Abnormal Pressure

Abnormal pressure is reservoir pore fluid pressure that differs significantly from the expected hydrostatic gradient for the depth at which it occurs. Normal hydrostatic pressure increases with depth at a rate determined by the density of the pore fluid: approximately 10.5 kilopascals per metre (0.465 pounds per square inch per foot) for freshwater and about 11 kilopascals per metre (0.487 psi/ft) for 80,000 milligrams per litre saline brine. Abnormal pressure is divided into two categories: overpressure (or supernormal pressure, geopressure), where the pore pressure exceeds the hydrostatic gradient, sometimes approaching lithostatic (overburden) pressure; and underpressure (or subnormal pressure), where the pore pressure is below the hydrostatic gradient. Overpressure is the more operationally dangerous condition and requires higher-density drilling mud to prevent a wellbore kick or blowout. Underpressure creates a risk of lost circulation if the drilling mud column pressure exceeds the formation's fracture gradient.

Key Takeaways

Overpressure develops through several mechanisms. The most common is rapid burial and compaction of shale: when shales are buried faster than pore water can escape through the low-permeability rock, the pore water supports part of the overburden stress that would normally be carried by the grain framework. The result is a formation where the pore pressure is higher than hydrostatic but lower than lithostatic pressure. Fluid expansion overpressure occurs when temperature increase causes thermal cracking of organic matter to gas (kerogen maturation), generating new fluid volume in a sealed compartment. Tectonic overpressure results from horizontal compression in thrust belt environments. Chemical diagenesis (clay mineral transformations such as smectite-to-illite conversion) can also generate overpressure by releasing water into a sealed pore system.
Seismic velocity is one of the most useful indicators of overpressure prediction before drilling. In normally pressured shale, porosity decreases and seismic velocity increases with depth as the shale compacts. In overpressured shale, compaction is retarded because the high pore pressure supports the grain framework against overburden stress, so the shale retains higher porosity and lower seismic velocity than expected for its depth. By comparing the observed velocity from seismic interval velocity analysis to the expected normal trend, the pore pressure analyst can predict where overpressure begins (the velocity reversal depth) and estimate its magnitude using Eaton's method or similar empirical relationships.
The equivalent mud weight (EMW) concept translates pore pressure from psi or kPa to the mud density required to balance it. If a formation has pore pressure equivalent to a 1.35 specific gravity fluid column at 3,000 metres depth, the driller must run mud at 1.35 SG or higher to prevent a kick. The difference between the pore pressure gradient and the minimum horizontal stress (fracture gradient) defines the mud weight window: the range of mud weights that will balance the formation without fracturing it. A narrow mud weight window (common in highly overpressured deep wells) requires very precise mud weight management and is one of the main technical challenges in drilling HP/HT (high-pressure, high-temperature) wells.
Subnormal pressure (underpressure) develops when fluids are produced from a reservoir faster than the pressure is supported by aquifer influx, or when pressure depletion from an old field has not recovered. It also occurs in formations that are hydraulically isolated from the current regional hydrostatic regime by faults or stratigraphic barriers. In the Deep Basin of west-central Alberta, many tight gas formations have subnormal pressures, with gradient gradients of 6 to 8 kPa/m rather than the regional 10.5 kPa/m. Drilling into subnormal pressure zones with mud that is too heavy causes lost circulation as the mud invades the thirsty formation.
Well control procedures are designed specifically to detect and manage overpressure. An increasing trend in pit volume (mud flowing back into the pit faster than the pump sends it down) while drilling is the primary surface indicator of an influx (kick) from an overpressured formation. Closing the blowout preventer (BOP) and applying the driller's method or wait-and-weight method circulates the kick fluid out while increasing mud weight to balance the formation pressure. All drilling personnel on a rig are trained in well control procedures, and Alberta and British Columbia regulations require certified well control competency for all drilling supervisors and company reps.

Recognizing Abnormal Pressure While Drilling

There is an old saying in the drilling industry: "The well will tell you if it's kicking." That is true, but the well sends earlier, subtler signals before a full kick develops. Recognizing these early warning signs allows the driller to take action before the situation becomes dangerous.

Rate of penetration (ROP) is one of the earliest indicators. If the drill bit suddenly starts drilling faster without any change in weight on bit or rotary speed, the formation below has higher porosity than the surrounding shale — often because compaction is retarded by overpressure. This phenomenon, called drilling break, is a standard early warning that the drilling engineer watches for. A drilling break of more than 20 percent in shale sections triggers a flow check (picking up the bit, stopping the pumps, and watching for flow from the formation into the wellbore).

Gas in the mud (drilling gas or connection gas) is another indicator. Gas-cut mud (mud returning to surface with gas bubbles) after drilling through a formation shows that gas is coming from the formation. Connection gas (a consistent increase in gas readings every time the pump is stopped to connect a new drill pipe joint) indicates that the formation is slightly above-balance and gas is slowly entering the wellbore when bottomhole pressure drops slightly during connections. Both signals indicate that the mud weight is close to the formation pressure and that an increase in mud weight may be needed.

Fast Facts

The Mackenzie Delta of Canada's Northwest Territories and the Beaufort Sea offshore are among the most overpressured exploration environments in North America, with pore pressures in some Tertiary shale sections reaching 80 to 90 percent of lithostatic pressure (equivalent mud weight of 1.8 to 2.0 specific gravity). The Foothills play of southwestern Alberta and northeastern British Columbia also encounters severe overpressure in the Mississippian and Devonian targets of the thrust belt, where tectonic loading has contributed to pressures 50 to 80 percent above hydrostatic. The Flat Lake blowout of 1988 in northern British Columbia occurred in an overpressured Triassic gas reservoir that was penetrated with inadequate mud weight control, resulting in a well that flowed uncontrolled for several months before a relief well killed it. This event led to significant revisions of AER and BC Oil and Gas Commission (now BCOGC) regulations governing mud weight programs in known high-pressure areas.

The Mud Weight Window

Every well has a mud weight window at every depth: a range of mud densities that are acceptable. The lower bound of the window is the pore pressure gradient (the minimum mud weight needed to prevent formation fluids from entering the wellbore). The upper bound is the fracture gradient (the maximum mud weight before the formation fractures and takes mud, causing lost circulation).

In a normal pressure well with a wide mud weight window, the driller has flexibility. A 12.0 ppg mud might be needed to control formation pressure, and the fracture gradient might be 15.0 ppg, giving a 3.0 ppg window — plenty of room for error.

In a highly overpressured well where the pore pressure gradient is 14.8 ppg equivalent and the fracture gradient is 15.2 ppg, the window is only 0.4 ppg. The driller must maintain mud weight within this narrow range precisely, monitoring continuously and adjusting mud weight in small increments as each new section is drilled. This situation, common in deep HP/HT wells in the Gulf of Mexico, North Sea, and some Alberta Foothills targets, requires real-time pore pressure monitoring from MWD formation pressure measurements, continuous mud weight monitoring at the flowline, and experienced drilling supervisors making rapid decisions.

Abnormal pressure is also called formation overpressure, geopressure, overpressure, or pore pressure anomaly (for overpressured situations). Underpressure is also called subnormal pressure or subhydrostatic pressure. Related terms include pore pressure (the pressure of the fluid in the pore space of a rock; normal pore pressure equals hydrostatic pressure at that depth; abnormal pore pressure deviates from hydrostatic above or below), fracture gradient (the pressure at which a formation will hydraulically fracture, expressed as a mud weight equivalent; the upper bound of the mud weight window; exceeded mud weight causes lost circulation), mud weight (the density of drilling fluid, measured in kilograms per cubic metre, specific gravity, or pounds per gallon; selected to maintain bottomhole pressure between pore pressure and fracture gradient; the primary tool for managing abnormal pressure), kick (an influx of formation fluid into the wellbore caused by bottomhole pressure falling below formation pore pressure; a kick precedes a blowout if not controlled; primary surface indicator is increasing pit volume), and well control (the set of procedures and equipment used to detect and manage a kick and prevent a blowout; the primary application of abnormal pressure management in drilling operations).

How an Undetected Overpressure Transition Caused a Montney Blowout Scare in Northeast BC

A Montney horizontal well was being drilled in the Dawson Creek area of northeast British Columbia. The well plan called for a vertical section through the Peace River arch to approximately 2,800 metres TVD (true vertical depth) before building to horizontal in the Montney. The pre-drill pressure prediction, based on seismic interval velocities, showed normal to mildly overpressured conditions (gradient of 11 to 12 kPa/m) through the vertical section, with a transition to a more overpressured Montney at approximately 2,500 metres TVD.

The mud program specified increasing mud weight from 1.15 SG in the shallow section to 1.30 SG at the Montney entry point, with a step increase at 2,400 metres TVD. During drilling at 2,350 metres TVD, the rate of penetration in a thick montmorillonite shale section increased by 35 percent over a 15-metre interval. The driller noted the drilling break but attributed it to a change in shale lithology rather than a pressure transition and continued without increasing mud weight or performing a flow check.

At 2,420 metres TVD, the pit volume indicator showed an 8-barrel gain over 15 minutes. The driller closed the BOP and performed a shut-in drill pipe pressure (SIDPP) measurement of 1,250 kPa, indicating a kick. The influx volume was calculated at approximately 6 cubic metres of gas. The kill operation proceeded using the driller's method, circulating gas out of the wellbore while incrementally increasing mud weight to 1.42 SG to balance the formation pressure, which had been reached 80 metres earlier than the pressure prediction had indicated.

The well was successfully killed without a surface release, but the unplanned shut-in took 18 hours and required a full BOP test, safety review, and regulatory notification before drilling could resume. The British Columbia Oil and Gas Commission (BCOGC) report on the incident found that the drilling break at 2,350 metres was a clear precursor sign that should have triggered a flow check and mud weight increase before continuing. Updated pressure prediction in the well showed that the seismic velocity-based prediction had underestimated the overpressure transition by 70 metres due to a local structural complication not captured in the pre-drill model. The 18-hour non-productive time cost the operator approximately CAD 220,000 at the rig day rate.