Significance of ECD and factors affecting it
It’s advisable to keep ECD under control. Efforts to minimize ECD assume even more significance in the narrow drilling margin scenarios where the difference between formation pressure and fracture pressure is small. This phenomenon is more prevalent in HPHT and deep-water wells. In such cases, the well will be stable in static conditions but could start losing as soon as the circulation begins.
High ECD could fracture formation, causing lost circulation and leading to well control situations. Ballooning or supercharging the formation is another ill effect of high ECD, where the well seems to be losing mud in dynamic conditions and gaining when static. Ballooning creates confusion and wastes rig time in diagnostic efforts.
ECD is also a critical factor in Horizontal and ERD wells. These wells typically have long Measured Depth (MD), which results in high APL, but shorter True Vertical Depth (TVD), which results in lower fracture strength.
Factors affecting ECD:
ECD is high if annular pressure losses (APL) are high. The longer the mud travels from the bottom of the well to the surface, the higher the APL. Hence, wells with higher Measured Depth (MD) will see higher ECD.
Annular Pressure Loss (APL) depends on the pump rate. The higher the pump rate, the higher the ECD.
APL also depends on annular clearance. The hole size and size of Bottom Hole Assembly (BHA) components define the annular clearance. Smaller annular clearance causes more restriction to flow and increases ECD.
Mud rheology affects ECD. Higher viscosity and gelation will result in higher frictional losses and higher pressure to break gels, increasing ECD.
A higher percentage of Low Gravity Solids (LGS) in the mud system tends to scramble mud rheology and result in higher ECD.
Low hole cleaning efficiency results in more suspended cuttings or caving in the annulus, increasing ECD.
Drilling practices like sudden acceleration or deceleration create pressure spikes that fluctuate ECD.