Successful drilling, especially in very deep wells, may be driven by hydraulic limiting parameters. Two new technologies, UnderBalanced Drilling (UBD) and Managed Pressure Drilling (MPD) have emerged as solutions to specific hydraulic issues during drilling. A hydraulic parameter can be defined as any factor, mechanical, structural or fluid that impacts the exertion of hydrostatic head on the open hole. Hydraulic parameters, as a group, must be planned for and managed during all drilling operations to prevent unwanted or unsafe conditions.

Many hydraulic parameters are documented thoroughly and therefore well-known in the upstream oil and gas industry. They include; Pump Rate; Drillstring and Hole Geometry; Mud Rheology (including surge effect, swab effect, standpipe pressure and hole cleaning); Surface Backpressure; and Rate of Penetration (ROP). These factors are commonplace and routinely addressed as part of a complete drilling program. Other hydraulic limiting parameters are lesser known and sometimes not addressed in the basis of design for unconventional drilling prospects.

It is paramount that drillers consider all hydraulic parameters that influence UBD or MPD operations or the project can end in failure or with unsuccessful consequences. This paper discusses in general some of the lesser-known hydraulic issues that might be encountered when drilling vertical wells using UBD or MPD techniques, especially to deeper horizons.

MPD / UBD Overview

Hydraulic limits occur in both conventional wells and unconventional wells. The more critical wells experience limits that are unmanageable with conventional techniques; thus the emergence of UBD and MPD. MPD is defined by the International Association of Drilling Contractors (IADC) as "an adaptive drilling process used to more precisely control the annular pressure profile throughout the wellbore." Simply put, drillers are concerned with the entire pressure profile in the open hole - including the annulus pressure at the casing shoe as well as bottomhole pressure (BHP). MPD does not encourage formation influx.

UBD operations involve drilling into any formation where the pressure exerted by the drilling fluid is less than the formation pressure. The technique reduces the hydrostatic pressure of the drilling fluid column so that the net pressure in the wellbore is less than the formation pressure. Consequently, the formation pressure may cause permeable zones to flow, if conditions allow flow at the surface. UBD can facilitate drilling of pressure-depleted formations and lessen formation damage for better productivity. UBD operations include formation influx in the operating plan except in the case of a hole being drilled for ROP applications in impermeable rock.

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