• BHP (Drilling) – This is the actual pressure at the bottom of the well. It is best described by the formula:
• BHP = Static mud weight + ECD component + Effects + Applied Surface Backpressure (using MPD choke manifold and RCD)
• BHP Variations – This describes the uncontrolled variation in BHP that will occur within the well. These variations are especially pronounced when the well is moved from dynamic to static condition or vice versa.
• Drilling Window – This is defined in terms of the differential pressure between the Pore Pressure and the minimum fracture pressure (minimum fracture gradient). This is sometimes referred to as the “drilling margin” or “operating margin”. Where there is uncertainty about the pore pressure the original pore pressure may be used which results in a ‘narrow’ drilling window.
• Dynamic condition – The condition in which the wellbore is being continuously circulated
• Dynamic flow check – The process during which backpressure is reduced on the well while continuing to circulate the well, and verifying that there is no net inflow (from the formation) or outflow (losses) from the well.
• ECD + Effects – Effects refer to temperature effects on mud weight and mud rheology that have a direct impact on downhole pressure. It also includes all additional effects on the Bottom Hole Pressure (BHP) that result from operations. E.g. rotation, torque, drilling, cuttings load, etc. Although these effects exist in all wells, their magnitude becomes significant in HPHT drilling when large temperature variations occur and high mud weights are in use.
• Equivalent Circulation Density (ECD) – The increase in BHP that results from the annular friction pressure created when well fluids are moved (circulated) along the wellbore.
• Gel Break – When heavy weight muds are allowed to remain static for any period of time, static gels are formed in the mud. Static gels are a required design feature of drilling mud to keep cuttings and other particles suspended during periods of no circulation. The combined effects of high temperatures and mud weights are such that the pressure required to break these gels, to initiate movement of the mud, can be considerable. A pressure surge occurs as the gels break and the mud begins to move. This pressure surge is applied directly to the exposed formations and can cause formation fracture. Gel break pressures do not exist when a CCS is used because the mud is never in a static state. Operating procedures ordinarily compensate for the gel break strength and can eliminate their affects.
• Managed Pressure Drilling (MPD) - an adaptive drilling process used to precisely control the annular pressure profile throughout the wellbore. The objectives are to ascertain the downhole pressure environment limits and to manage the annular hydraulic pressure profile accordingly (IADC definition).
• Original Pore Pressure – Pore pressure in reservoir prior to depletion this is sometimes referred to as Initial Pore Pressure.
• Pore Pressure – The actual pore pressure of the formation being drilled, considering any depletion. Values can be determined by stethoscope (pressure probe inserted onto formation), PVT, standard methods of determining pore pressure, etc.
• Prognosed Pore Pressure – The pore pressure forecast by reservoir simulation to be the most likely pore pressure in the interval based on produced volume and reservoir model.
• Static condition – The condition in which circulation is stopped e.g. during a conventional connection, trip or during well control when the BOP is closed and there is no increase of pressure from an influx or drilling fluid losses to the formation.
• Static flow check – The process during which circulation in the well is stopped, removing the ECD component, and verifying that there is no net inflow or outflow from the well.
• Static Mud Weight – The actual mud weight in use, sometimes this is called ESD or Equivalent Static Density. Although this weight does vary with the wellís temperature profile, for simplicity this can be assumed to be lowest average mud weight in the well during any operations. Measured in Specific Gravity (SG).
• Trip Margin (TM) – Trip Margin is an addition to the BHP above the Pore Pressure as a safety margin. Its only function is to compensate for swab pressure while the drill pipe is being lifted from the well either by pulling out or by heave. This addition can be additional static mud weight for conventional or additional backpressure for MPD.
• MPD Automated Choke Manifold: a set of high-pressure valves and associated piping that includes two automated chokes, arranged such that one automatic choke may be isolated and taken out of service for repair and refurbishment while well flow is directed through the other one. It also includes a pressure gauge upstream and a Coriolis flow meter on the manifold. Both chokes are automated and hydraulically controlled by a PLC system using real-time control system software, surface pressure, temperature and flow rate data and the MWD BHP to maintain tight control over the BHP. The chokes are designed for continuous use while MPD, with cuttings and mud returns.
• Mud Gas Separator: MPD or rig MGS device that removes air or gases from drilling liquids. This works by increasing the surface area available to the mud so that bubbles escape (through the use of various cascading baffle plates). Gas is routed away from the rig up to the top of the derrick via the vent line on an offshore installation, or to a flare pit on a land rig.
• RCD: Rotating Control Diverter. A rotating sealing element / diverter which allows drill pipe to enter and exit the well bore whilst maintaining pressure in the annulus, also allows rotation of the drill pipe whilst containing annulus pressure. The sealing elements rotate with the drill string, as they are mounted in a bearing assembly. They divert flow from the well to the MPD system, via a flow spool under the RCD.

List of Abbreviations

API	American Petroleum Institute
B/U	Bottoms Up
BHA	Bottom Hole Assembly
BHCT	Bottom Hole Circulating Temperature
BHP	Bottom Hole Pressure
BHST	Bottom Hole Static Temperature
BMP	Balanced Mud Pill
BOP	Blow Out Preventor
BOD	Basis of Design
BP	Back Pressure
BRT	Below Rotary Table
CCS	Continuous Circulation System
DBP	Dynamic Back Pressure
DP	Drill Pipe
DW	Drilling Window
E&B	Lines Equalisation and Bleed Lines
ECD	Effective Circulating Density
EMW	Equivalent Mud Weight
EPP	Estimated Pore Pressure
ESD1	Equivalent Static Density
ESD2	Emergency Shut Down