Field Verification of the Effect of Differential Pressure on Drilling Rate
Authors: Vidrine D.J. , Benit E.J. 
Published: December 2007

Abstract
A field study was conducted on eight South Louisiana wells to determine the effect of differential pressure on the instantaneous rate of penetration in shale.
Drilling rate is affected significantly by changes in differential pressure and may be reduced as much as 70 percent as the differential fluid pressure is increased from 0 to 1,000 psi. When the formation pressure becomes greater than the mud-column pressure, drilling rates continue to increase, sometimes at an increasing rate. The sensitivity of drilling rate to differential pressure depends upon the magnitude of the bit load. Overburden pressure and the hydrostatic head of the mud column over the intervals studied had no detectable erect on penetration rate.
Increases in formation pressure can be detected by an increase in penetration rate. The careful observation of drilling rate can help avoid kicks and allow the well to be maintained at minimum overbalance hydrostatic conditions. Decreased well-drilling costs and an increased drilling rate will result.

Introduction
As well depth increases, drilling rates decrease. The number of deep wells being drilled annually is increasing. Associated with these wells are low penetration rates and high costs. The reduction in drilling rate with depth is primarily a function of increased differential pressure. The paper presents a field study of the effect of pressure on the instantaneous rate of penetration in Gulf Coast Louisiana shales.
On the basis of laboratory investigations, several authors agree that drilling rate is decreased as the differential between the formation pore pressure and the hydrostatic pressure of the mud column is increased. Maurer conducted crater-volume tests while varying overburden, hydrostatic and formation-pore pressure. He concluded that drilling rate is influenced by a combination of both hydrostatic and differential pressure, but is independent of overburden pressure. Cunningham and Eenink conducted microbit drilling tests at varying hydrostatic, formation, and overburden pressures and found that only differential pressure affects drilling rate. Eckel noted that a pressure differential between hydrostatic and formation pressure is the only pressure that affects drilling rate. Garner et al. conducted crater-volume tests on dry limestone and implied that drilling rate is influenced by both overburden and differential pressure. Robinson found that the yield strength of jacketed rock samples increased as the differential between confining pressure and pore pressure increased. He indicated that this increase in strength may be an important factor in reducing drilling rate. Garnier and van Lingen determined that differential pressure affects both rock strength and chip hold-down, that chip hold-down forces are of a static and dynamic nature, and that these forces are a complex function of almost all known drilling parameters.
These studies have provided useful data on crater formation and microbit drilling rates under simulated downhole conditions; however, there has been very little work reported on relatively impermeable rocks such as shale where the actual internal pore pressure is known. In addition, there have been very few field studies reported on the effect of pressure on drilling rate.
This study was conducted on eight South Louisiana wells. The depth of the intervals studied ranged from 8,200 to 14,800 ft; hydrostatic pressures varied from 5,000 to 13,900 psi and the formation pressure ranged from 3,900 to 13,200 psi. Differential pressure between formation and mud-column pressures ranged from a positive differential of 1,500 psi (pm greater than pf) to a negative differential of 930 psi (pm less than pf). The average test interval for the eight wells studied was 1,400 ft.
A unique feature of this study is the pressure control afforded by the use of shale resistivity methods to predict formation pore pressure. The development and increased accuracy of formation pressure calculations from log-derived shale properties provide a method and a degree of control never before available in field studies of this nature. These methods aid in the field evaluation of the effect of many drilling parameters on drilling rate.