Hydraulic Parameters of Underbalanced and Managed-Pressure Drilling

Abstract

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 114667, "Some - times-Neglected Hydraulic Para meters of Underbalanced and Man aged-Pressure Drilling," by Charles R. Stone, SPE, and Shifeng Tian, SPE, Signa Engineering Corporation, originally prepared for the 2008 SPE/IADC Managed Pressure Drilling and Underbalanced Operations Conference and Exhibition, Abu Dhabi, UAE, 28-29 January. The paper has not been peer reviewed. 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 hydraulics issues during drilling. A hydraulic parameter can be defined as any factor—mechanical, structural, or fluid—that affects 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. Introduction 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 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 and the 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 at the surface allow flow. 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 rate-of-penetration (ROP) applications in impermeable rock. In MPD, the driller seeks to stay slightly above or "at-balance" with the downhole pore pressure (PP), or as close to near balance as possible during the entire section of problem hole, during both drilling and connections. Precise control of downhole pressure allows the driller to drill within the window between PP and fracture gradient (FG) without setting casing prematurely.