Managed Pressure Drilling for Subsea Applications; Well Control Challenges in Deep Waters

Abstract

Abstract This paper describes a new drilling riser concept and drilling method that will remove some of the well control challenges presently encountered and provide improved well control procedures, when handling deepwater kicks and deep formation gas flow into a well being drilled. The new system will also allow for longer hole sections to be drilled in deepwater, thus reducing the number of casing strings required in the well and reduce the chances of hydrate plugs forming at seabed. The main element in the system is based on using a smaller size (14"-12.5" ID) high pressure drilling riser with a split BOP between surface and subsea, a subsea mud pump connected to the high pressure drilling riser, taking returns from a lower level in the riser. The mud level in the riser is dropped down to a level considerably below sea level to create a mud/air interface ("mud cap") that can be continuously adjusted up or down by the mud-lift pumping system. As a consequence, the bottom hole hydrostatic pressure will be controlled. One of the main purposes of this system is to mitigate the inherent problems with a conventional 21" marine drilling riser during well control scenarios in deepwater operations. It will adjust the bottom hole pressure accordingly and compensate for frictional pressures due to circulation. Introduction Experiences from deepwater drilling operations in geo-pressured environments such as the Gulf of Mexico (GOM) have shown that the upper layers of the subsurface having fracture strength close to the hydrostatic pressure of seawater. The small margin between the pore pressure and the formation strength dictates that frequent and multiple casing strings (4–5 below the surface casing) have to be set when drilling with a conventional marine riser system. In HPHT fields and in drilling through salt intrusions, small windows between pore pressure and formation strength can be experienced. In some instances the added pressure at the bottom of the well caused by circulation (Equivalent Circulating Density, ECD) is enough to dictate casing settings. Loss of circulation is often a problem experienced in deepwater areas, in HTHP wells, when drilling in highly faulted and fractured formations and when drilling through depleted formations, etc. The process of repairing losses is costly (time & money). During a well control event, the kick is circulated out through the choke line. This line has a small diameter and in deepwater the friction in this line is of major importance whilst circulating out a kick. As a consequence more than 75% of all deepwater kicks experience formation ballooning, partial losses and other down hole problems.1 In severely depleted reservoirs, drilling operations are often conducted in the small margin between formation fracture and hole stability. The challenges in these situations can restrict the ability to drill under balanced unless the bottom hole pressures can be controlled fast, safe and effectively. Consequently, conventional well control procedures can cause severe loss circulation or hole stability problems which are extremely costly in deep waters. In deepwater, low mudline temperature and high pressure may lead to hydrate formations if gas is present. Hydrate plugs can cause delay in operations and cause severe well control challenges.2 In this paper, three different methods of pressure control will be discussed. One method is the conventional way of controlling pressure in an open system with a high pressure riser and a surface BOP. Second method is the closed loop method of managed pressure drilling (MPD) with a surface BOP and the third method is the method here referred to as the "controlled mud cap" (CMC) with a split BOP between subsea and surface. For comparison of the methods, reference will be made to Figure 1 which shows pore pressure and fracture pressure vs. depth for an example well in deeper water.