Hydraulic Wellbore Erosion While Drilling

Abstract

Abstract This study shows that nozzle velocity and not annular velocity causes hole enlargement in competent formations. Five similar wells were selected where pump rates and nozzle sizes could be controlled to evaluate the effect of nozzle shear rate and annular velocity. After the wells were drilled, caliper logs evaluated hole erosion. These comparative studies revealed that annular velocity did not cause erosion. Holes in which the nozzle velocity was low and annular velocities high eroded less than holes with high nozzle velocities and low annular velocities. Two wells were drilled in competent formations with an oil-base drilling fluid and three were drilled with a water-base drilling fluid. Below surface casing, all wells used IADC code 427, 517 and 537 drill bits. The data indicates that even with oil-base drilling fluid, high nozzle shear-rates eroded the wellbore more than holes drilled with lower nozzle shear rates. High annular velocities did not erode the hole. Adequate bottomhole scavenging was maintained even with lower nozzle velocities. Drilling rates did not decrease because of inadequate bottomhole cleaning. This paper describes the drilling conditions, and the caliper logs, and discusses sequences of events that could explain the observations. Some preliminary guidelines are presented so that drillers can prevent erosion of the wellbore from high shear rates at the bit nozzles.