In determining a "normal" downhole pore pressure, engineers often compute pressure using a hydrostatic gradient based on the weight of seawater. Such a well would require a depth of more than 10,700 m [35,000 ft] to reach the 15,000 psi HPHT threshold. However, because of geologic features and variable overburden forces, a higher hydrostatic pressure than that which the normal pressure gradient would predict is often required to overcome reservoir pore pressure. Drilling high-pressure wells using mud weights that are more than twice that of seawater is not uncommon. Overpressured formations, those having higher than normal pore pressure, can be present even at shallow depths.
Ultradeep wells being drilled today may reach depths beyond 10,700 m, and their hydrostatic pressure can exceed 207 MPa [30,000 psi]. Drilling assemblies, LWD tools, wireline logging equipment, well testing tools, completion hardware and well intervention tools are exposed to these extreme pressures. To mitigate the effects of high pressure, design engineers focus on metallurgy and sealing. Metals and alloys commonly used in the aerospace and nuclear power energy have been adopted by the oil and gas industry. However, use of these materials in oil and gas applications is often constrained by wellbore size limitations. his is especially true for deepwater wells in which some of the highest pressures are encountered—logging and drilling tools must withstand high pressure extremes and also fit into small diameter wellbores that are typical of ultradeep wells. Materials used for sealing elements must seal against extreme pressure, often under high temperature, and they may have to undergo multiple pressure cycles without failing.
The risks associated with downhole pressure are not only for the equipment used there. When completions, testing and production operations are performed with high pressure at the surface, a risk potential to personnel working with the equipment exists. To manage this risk and allow wellsite operations to be performed safely, engineers use equipment that is designed to function above the anticipated maximum pressure. The maximum pressure of the full system depends on the lowest rated component in the full containment string. To ensure that properly rated equipment is used, operators must know the maximum pressure potential in advance.
Pressure control requirements directly affect choices of equipment engineering and design. Pressure equipment is rated for maximum anticipated pressure, and these ratings determine material selection and thickness, elastomer configuration, sealing mechanisms and pressure control components. To ensure operations can be performed safely, the equipment is function tested above the maximum anticipated pressure prior to its use.