In highly deviated wells and horizontal wells, gravity increases the contact area between the drill string and the formation, and proper control of the lubricity of the drilling fluid and the quality of the filter cake in the permeable formation can help reduce the occurrence of stuck pipe accidents.
The mechanical causes of pipe sticking include: isolation caused by poor wellbore cleaning, shale expansion, wellbore collapse, formation plastic flow (that is, plastic fluid in the wellbore), etc. Preventing stuck pipe requires close monitoring of early warning signs such as increases in torque and drag, signs of excessive cuttings loading, sticking while running in, and loss of circulation while drilling.
Depending on what is causing the stuck pipe, the drilling fluid density needs to be increased (to stabilize the expanding shale) or decreased (to protect the depleted zone and avoid differential sticking). The coefficient of friction of the fluid is an important factor in effectively preventing stuck pipe and/or allowing stuck pipe to work freely.
Oil and synthetic bases provide maximum lubricity; inhibited water bases can be treated with lubricant (typically 1% to 5% by volume) and formulated into a thin, impermeable filter cake for added protection from sticking .
