During the oil extraction process, line pipe is not the only tool needed for successful drilling. Among the materials used with line pipe are drill collars. Drill collars are used to provide weight to the
drill bit. They are the tubular section of the drill string, with thick walls, made of steel or special alloys. Another major function of the drill collar is to provide a path for the drilling fluid that is pumped in. The drill collar provides drilling weight to the drill bit and prevents the drill string from bending. In addition, the
drill pipe should not be run in a compressed state, otherwise it will be seriously damaged, so we need to know the weight of the drill collar that is enough to provide weight to the drill bit.
Drill Collar Dynamics
The working principle of the drill collar is that gravity acts on the drill collar. Due to the large mass of the drill collar, gravity will pull the drill collar down and provide the downward force required for the drill bit to penetrate the ground. Multiple drill collars can also be assembled together to form what is called a bottom hole assembly. During the drilling process, the downward force provided by the drill collar is formed by lowering the drill bit until it touches the bottom, which also puts more weight on the drill bit. Given how these drill collars work, it is critical that their materials provide the weight and bending strength required for their function.
Drill Collar Size Standards
Most drill collars conform to American Petroleum Institute (API) size standards, such as API 5DP and 7-1. Key measurements include outside diameter (OD), inside diameter (ID), and wall thickness. The OD typically ranges from 4 1/2 inches to 9 5/8 inches. Common IDs range from 2 to 4 1/2 inches, and wall thicknesses typically range from 3/4 inch to 2 inches. Heavier, narrower-walled drill collars provide more rigidity.
How to calculate the weight of a drill collar?
The weight of a drill collar depends on its density, length, and cross-sectional area. Here are the steps to calculate the weight of a drill collar:
Get the OD, ID, and length in feet.
Calculate cross-sectional area: Cross-sectional area = π (OD² - ID²) / 4
Determine the appropriate density based on the material density. E-75 alloy steel uses 490 lb/ft³.
Multiply the length and cross-sectional area to calculate the volume.
Multiply the volume and density to get the total weight.
For example, let's calculate the weight of a 60-foot, 7-inch OD x 3-inch ID, E-75 collar:
Cross-sectional area = π (7² - 3²) / 4 = 15.904 in²
Volume = Length x Area = 60 feet x 15.904 square inches = 954.24 square inches
The density of E-75 is 490 lbs/ft3
Weight = Volume x Density = 954.24 cubic inches x (490 lbs/ft3 / 1728 in3/ft3) = 26,000 pounds
So, the total weight of this 60-foot long, 7-inch OD collar is 26,000 pounds or 13 tons.
Steps to Determining Drill Collar Weight
Determining the correct
drill collar weight requires a systematic approach that considers a variety of factors. Here are the steps to determine the weight of drill collars:
Gather necessary information: Collect drilling data, including information about the geological formations being drilled. You must also consider drilling fluid property data, such as density and viscosity. Consider wellbore conditions, including diameter and inclination.
Calculate effective weight: Use the information collected to calculate the drill collar weight. Also, consider the hardness of the rock formation and adjust the weight accordingly. Consider the properties of the drilling fluid to determine the appropriate downward force.
Consider safety factors: Use safety margins to account for unexpected changes or uncertainties in good conditions. Also, ensure that the calculated weight provides a strong stability margin to prevent problems during drilling.
Monitor and adjust during drilling: Continuously monitor drilling parameters and conditions in real time. Also, adjust the drill collar weight at any time based on changes in geological or operating conditions.
Use weight indicators and measurements: You must use weight indicators on the drilling equipment to measure the force applied. Use downhole measurements to evaluate the actual downhole conditions and make necessary adjustments.
Review and improve: Regularly review drilling performance and evaluate the effectiveness of the drill collar weights used. Improve weight calculations based on ongoing drilling experience and data feedback.