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Summary The annular cement sheath is one of the most-important well-barrier elements, both during production and after well abandonment. It is, however, well-known that repeated pressure and temperature variations in the wellbore during production and injection can have a detrimental effect on the integrity of the cement sheath. A unique laboratory setup with downscaled samples of rock, cement, and pipe has been designed to study cement-sheath-failure mechanisms during thermal cycling, such as debonding and crack formation. With this setup, it is possible to set the cement under pressure and subsequently expose the cement to temperature cycling under pressure as well. Cement integrity before and after thermal cycling is visualized in three-dimensional by X-ray computed tomography (CT), which enables quantification of and differentiation between debonding toward the casing, debonding toward the formation, and cracks formed inside the cement sheath itself. This paper describes in detail the development and functionality of this laboratory setup along with the experimental procedure. Several examples to demonstrate the applicability of the setup, such as tests with different types of casing surfaces and different rocks, are also shown.

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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.

Summary This paper describes a new drilling-riser concept and drilling methodology for deepwater operations that will remove some of the well-control challenges and limitations currently experienced when handling kicks and deep gas influxes in deepwater regions, with the following results:Providing improved and more flexible well-control procedures.Reducing the potential of hydrate plug formation during well-control operations.Allowing for drilling longer hole sections than normally considered feasible when using conventional drilling methods, thus reducing the number of casing strings required in the well.Allowing for improved drilling performance in depleted formations. The main elements in the system are based on using a small, high-pressure drilling riser [14-in. outer diameter (OD), 12.5-in. inner diameter (ID)] with a split surface/subsea blowout preventer (BOP) and a subsea mud-lift pump connected to the drilling riser and a separate mud-return line. During drilling and well-control operations, the mud level in the riser is maintained considerably below sea level to create a mud/air interface (i.e., a "mud cap") that can be continuously adjusted up or down by the mud-lift pumping system. As a consequence, the bottomhole hydrostatic pressure will be controlled. One of the main purposes of this system is to mitigate the inherent problems with a conventional 21-in.-OD marine drilling riser during well-control scenarios in deepwater operations. The system will compensate for frictional pressures resulting from circulation and adjust the bottomhole pressure (BHP) accordingly. Introduction Experiences from deepwater drilling operations in geopressured environments such as the Gulf of Mexico (GOM) have shown that the upper layers of the subsurface have low fracture strengths close to the hydrostatic pressure of seawater. The resulting small margin between the pore pressure and the formation strength typically requires four to six or more casing strings to be set below the surface casing when drilling with a conventional marine riser system. When drilling in high-pressure/high-temperature (HP/HT) fields, or through salt intrusions, small windows between pore pressure and formation strength can be experienced. In some of these cases, after drilling only a short interval, the incremental BHP caused by circulation [i.e., the equivalent circulating density (ECD) effect] is high enough to require setting a casing string to maintain adequate well-control margins. Lost circulation is a problem experienced frequently when conventionally drilling in deepwater areas, HP/HT areas, highly faulted and fractured formations, and in depleted formations. The remedial process can be costly (in both time and money). When drilling with a conventional large riser system during a well-control event, the kick is circulated out through the chokeline. This line has a small diameter, and in deepwater wells, the friction in this line can be a significant factor while circulating out a kick, even at low pump rates. As a consequence, more than 75% of all deepwater kicks experience formation ballooning, partial losses, and other downhole problems (Skalle et al. 2002). In severely depleted reservoirs, drilling and well-control operations are often conducted within the small pressure region between formation fracture and wellbore instability (collapse). The resulting challenge can restrict the ability to drill underbalanced unless the BHPs can be controlled in a fast, safe, and effective manner. The inherent limitations of conventional well-control procedures can, as a consequence, cause severe lost-circulation or hole-stability problems, which are extremely costly in deepwater operations. In deep water, the low mudline temperature and high pressure may lead to hydrate formation, if gas is present. Hydrate plugs can cause delay in operations and can cause severe well-control challenges (Barker and Gomez 1989). In this paper, three different methods of pressure control will be discussed. The first method is the conventional way of controlling pressure in an open system with a high-pressure riser and a surface BOP. The second method is the closed-loop method of managed-pressure drilling (MPD) with a surface BOP, a rotating control device (RCD), and a pressurized riser, and the third is the method referred to as the "controlled mud cap" (CMC) with a split BOP between seabed and surface.