Petroleum Engineering Laboratory: Rheological Properties Of Drilling Fluid

Abstract

Viscosity is one of the primary rheological properties of drilling fluid, crucial for monitoring drilling operations and enhancing well cleaning efficiency. This report presents experimental data on shear stress, apparent viscosity, shear rate, and gel strength of test fluids, measured using a rotary viscometer. Viscosity measurements can vary depending on equipment accuracy and fluid sources. The report analyzes rheological properties through laboratory testing and trend line modeling of rheological equations.

Introduction

Rheology is the study of matter's flow and deformation, particularly non-Newtonian flow of liquids.

Viscosity is a fluid's resistance to deformation at a given rate. This experiment focuses on mud rheology properties, crucial for successful drilling operations.

Aim of the Experiment

The primary aim of this experiment is to determine the viscosity of three different mud samples using a rotary viscometer.

Objectives

• Investigate the effect of density on viscosity.
• Illustrate the use of a rotary viscometer for different fluids.
• Determine viscosity of Newtonian liquids.
• Compare apparent viscosity and plastic viscosity.
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• Calculate viscosity, yield point, and gel strength.
• Understand fluid behavior and viscosity using a rotary viscometer.
• Learn how to operate rotary viscometer equipment.

Scope of the Experiment

The scope of this experiment encompasses various aspects of mud rheology. It includes measuring bentonite and barite using an electronic balance, measuring water with a graduated cylinder, preparing mud samples using an electric mud mixer, and measuring mud viscosity using a rotary viscometer at room temperature.

Limitations of the Experiment

• This experiment is designed to work under specific conditions.
• It lacks temperature control capabilities, limiting its applicability to temperature-sensitive fluids.
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• The experiment is not suitable for analyzing non-Newtonian fluids.

Theory

Viscosity is a fluid's resistance to flow and plays a pivotal role in chemical and biological engineering processes. In this experiment, we aim to measure mud viscosity, a crucial property that influences wellbore pressure and drilling operations. The Bingham plastic model serves as the foundation for calculating rheological properties such as apparent viscosity, plastic viscosity, yield point, and gel strength.

One significant application of mud viscosity in drilling operations is the control of wellbore pressure. As the depth of a well increases, the formation pressure also rises. To ensure that the wellbore pressure surpasses the formation pressure, drilling fluids are engineered to increase hydrostatic pressure at greater depths. This is often achieved by introducing materials like clay solids or barite with a specific gravity of 4.2 into the drilling fluid, a cost-effective approach to enhance hydrostatic pressure.

Apparatus and Materials

The experiment employed various equipment and materials, essential for measuring the viscosity of mud samples. The key components include:

1. Rotary Viscometer (Analog): This device utilizes the torque on a rotating shaft to measure a fluid's resistance to flow. It allows for adjustable shear rates by modifying rotor speed, dimensions, and rotor-stator gaps.
2. Electronic Balance: An electronic balance is an accurate instrument for measuring the weight of materials. It played a crucial role in measuring bentonite and barite precisely.
3. Electric (Mud) Mixer and Steel Vessels: The mud mixer facilitated the preparation of mud samples by mixing water with materials like bentonite or barite.
4. Measuring Cylinder: This common laboratory tool was used to measure the volume of liquids, particularly for measuring water in this experiment.
5. Stopwatch: A stopwatch was employed to record timing during specific parts of the experiment.
6. Sodium Hydroxide: Sodium hydroxide, also known as lye and caustic soda, was used as a chemical reagent in the experiment.

Experimental Procedures

Rotary Viscometer (Analog)

The rotary viscometer, an essential component of this experiment, measures flow characteristics by analyzing shear rate and shear stress. It offers eight precisely regulated test speeds, ranging from 3 to 600 RPM. Gel strength, a critical rheological property, was measured both immediately after agitation and following a ten-minute rest. The difference between the two readings provides insights into the thixotropy of the mud system.

Preparing the Mud

1. Begin by placing a clean, dry beaker on the electronic balance and reset the balance to zero since the beaker has its weight.
2. Measure the required quantities of bentonite, salt (Sodium hydroxide), and barite using the electronic balance.
3. Measure 400ml of water using a graduated cylinder to prepare the mud samples.
4. Pour the water into a steel cup and gradually add the bentonite while continuously mixing.
5. Wait for a few minutes for the mixture to reach a uniform consistency.
6. Inspect the mixture for lumps using a spatula. If lumps are present, return the steel cup to the mixer for further mixing.
7. If there are no lumps, remove the cup from the mixer and transfer the mud into a clean bowl. Repeat this process approximately four times.
8. Fill the viscometer with mud, ensuring that the mud is poured through a filter to remove impurities.
9. Use a timer, a cup, and a funnel to measure and record the filling time as you pour water into the cup until it reaches 960ml.
10. Repeat the above procedures for the other two mud samples.

Procedure

Operate the rotary viscometer by attaching the splash guard and the appropriate bob, then immerse the rotor sleeve up to the fill line with the mud sample. Tighten the lock nut on the platform and turn on the instrument. Rotate the speed selector knob to mix the sample and then set it to different RPM settings, waiting for the dial to stabilize before recording the readings. Gel strength is measured after specified durations of rest.

Maintenance

After each test, disassemble the viscometer, remove the bob and splash guard, and wipe down the bob shaft. Clean all components with soap and water, and ensure they are thoroughly dry before the next use.

Results

The experiment yielded data on the viscosity of mud samples A, B, and C at various RPM settings. The results are summarized in the table below:

Gel Strength (lb/100ft²):

• Sample A: 10-second = 1, 10-minute = 0
• Sample B: 10-second = 3, 10-minute = 1
• Sample C: 10-second = 16, 10-minute = 8

Plastic Viscosity (cp): PV = 6.0 - 4.0 = 2.0

Yield Point (lb/100ft²): YP = 4.0 - 2.0 = 2.0

Apparent Viscosity (cp): AV = 6.0 / 2.0 = 3.0

Discussion

The experiment successfully measured the viscosity of mud samples A, B, and C. The plastic viscosity, yield point, and apparent viscosity were calculated from the data. The gel strength was also determined, reflecting the thixotropy of the mud samples.

Viscosity is a critical property in drilling operations, as it impacts wellbore pressure and drilling efficiency. Understanding the rheological properties of drilling mud is essential for safe and effective drilling.

One significant observation is the difference in gel strengths between the 10-second and 10-minute measurements for the mud samples. This difference indicates the thixotropic nature of the mud, with the gel strength increasing over time. Thixotropy is an important property for drilling fluids, as it affects the fluid's ability to suspend cuttings and maintain wellbore stability.

Conclusion

This experiment aimed to measure mud viscosity using a rotary viscometer. The results provided valuable insights into the rheological properties of the mud samples, including plastic viscosity, yield point, and gel strength. Understanding these properties is crucial for efficient drilling operations, as they impact wellbore pressure and drilling performance. Overall, this experiment enhances our knowledge of fluid behavior and viscosity analysis using a rotary viscometer.