Drilling Fluids, Circulation and Wellbore Stability
Domain focus
This domain tests mud properties, circulation, and wellbore stability. Expect questions on density, viscosity, gel strength, filtration, pH, solids, equivalent circulating density (ECD), annular velocity, and the balance between formation pressure and fracture pressure.
Core rules
- Mud functions: cool and lubricate the bit, carry cuttings, suspend solids, control formation pressure, stabilize the wellbore, and help seal permeable zones.
- Hydrostatic pressure in psi equals mud weight in ppg × 0.052 × true vertical depth in ft. In metric units, MPa equals specific gravity × 0.00981 × TVD in m.
- ECD includes annular pressure loss; it is higher than static mud weight when circulating. Check ECD against fracture pressure, especially in narrow margins.
- Lost circulation, swelling clays, stuck pipe, and poor hole cleaning are stability symptoms; treat the cause, not only the symptom.
- Add barite or base fluid in controlled amounts and circulate/condition fully before interpreting a property change.
Worked example
Mud is 1.25 sg at 2,500 m TVD. Hydrostatic pressure is 1.25 × 0.00981 × 2,500 = 30.66 MPa. If formation pressure is 31.00 MPa, the well is underbalanced by 0.34 MPa, so a kick is possible. The static kill weight needed is 31.00 ÷ (0.00981 × 2,500) = 1.264 sg. If annular loss adds 0.80 MPa while circulating, ECD becomes about 1.283 sg, which must be compared with fracture pressure.
Traps
Confusing mud weight with viscosity; ignoring ECD and using static density only; adding barite without checking rheology and solids; assuming a low pump pressure means good hole cleaning.