Excessive torque and drag is a common problem in the drill string. It can cause a variety of problems that negatively impact well efficiency. The primary cause of often mechanical friction between the wellbore and tubulars. The solution is usually to add lubricants to the drilling fluid.
Prior to choosing any lubricant, it is important to evaluate all available options to see which one lubricates the best and has the best chemical compatibility with the drilling fluid. The most direct test method involves determining the coefficient of friction of the base fluid and comparing it to the coefficient of friction with the lubricant.
The lubricating properties of drilling fluids are an important factor in well design. Deviated holes are particularly susceptible to excess drag between the drill string and casing. Serious casing wear can also occur in deep and ultra-deep wells and is further complicated by tripping in and out of the hole.
The challenge for mud engineers is to design a drilling fluid system that adequately lubricates the drill string without compromising other physical properties such as density, rheology, and filtration. To test the drilling fluid, engineers use a specially designed Lubricity Tester. This instrument simulates the rotation of the drill string and the pressure of the drill pipe against the casing. A stationary block is pressed against a rotating ring while both are submerged in the test fluid. The control board measures the torque necessary to turn the ring at a constant speed and converts that into the coefficient of friction.
Excessive torque and drag (ETD) is a common drilling problem, especially in extended-reach wells. ETD can cause lower penetration rates, increased hook-load, damage to downhole tubulars, and failure to reach target zones. While it can have many causes, mechanical friction between the wellbore and tubulars is often to blame. An effective and economical way to reduce mechanical friction is to add lubricating agents to the drilling fluid.
Before choosing a lubricant, it is important to test several options to determine which has the best lubricating properties and chemical compatibility with the drilling fluid. Performance testing involves determining the coefficient of friction (Cf) of the base drilling fluid and comparing it to the Cf with each additive.