pressure hunt, Oil & Gas Glossary

What Is a Pressure Hunt?

A pressure hunt is a formation pressure testing procedure in which the wireline formation tester tool (MDT, RFT, FMT) is pressed against the borehole wall at a potential reservoir zone and a small pretest piston withdraws a controlled volume of fluid while monitoring the pressure response, with the shape and rate of the pressure recovery used to assess whether the formation is permeable enough to flow measurable fluid and whether the measured pressure represents the true formation pore pressure.

Key Takeaways

A pressure hunt sequence tests multiple depths in a zone to identify the depths with best pore pressure communication and permeability.
Good pressure recovery after pretest drawdown indicates permeable formation; supercharging indicates tight formation with elevated near-wellbore pressure from mud filtrate.
Pressure hunt data identifies the optimal depths for large-volume sampling before committing to expensive MDT sampling stations.
The formation pore pressure gradient (pressure vs. depth slope) identifies fluid type: oil gradient ~0.35-0.45 psi/ft, water ~0.44-0.48 psi/ft, gas ~0.05-0.15 psi/ft.
A "wet" zone below the OWC shows water gradient; a "pay" zone shows oil gradient — the crossover identifies the contact.

How a Pressure Hunt Is Conducted

During a pressure hunt programme, the wireline formation tester (MDT, Modular Formation Dynamics Tester, or its predecessors the RFT and FMT) is set at successive depths in a reservoir interval. At each station, the probe is extended from the tool and pressed against the borehole wall with sufficient force to achieve a mudcake seal. A small pretest piston in the tool withdraws a fixed volume of fluid (typically 3-10 cm³ in 2-4 seconds), creating a rapid pressure drawdown. After the pretest withdrawal stops, the pressure is monitored as it recovers toward the formation pore pressure. The rate of pressure recovery reflects the formation permeability around the test point: fast recovery indicates high permeability (the formation can easily supply fluid to fill the small volume withdrawal); slow recovery indicates low permeability; no recovery indicates essentially impermeable rock.

The final pressure to which the test recovers is the formation pore pressure at that specific depth — provided the formation has sufficient permeability to equilibrate during the test. In tight formations, the measured pressure may be higher than true pore pressure due to supercharging: mud filtrate invasion has pressurised the formation around the borehole above the true reservoir pressure, and the tool measures this elevated near-wellbore pressure rather than the virgin formation pressure. Supercharging can be identified by running multiple pretest volumes at the same depth; a supercharged formation shows inconsistent pressure readings that decrease with each successive pretest as mud filtrate pressure is bled off. A true formation pore pressure measurement shows consistent, reproducible recovery to the same final pressure regardless of pretest volume.

Pressure Hunt Applications Across International Jurisdictions

In Canada, pressure hunt programmes are conducted in WCSB exploration and appraisal wells to define the fluid pressure gradients in Cardium, Viking, Montney, and Devonian carbonate reservoirs. AER well test approval requirements for open-hole wireline formation testing are less onerous than for DST, making MDT pressure hunts the preferred method for obtaining formation pressure data in exploration wells where full DST rig-up cost is not warranted. The pressure gradient from pressure hunt data across an oil column defines the oil density and API gravity, which together with the water gradient below the OWC provides the OWC depth without requiring a produced fluid sample. Montney horizontal well programmes use pressure hunt data from vertical pilot wells to define the pressure gradient and confirm normal versus abnormal pressure before designing the horizontal well mud weight programme.

In the United States, MDT pressure hunt surveys are standard practice for Gulf of Mexico deepwater exploration wells where the combination of pressure gradients from multiple zones in a single well run defines the fluid contacts and connectivity between reservoir compartments. BSEE formation evaluation requirements for OCS exploration wells include formation pressure data; pressure hunt surveys satisfy this requirement without requiring DST production testing. In Norway, pressure hunt programmes on NCS exploration wells provide pressure gradient data that is submitted to Sodir's Diskos national data repository; the pressure data informs basin-wide pore pressure modelling and trap pressure integrity assessment. In the Middle East, Saudi Aramco uses MDT pressure hunt surveys in Arab Formation exploration and appraisal wells to map oil-water contacts across multiple reservoir layers (Arab C, D, E) and to calibrate the reservoir pressure model used for production planning at Ghawar.

Fast Facts

The pretest volume withdrawn during a pressure hunt station is approximately 3-10 cm³ — a volume smaller than a teaspoon. Despite this tiny withdrawal volume, the drawdown-recovery test provides formation permeability estimates over a range of approximately 0.01 mD (very tight) to greater than 1,000 mD (high permeability) from the shape of the recovery curve. The mobility (permeability/viscosity ratio) is extracted from the recovery rate using spherical flow analysis for standard probe geometry; for tight formations where the drawdown recovery is very slow, extended pretest times (up to 30 minutes) are used to obtain sufficient pressure recovery for reliable mobility estimation.

Fluid Gradient Analysis from Pressure Hunt Data

The most powerful use of pressure hunt data is the construction of pressure versus depth (P-Z) plots for multiple zones within the same well. When pressure measurements are taken at numerous depths across a formation, the slope of the best-fit line through the pressure points on the P-Z plot gives the fluid pressure gradient in pressure per unit depth. This gradient is directly related to the fluid density: gradient (psi/ft) = fluid density (g/cm³) × 0.4335. A gradient of 0.43 psi/ft corresponds to fresh water (1.0 g/cm³); 0.35-0.40 psi/ft indicates oil (0.8-0.92 g/cm³); 0.05-0.15 psi/ft indicates gas. When the P-Z plot shows a clear break in gradient — a steeper (water) gradient below a shallower (oil) gradient — the crossover depth is the oil-water contact. This direct determination of fluid contacts from pressure hunt data is one of the highest-value outputs of a wireline formation test programme.

Tip: When planning a pressure hunt programme for OWC determination, aim to bracket the expected contact depth with pressure stations on both sides — several stations clearly in the oil gradient above the contact and several stations clearly in the water gradient below. If only stations above or below the contact are obtained, you can define the gradient of one fluid type but cannot determine the contact depth precisely. At the minimum, you need one confirmed oil-gradient station and one confirmed water-gradient station on either side of the expected contact depth. The contact is where the two gradient lines intersect on the P-Z plot. If your stations cluster too tightly in a single fluid, you may miss the contact altogether in a thin transition zone.

Pressure hunt is also referenced as:

Pretest survey — the term used in Schlumberger and Halliburton service company documentation for the same procedure; "pretest" refers to the small drawdown test used at each station during the survey
Wireline formation test (WFT) — the broader category encompassing pressure hunt surveys, fluid sampling, and permeability testing using any wireline formation tester tool; "pressure hunt" refers specifically to the multi-station pressure measurement objective
MDT pressure survey — the trade-name-specific form (Modular Formation Dynamics Tester is Schlumberger's tool); used when Schlumberger tooling is specifically being used; equivalent tools include Halliburton's RDT and Baker Hughes' RCI

Frequently Asked Questions

How does supercharging affect pressure hunt measurements in tight formations?

Supercharging occurs when the drilling fluid filtrate invades a low-permeability formation at higher pressure than the true formation pore pressure. In tight rocks (permeability below 0.01-0.1 mD), the filtrate invasion front during drilling builds up a pressure excess in the near-wellbore zone because the filtrate cannot flow away from the wellbore fast enough to equilibrate with the true formation pressure. When the formation tester probe samples this supercharged zone, it measures the elevated near-wellbore pressure rather than the virgin formation pressure. Supercharged pressure readings are typically 0.5-5 MPa above the true formation pressure and can be identified by: (1) pressures that plot above the expected fluid gradient line on the P-Z plot; (2) pretest responses that show declining recovery pressure with successive pretest volumes as the elevated near-wellbore pressure is bled off; or (3) direct comparison with independently measured reservoir pressures (DST, RFT in the same well). In tight reservoirs, supercharging corrections must be applied before using the MDT data for fluid gradient analysis.

What mobility can be estimated from a standard pretest?

The pressure recovery of a standard pretest (3-10 cm³ at a few cm³/s) responds to the spherical flow mobility within a radius of approximately 10-50 cm around the probe. Mobility (k/μ in mD/cp) can be estimated from the slope of the Horner pressure recovery plot as: k/μ = 5,660 × q / (m × rp), where q is pretest flow rate (cm³/s), m is the slope of the Horner plot (psi per log cycle), and rp is the probe radius (approximately 0.5 cm). For typical pretest conditions, mobilities between 0.01 and 10,000 mD/cp can be estimated. At the lower end (0.01 mD/cp), the pressure recovers very slowly and the formation may appear "tight" on the standard 60-second pretest; extended pretests of 5-30 minutes are needed to achieve recovery for mobility estimation. At the high end (above 1,000 mD/cp), the pretest recovery is essentially instantaneous and mobilities must be estimated from the very early time data or from a separate in-situ permeability test module.

Why Pressure Hunts Matter in Oil and Gas

The determination of formation fluid type, fluid contacts, and pore pressure from a single wireline logging run is one of the highest value-to-cost ratios in oilfield data acquisition. A pressure hunt programme conducted during the wireline logging phase of an exploration well adds modest cost (typically USD 10,000-100,000 depending on the number of stations) relative to the total well cost, yet provides fluid gradient data that directly defines the OWC, GOC, and compartmentalisation of the reservoir — information that previously required expensive DST operations or pilot production. For appraisal decisions involving potential reserves worth tens to hundreds of millions of dollars, the pressure hunt data that defines fluid contacts and pressure communication between reservoir compartments is among the most cost-effective data that can be acquired during the well programme.

Pressure Hunt: Definition, Formation Pressure Testing, and Permeability Indicators