Page 288 - Drilling Technology in Nontechnical Language

Chapter 11 – WELL CONTROL 279

High-pressure, high-temperature (HPHT) wells

As most of the shallow, easily exploitable oilfields have probably been

found, the exploration for hydrocarbons has moved to remote areas and
also to deeper prospects in mature areas. This has made HPHT wells

much more common over the last few years. An HPHT well is defined
as a well where wellhead pressure could exceed 10,000 psi and where the
undisturbed bottomhole temperature exceeds 150°C. Though definitions in

the industry do vary a little, this gives an idea of the scale of the problem.
From a well control point of view, hole sections that meet the HPHT
definition require extensive planning, careful training of drill crews, and

special equipment to detect and handle kicks.
Kick tolerance was mentioned in chapter 3. In HPHT wells, the
formation fracture gradient and pore pressure gradient are often fairly
close together, meaning the kick tolerance will be small. Therefore, very
sensitive kick detection systems are necessary, together with crew training

and regular drills to ensure that at the first sign of a kick, the well is closed
in quickly (without visually checking for a flow). The well is then watched

for any buildup of pressure.
Another problem in HPHT wells is that oil-based muds are often
preferred due to temperature limitations of some water-based systems.
Gas at high pressure (above around 6,000 psi) is completely soluble in oil
muds, so a gas influx can enter the well as a liquid. A small volume can

enter undetected. However, as this travels up the annulus and the pressure
reduces (because the hydrostatic head of fluid above it reduces), the gas can

suddenly come out of solution and expand to many times its liquid volume.

The result would be a very rapid increase in flow detected at the surface
and a large volume of gas to circulate out when killing the well. Under
these circumstances, the kicking formation probably gave a continuous
slow stream of dissolved gas into the mud so that all of the mud between
the kicking formation and the point where gas comes out of solution is
contaminated. More gas will come out of solution during the well kill.
Special seals are required for the BOPs that can handle the high
temperatures at which they have to operate. Also, as gas moves through
the chokes, it experiences rapid expansion due to the reduction from high
pressure down to atmospheric pressure. As gas expands, it cools down,
and it is possible for the temperature downstream of the chokes to be well
below freezing point. This requires consideration of low-temperature

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