Definition - What does Mud Weight mean?
Mud weight is measured as the weight of the drilling fluid against the weight of water. Mud weight is an important property of drilling fluid and plays a crucial role in the drilling process. It needs to be maintained within limits to prevent failure of the borehole. Trenchless technology utilizes HDD and other forms of horizontal drilling methods in which mud weight becomes a critical factor because the original equilibrium of the rock mass in a horizontal bore gets disturbed. The soil and rock that surround the borehole have to sustain part of the load that was previously supported by the displaced material. It's the drilling fluid that exerts pressure and supports the surrounding soil from collapsing.
Trenchlesspedia explains Mud Weight
During the drilling process, mud exerts pressure on the borehole wall, and affects the stresses that act on it. The well bore can fail due to toric shear failure, elongated shear, helical shear or tensile failure depending on whether the mud pressure exceeded the limit or fell short. When the mud pressure falls below the lower limit, the borehole becomes unstable and can collapse due to lack of support. When the mud pressure exceeds the upper limit, the borehole can collapse due to excessive mud pressure, the drilling fluid can flow out into formation fractures, the rate of penetration decreases, the drill string may get stuck in permeable rocks, and the amount of mud invading porous rock formation surrounding the borehole will increase. The limiting values of mud weight are known as critical mud weight and have to be maintained within these two limits to keep the bore hole in safe working condition.