropriate level, and take the guess work out of fluid system design and delivery.
Written by: Tyler Etheredge, RheoSense Technical Sales Representative
- Hayes (2022). Fracking. Investopedia – Commodities – Oil. https://www.investopedia.com/terms/f/fracking.asp
- Montgomery (2013). Fracturing Fluid Components. Effective and Sustainable Hydraulic Fracturing. https://cdn.intechopen.com/pdfs/44660/InTech-Fracturing_fluid_components.pdf