Yield point (YP) is a rheological property of drilling fluid representing the resistance to initial flow. Measured in lb/100ft² using a Fann viscometer, yield point indicates the fluid's ability to suspend cuttings and weighting materials when circulation stops.
API Operating Range
API 13B-1 specifies: 5 ≤ YP ≤ 3×PV (lb/100ft²), where PV is plastic viscosity. Recommended PV range is 8-35 cP, with 10-minute gel strength between 2-35 lb/100ft².
YP/PV Ratio and Hole Cleaning
The relationship between YP and PV significantly impacts cuttings transport: Higher YP/PV ratios improve cuttings carrying capacity in laminar flow (0-45° inclination). Lower YP is preferred in turbulent flow regimes (55-90° inclination). The 45-55° inclination range represents a transitional zone requiring balanced approach.
Research by Hussaini and Azar (1983) demonstrated that medium rheology (PV=14, YP=14) outperforms both low and high extremes in subcritical flow conditions.
Cuttings Transport Efficiency
A Cuttings Transport Ratio (CTR) of 75% or greater is considered sufficient for effective hole cleaning. Transport velocity must exceed cutting slip velocity—a relationship directly influenced by yield point.
ECD Trade-off
High yield point improves cuttings transport but increases equivalent circulating density. Optimal practice uses low PV combined with high YP to maximize hole cleaning with minimal ECD impact.
Automation Connection
Drilling optimization technology must account for fluid rheology when optimizing parameters. Autonomous systems adjust flow rate and rotation speed based on real-time hole cleaning indicators, working within the constraints established by mud properties including yield point.