with depth, as does the pore pressure (the pressure exerted by the saline water or hydrocarbons in the pore space of rock).5 Rig personnel use dense fluids during drilling (i.e., drilling mud) and different types of barriers inside the well after drilling to control subsurface pressure and prevent unintended hydrocarbon flow from geologic formations into the wellbore.
As the well is being drilled, drilling mud is pumped into the drill pipe connected to a drill bit. Mud flows out of nozzles in the bit and then circulates back to the rig through the space between the drill pipe and the sides of the well (the annular space), carrying away cutting debris and cooling and lubricating the bit and wellbore. In addition, drilling mud is used to control pressures inside the wellbore.
The pore fluids are contained in the reservoir rock by using the weight of a column of drilling mud to create hydrostatic pressure at the reservoir that is higher than the pore pressure. The crew monitors and adjusts the mud weight to keep the pressure exerted by the mud inside the wellbore between the pore pressure and the fracture pressure. Should the mud weight be lower than the pore pressure, an undesired flow of reservoir fluids will enter the wellbore (an event known as a kick). If a kick occurs, a blowout could result if proper well control procedures are not followed.
As the well is drilled deeper, an increase in the mud weight may be necessary to prevent kicks. However, the mud weight must not be so high that the hydrostatic pressure in the wellbore exceeds the fracturing pressure of the exposed rock at any point in the wellbore. If a fracture occurs, drilling mud will flow out of the well into the geologic formation so that mud returns are lost instead of circulating back to the surface. Should lost circulation occur, drilling cannot be continued until the mud losses are stopped. Severe lost circulation can cause the pressure in the well to become too low to prevent reservoir fluids from entering the wellbore. The well may also become unstable and collapse.
The fracture pressure and pore pressure can be difficult to predict in advance of drilling the well, and some formations in the Gulf of Mexico have pore pressures and fracture gradients that can be either higher or lower than anticipated. The pore pressure can be close to the fracture pressure, as was seen in drilling the Macondo well, presenting a substantial challenge to the overall safety of the drilling operation (see Chapter 2).
For cases where the pore pressure is close to the fracture pressure, which is common in the deep water of the Gulf of Mexico, attention is paid to any increases in well pressure that might be caused by drill pipe movement or pumping fluids. Each of these factors can cause the pressure in the wellbore to exceed the fracture pressure, creating well control problems such as lost circulation and possibly a kick.
5 Additional information about designing and constructing offshore wells is given by sources such as Maclachlan (2007), Bommer (2008), and Zoback (2010).