Ethan Clarke
Plastic viscosity is a crucial parameter in drilling operations. It is the resistance to the flow of a fluid in bores, caused by mechanical friction within the drilling fluids between solids and liquids, and the deformation of liquid under shear stress/pressure. Monitoring plastic viscosity is essential to prevent drilling problems and achieve successful operations. High plastic viscosity is undesirable as it causes increased torque and drag, low bit penetration rate, and increased surge and swab pressures. The plastic viscosity of drilling mud is affected by the type of base fluid, solids concentration, temperature, and other factors.
| Characteristics | Values |
|---|---|
| Definition | Plastic viscosity (PV) is the resistance to the flow of a fluid in bores. |
| Calculation | The Bingham Plastic mathematical rheological model generated from rheometer reading (R300 and R600) in a lab. |
| Testing methods | Marsh cup and funnel, Fann VG Viscometer, March funnel |
| Importance | Monitoring plastic viscosity is essential to prevent various drilling problems and achieve successful drilling operations. |
| Desirable level | Low plastic viscosity is desirable as it results in higher energy at the drilling bit, better hole cleaning, and less wear on equipment. |
| Consequences of high viscosity | Increase in torque and drag, low bit penetration rate, increase in surge and swab pressures, the possibility of pipe sticking, increased equivalent circulating density, decreased rate of penetration |
| Related concepts | Apparent viscosity, yield point, gel strength, shear thinning, Newtonian, non-Newtonian |
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What You'll Learn
Plastic viscosity is undesirable at high levels
Plastic viscosity is the viscosity that a fluid has at a very high shear rate, such as when it flows through a bit nozzle. It is the resistance to the flow of a fluid in bores, caused by mechanical friction within the drilling fluids between solids and liquids, and the deformation of liquid under shear stress/pressure.
High plastic viscosity is not desirable in drilling fluid as it causes several issues. Firstly, there is an increase in torque and drag, resulting in a low bit penetration rate. Secondly, there is an increase in surge and swab pressures, which can lead to pipe sticking. These problems are associated with wellbore issues, indicating that high plastic viscosity is detrimental to drilling operations.
A fluid with high plastic viscosity can also cause an increase in equivalent circulating density due to increased pump pressures required to pump the fluid in the wellbore. This results in a decreased rate of penetration, as the energy required to pump the fluid increases with increased viscosity. Therefore, high plastic viscosity negatively impacts the efficiency of drilling processes.
Additionally, high plastic viscosity can lead to viscosity problems if drilled solids are not controlled. The viscosity of the fluid phase is influenced by temperature, with plastic viscosity decreasing as temperature increases. This relationship is particularly evident in water-based fluids, where the viscosity of water decreases with higher temperatures. Oil emulsified in water-base fluids can also act as a solid, impacting the plastic viscosity of the fluid.
In summary, high levels of plastic viscosity in drilling fluids are undesirable due to the resulting increase in torque and drag, surge and swab pressures, possibility of pipe sticking, decreased rate of penetration, increased pump pressures, and potential viscosity problems. These issues can hinder the success of drilling operations, highlighting the importance of monitoring and controlling plastic viscosity during drilling processes.
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Monitoring viscosity during drilling
Drilling fluids, also known as drilling muds, are critical to the drilling process. They serve multiple functions, including cooling the drill bit, carrying rock cuttings to the surface, and maintaining wellbore stability. One of the key properties of drilling fluids is their viscosity, which is a measure of a fluid's resistance to flow.
Viscosity is a critical parameter that directly influences drilling performance, borehole stability, and operational safety. It is essential for hole cleaning, ensuring cuttings are efficiently transported to the surface. During drilling, the fluid acts as a lubricant and cools the drill. When drilling stops, the fluid prevents the well's cuttings from refilling the well hole by applying pressure to the well's walls.
To monitor viscosity, various methods and instruments are available, each with its advantages and limitations. Rotational viscometers, for example, measure viscosity by detecting the torque required to rotate a spindle or bob in the fluid at a given speed. The most common type is the Couette viscometer, with two concentric cylinders. Rheonics also offers inline viscosity and density sensors that provide continuous, accurate, real-time data on mud density, viscosity, and temperature.
By analysing viscosity data, drilling operators can optimise drilling fluid properties. For instance, if the viscosity is too high, a thinner may be added or the solids content reduced. Monitoring viscosity helps prevent drilling issues and improves well-cleaning efficiency. It is key to a successful well drilling process, allowing operators to recognise changes, analyse data trends, and act accordingly.
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Rheological models and categorisation
Rheology is the study of the deformation and flow of matter under stress or force. Rheological models are used to simulate the characteristics of drilling mud under dynamic conditions. They are critical for a drilling fluid study because they help predict the flow behaviour of drilling mud.
Drilling fluids are categorised into three types based on the base fluid used: Water-Based (WB), Oil-Based (OB), and Gas-Based (GB) drilling fluid. WB drilling fluids are preferable because they are cheaper, environmentally friendly, and capable of mitigating well-control problems.
The Bingham plastic model is a commonly used rheological model with two parameters: plastic viscosity and yield point. In this model, the curve of shear stress versus shear rate is a straight line that does not cross the origin. The Bingham plastic model is suitable for predicting the rheology of non-Newtonian fluids like drilling mud. However, it is not accurate enough for hydraulic calculations. Other rheological models include Newtonian, Power Law, and Hershel-Bulkley. The Newtonian fluid model is the simplest, where the viscosity is constant and does not change with the applied force.
The rheological properties of drilling mud include plastic viscosity, apparent viscosity, gel strength, and yield stress. Monitoring these properties during drilling operations is essential to prevent drilling problems and achieve successful operations. For example, high plastic viscosity in drilling fluid is undesirable as it increases torque and drag, reduces the rate of penetration, and can cause pipe sticking. On the other hand, low plastic viscosity and high yield point improve cuttings transport.
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The effect of temperature on viscosity
Plastic viscosity is the resistance to the flow of a fluid in bores. This resistance is caused by the mechanical friction within the drilling fluids between solids and liquids, and the deformation of liquid under shear stress/pressure.
Drilling fluid plays a crucial role in the drilling operations of petroleum wells. Drilling fluids are categorized into three types based on the base fluid used: Water-Based (WB), Oil-Based (OB), and Gas-Based (GB) drilling fluid. WB drilling fluids are preferable because they are cheaper, environmentally friendly, and capable of mitigating well-control problems.
Apparent and plastic viscosities are two of the significant rheological properties of drilling fluid. Monitoring the apparent and plastic viscosities during drilling operations is essential to prevent various drilling problems and achieve successful drilling operations.
The Bingham Plastic model is a mathematical rheological model that is often used to determine the apparent and plastic viscosities of drilling fluids. The model assumes that the curve of shear stress versus shear rate is a straight line but does not cross the origin. However, it has been observed that present-day drilling fluids do not follow the Bingham plastic flow model, and the flow equations expressing the relations of shear stress to shear rate vary from simple formulas to complex polynomials.
The effect of temperature on the viscosity of drilling fluids is significant. Investigations have shown that drilling fluids exhibit dramatic differences in viscosity at various temperatures. For example, when mud samples from a hot rolling oven were tested, the Farm meter was inadequate for measuring flow properties at elevated temperatures. This indicated the need for a more elaborate viscosimeter to conduct more comprehensive investigations into the flow properties of drilling fluids at elevated temperatures.
Additionally, the addition of nanoparticles to drilling fluids leads to a notable change in their rheological properties depending on the temperature. As the temperature increases, the yield stress and consistency index of drilling fluids with nanoparticles increase, while the behaviour index decreases. The particle size of the nanoparticles also plays a role, with larger particles having a greater influence on the temperature dependence of the drilling fluid's viscosity.
In summary, the effect of temperature on the viscosity of drilling fluids is complex and depends on various factors such as the type of drilling fluid, the presence of nanoparticles, and the range of temperatures encountered. Monitoring the apparent and plastic viscosities of drilling fluids during operations is essential to ensure successful drilling and prevent problems.
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The role of drilling fluid in petroleum wells
Drilling fluid, also known as drilling mud, is used to aid the drilling of boreholes into the earth. It is a general term for circulating fluid throughout the entire oil and gas production process. The drilling fluid stays in contact with the wellbore during the entire drilling process. Its main function is to design the drilling fluid formula so that the drilling project can be successfully completed in the wellbore to reach the expected petroleum formation.
The drilling fluid carries the rock excavated by the drill bit up to the surface. Its ability to do so depends on cutting size, shape, and density, and the speed of fluid travelling up the well (annular velocity). The drilling fluid should have sufficient suspending properties to ensure that cuttings and solid phases do not settle. The main functions of liquid drilling fluids are to exert hydrostatic pressure to prevent formation fluids from entering into the well bore, and carrying out drill cuttings as well as suspending the drill cuttings while drilling is paused.
Drilling fluids are categorised into three types based on the base fluid used: water-based, oil-based, and gas-based. Water-based drilling fluids are preferable because they are cheaper, environmentally friendly, and capable of mitigating well control problems. A typical water-based drilling fluid contains water as the base fluid and other additives such as perlite to control filtration loss. Oil-based muds are used for increased lubricity, enhanced shale inhibition, and greater cleaning abilities with less viscosity. They also withstand greater heat without breaking down.
The rheological properties of drilling fluids include plastic viscosity, apparent viscosity, gel strength, and yield point. Plastic viscosity is the resistance to the flow of a fluid in bores. This resistance is caused by the mechanical friction within the drilling fluids between solids and liquids, and the deformation of liquid under shear stress/pressure. Apparent viscosity is the shear stress applied to a fluid divided by the shear rate. Monitoring the apparent and plastic viscosities during drilling operations is essential to prevent various drilling problems and achieve successful drilling operations.
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Frequently asked questions
Plastic viscosity is the resistance to the flow of a fluid in bores. This resistance is caused by the mechanical friction within the drilling fluids between solids and liquids, and the deformation of liquid under shear stress/pressure.
Monitoring the apparent and plastic viscosities during drilling operations is essential to prevent various drilling problems and achieve successful drilling operations.
A fluid with high plastic viscosity is undesirable and could result in increased equivalent circulating density due to increased pump pressures to pump the fluid in the wellbore. Therefore, the plastic viscosity should be kept as low as possible.
Any increase in solid content in drilling mud will result in higher plastic viscosity. To lower the plastic viscosity, solid content must be removed by using solid control equipment and/or diluting drilling mud with the base fluid. Additionally, the viscosity of the base fluid decreases as the fluid temperature increases, resulting in a decrease in plastic viscosity.
Plastic viscosity is calculated using the Bingham Plastic mathematical rheological model generated from rheometer readings (R300 and R600) in a lab. The Bingham Plastic model assumes that the curve of shear stress versus shear rate is a straight line but does not cross the origin.
