High pressure, high temperature (HPHT) is a term used to define any well having a pressure greater than 15,000 psi (103MPa); or a well whose temperature is greater than 350 degree F (177 degree C). This definition was given in 2012, by the American Petroleum Institute (API) in its technical report.
In terms of well control, the term HPHT refers to any high pressure well requiring well control equipment to have a working-pressure-rating in excess of 15,000 psi. Similarly, a high temperature well is one whose fluids are flowing with temperature greater than 350 degree F at the surface.
A well does not have to meet both of the pressure and temperature limits set in these definitions to be called a HPHT well because the term HPHT may also be applied to wells having either high pressure or high temperature. In reality, only few wells are really operating under high pressure and high temperature simultaneously.
Most wells are either operating at a high pressure or a high temperature but are still referred to as high pressure, high temperature (HPHT) well. The temperature thresh-hold for a HPHT well is 350 degree F and the pressure thresh-hold is 15,000 psi.
Reasons Why Certain Shallow Wells Operate Under HPHT Conditions
Using normal hydrostatic pressure gradient of 0.43 psi per foot, before any well can get to the high pressure thresh-hold, then such a well must be at least 35,000 feet deep. But this may not always be the case, some wells have been found to be operating under high pressure below 35,000 feet. This is as a result of variation in overburden forces and geologic features around the earth.
Also, with respect to the Earth’s geothermal gradient of about 1.4 degree F per 100 feet (2.55 degree C per 100 meter), then for a well to be operating at high temperature (350 degree C) that well must be at least 19, 700 ft (600 m).
However, downhole temperatures sometimes do not follow this rule and a well drilled close to a localized geothermal hotspots will be operating at an abnormally high temperature. This is why some shallow wells may be found operating under high temperature conditions.
Also, wells drilled close to other wells using steam injection to reduce heavy oil viscosity will have a reservoir temperature higher than normal because the injected steam will have raised the temperature of surrounding formations.
Challenges Posed by HPHT Wells
High pressure, high temperature (HPHT) wells present unique challenges previously not experienced with conventional wells. For example, HPHT wells are prone to casing buckling and rock collapse. HPHT wells also pose a challenge to downhole tools not experienced with conventional wells.
For high temperature wells, carefully selected metal alloys should be used to manufacture downhole tools like logging-while-drilling-tools, completion components, wireline and well testing equipments to avoid poor performance or failure under HPHT environment.
For high pressure wells, equipment should be pressure tested to ensure the seals can hold under high pressures so as to prevent leakages. Perhaps, one significant challenge with HPHT wells is in drilling fluid selection. Under high temperature and pressure, water based mud will flocculate and gel, leading to high fluid loss and requiring costly dilution and fluid conditioning.
Usually, HPHT wells are drilled with oil-base fluids (OBF) or synthetic-base fluids (SBF) to avoid this challenge but then due to high temperature oil base fluid could thin. This prompts the need to constantly check OBF or SBF fluid’s density for consistency and increasing density when required.
The oil-base fluid (OBF) could also heat up under high temperature requiring the installation of a cooling system at the surface before pumping the drilling fluid back into the hole. Besides OBF and SBF, water based mud can still be used to drill HPHT wells but certain adjustments have to be made to the mud to make it suitable for the HPHT environment.