OTC 21208--Managed Pressure Drilling Applications on Offshore HPHT Wells

摘要

High pressure-high temperature (HPHT) prospects are unquestionably the most challenging to drill safely and efficiently due to the nature of their drilling hazards and elevated consequences if not mitigated with appropriate risk management processes and best available technology. Several of the more predominant drilling hazards are kicks as a result of encountering unexpected formation pore pressure, swabbing effects when tripping out that invite an influx of reservoir fluids, and kick-loss scenarios common to drilling in narrow or relatively unknown pressure environments. These types of drilling challenges are required components of a “base case” that must be assigned values during a disciplined risk management process whose purpose is to establish the probability and consequences of a loss of well control. The probability values assigned may be similar to other complex offshore prospects, but the consequences of an occurrence are typically a magnitude higher on HPHT wells. This paper addresses the hydraulics of circulating fluids systems as applied to drilling such prospects in marine environments where many pose the risk of not being technically, economically and/or safely drillable with conventional methods. A key message will challenge the conventional wisdom of drilling such prospects with open-to-atmosphere mud returns under the rig floor, in the absence of real-time high resolution knowledge of the actual down-hole pressure environment and without precise mass flow in/out technology. The hydraulics of rotary drilling with jointed pipe and a conventional circulating fluids system requires that the annulus returns arriving under the rig floor be at atmospheric pressure for drilling to proceed without interruption. As remaining prospects become progressively more complex to drill, the offshore industry has shown a remarkable tolerance for the time, cost, and HSE consequences of drilling with open mud returns systems. However, whether drilling ahead or shut in to make connections, if the returns overflow excessively or the primary well control barrier (fluid column) falls precipitously, excessive non-productive time (NPT) and mud costs may be the least of the consequences on HPHT wells. Many years ago the process industries (chemical plants, refineries, etc.) recognized the benefits of bottling up their systems and leaving little open to atmosphere. The initial driver in many cases was to isolate product from exposure to the atmosphere for HSE reasons and often with regulatory encouragement. Several decades ago they began to use PLC’s and associated software for monitoring and precise control of critical parameters within closed systems and vessels. The benefits are widely known today as a major contributor to higher and more consistent product quality, less energy consumption, less manpower, less waste, and lower equipment maintenance cost. MPD brings this hallmark of the process industries to the drilling industry. One may view the circulating fluids system as a pressure vessel whose critical parameters may be precisely monitored and adjusted to meet changing conditions. Annulus returns are closed under the rig floor and positively diverted for HSE reasons. Desired amounts of surface backpressure may be applied as actual wellbore pressure profiles dictate. The equivalent weight of the mud in the hole at the time may be manually, semi-automatically or automatically adjusted to drill deeper within the encountered drilling window and without an interruption to the drilling progress. Associated benefits are enhanced well control, less drilling-related non-productive time (NPT) and mud costs. Therefore, MPD joins Drilling with Casing (DwC), Drilling with Liners (DwL) and Solid Expandable Systems (SES) as key drilling hazard mitigation technologies uniquely suitable for application on offshore HPHT prospects.