When drilling HDD bores, the measured depth is typically shallow at around 120 feet. A 3,500-foot underground crossing may pass under a river, road, sharp freeway or a sensitive area, and sometimes with certain soil types, the soil will break through to the surface: this is known as an inadvertent return, or IR.
Mud engineers help correct these types of problems with lost circulation. In some situations, mud might break through the fracture from the wellbore all the way up to the surface. Engineers not only administer fluids for the wellbore, but they're also responsible for fixing IRs.
When engineers realize that they're losing returns, they must determine if it's coming up through the ground or if it's due to a fracture underneath. In a typical application, engineers spot a pill and squeeze it with lost circulation material that consists of an environmentally safe sealant that has expandable material that is designed to correct IRs. The sealant consists of natural plant fibers. They’re compressible particles that can form to the pores and fractures to provide a bridge to seal to prevent any further loss of drilling mud.
This is done by sending a pill (about 25 barrels) down the drill pipe, chasing it out of the bit and spotting the pill. By backing up the pipe, you can get on top of the pill (about 40 or 50 feet on top of it where the pill is set) and then the formation is squeezed. The purpose of the squeeze is to seal those fractures and micro-fractures.
This process is usually done at the onset of an IR, and the squeeze lasts for a couple of hours. Another squeeze will follow to prevent any further IRs.