Gas-Oil Contact

The gas-oil contact is the boundary surface in a reservoir that separates the gas cap above from the oil column below. Above the contact, the pore space is saturated predominantly with gas. Below it, the pore space holds oil (with connate water). The gas-oil contact forms because gas is less dense than oil: in a reservoir at equilibrium, the less-dense fluid floats to the top. In pressure-depth terms, the gas-oil contact is the depth at which the pressure in the gas phase equals the pressure in the oil phase. Accurate knowledge of the gas-oil contact depth is needed to calculate initial gas and oil volumes in place and to plan where to perforate production and injection wells.

Key Takeaways

The gas-oil contact is determined from pressure data measured in the well. Gas and oil have different pressure gradients (gas pressure decreases less steeply with depth than oil). By measuring reservoir pressure at multiple depths with a wireline formation tester (MDT or equivalent), the interpreter plots pressure versus depth and identifies the depth where the gas gradient line crosses the oil gradient line. That intersection is the gas-oil contact.
In practice, the gas-oil contact is not an infinitely sharp boundary. A transition zone of varying gas saturation sits above and below the nominal contact depth, determined by capillary pressure forces in the rock. Finer-grained or lower-permeability rock has a broader transition zone; coarser-grained rock has a sharper transition.
A gas cap that is in pressure equilibrium with the oil column is a strong driver of oil production during primary recovery. As reservoir pressure declines during production, the gas cap expands and pushes oil toward the producers. Gas cap drive is one of the most efficient natural drive mechanisms in conventional reservoirs.
Coning occurs when a producer near the gas-oil contact draws the contact down into the perforation interval. Gas coning reduces oil production and can permanently damage well productivity. Gas-oil contact management is therefore important in field development: producers should be completed well below the gas-oil contact.
The gas-oil contact depth can shift over time as the reservoir is produced. A declining reservoir pressure may cause the gas cap to shrink or expand depending on whether gas is produced from the cap directly. Monitoring the contact movement with repeat formation tests or 4D seismic helps reservoir engineers manage sweep efficiency.

What Is the Gas-Oil Contact and Why Does It Matter?

Pour water, cooking oil, and air into a glass jar. The air sits on top of the oil; the oil floats on the water. The surface between the air and oil is the gas-oil contact. The surface between the oil and water is the oil-water contact. Inside an oil reservoir, the same layering happens, except the pressures are thousands of kilopascals higher, the fluids are hydrocarbons rather than household liquids, and the boundaries sit inside porous rock rather than in open space.

The position of the gas-oil contact determines how much oil and how much gas sits in the reservoir. A field with a large gas cap relative to the oil column may produce efficiently by gas cap expansion but also risks early gas breakthrough if wells are completed too close to the contact. A field with no gas cap relies on solution gas or water drive for pressure support instead. The gas-oil contact is one of the first things a reservoir engineer identifies when characterizing a new discovery.

In the Viking fields of central Alberta, some reservoirs have both a gas-oil contact and an oil-water contact. The oil column between the two contacts may be only 5 to 15 metres thick. Every metre of oil column matters for reserves calculations. A gas-oil contact placed 3 metres too shallow by a misread pressure gradient means overstating oil reserves by 20 to 40 percent. This is not a minor interpretive detail.

Fast Facts

On the Prudhoe Bay field on Alaska's North Slope, the gas-oil contact in the Sadlerochit reservoir sits at approximately 2,400 metres below sea level. Above it, a massive gas cap holds an estimated 25 trillion cubic feet of gas. Below it, the oil column contains approximately 14 billion barrels of original oil in place. The gas cap has been carefully managed since first production in 1977 by reinjecting produced gas into the cap to maintain pressure and preserve the oil column. Mismanaging the gas-oil contact in a field of this size would mean billions of barrels of stranded oil below a collapsed gas cap.

Measuring the Gas-Oil Contact in a Well

Formation pressure tests are the definitive method for locating the gas-oil contact. A wireline formation tester (such as Schlumberger's MDT or Halliburton's RCI) measures reservoir pressure at multiple depths as the tool is moved up the wellbore on electric wireline. Each pressure measurement gives a data point on a pressure-depth plot.

Gas has a pressure gradient of approximately 1 to 3 kilopascals per metre of depth, compared to 6 to 7 kPa/m for oil and 10 kPa/m for water. On the pressure-depth plot, the data points from gas zones fall on a line with a shallower slope than the data points from oil zones. Where the two trend lines cross is the gas-oil contact depth.

This method requires that at least some measurements come from both the gas cap and the oil column. In exploration wells where the full fluid column is drilled, this is usually achievable. In development wells that do not drill through the full oil column, the contact must be extrapolated from the gradients, which introduces uncertainty.

Resistivity logs can also indicate the gas-oil contact: gas has a slightly different resistivity contrast with the formation water than oil does, though in many reservoirs this difference is too subtle to be diagnostic on its own. Density logs show the gas-oil contact as the depth where the density reading shifts from the low-density gas value toward the higher-density oil value.

Gas-Oil Contact Management in Producing Fields

Once a field is in production, the gas-oil contact is no longer static. If producers complete in the oil column and no gas is injected back into the gas cap, the declining reservoir pressure allows the gas cap to expand as gas comes out of solution in the oil. This expansion drives oil toward the producers, which is the desired gas cap drive mechanism.

Problems arise when the gas-oil contact drops below the depth that was assumed in the completion design. A producer perforated at the oil column midpoint based on an initial contact depth of 2,500 metres may find itself producing predominantly gas if the contact has risen to 2,450 metres after three years of production. The gas-oil ratio rises and the well's economic value drops.

In North Sea Brent reservoirs, 4D seismic surveys (seismic data acquired at two or more times in the life of the field) have been used successfully to track gas-oil contact movement. The acoustic impedance contrast at the gas-oil contact produces a seismic reflection that shifts in depth as the contact moves. Comparing time-lapse seismic surveys reveals where the gas cap has expanded and where unswept oil remains, allowing better targeting of infill wells.

The gas-oil contact is abbreviated GOC in reservoir engineering documents. It is also called the gas-oil interface or the gas cap base. Related terms include oil-water contact (the boundary surface below the oil column where oil saturation gives way to water saturation; the oil column sits between the oil-water contact below and the gas-oil contact above), gas cap (a zone of free gas above the oil column in a reservoir, held in place by the density difference between gas and oil; gas cap drive is a primary recovery mechanism), transition zone (the interval of varying fluid saturation around the oil-water contact or gas-oil contact, controlled by capillary pressure; wider in lower-permeability rock), formation tester (a wireline tool that measures reservoir pressure at discrete depths; the pressure-depth gradient crossover is the most reliable method for locating the gas-oil contact in a new well), and gas cap drive (the reservoir energy mechanism in which an overlying gas cap expands as reservoir pressure declines, pushing oil downward toward the production wells).

How a Misread Gas-Oil Contact Added CAD 85 Million to Reservoir Development Costs in Alberta

An operator was developing a Cardium sandstone field in west-central Alberta. The discovery well had measured formation pressures in the oil zone but had not been tested through the gas-oil contact because drilling stopped approximately 8 metres above the interpreted contact depth based on mud log gas readings. The reservoir engineer extrapolated the oil gradient upward and placed the gas-oil contact at a depth that was 11 metres shallower than the actual contact.

Development wells were drilled and perforated based on the erroneous contact depth. The top perforations in three wells were placed within 2 metres of the actual gas-oil contact rather than the intended 9 metres below it. Within six months of production, all three wells were producing gas-oil ratios five to eight times the expected value as the gas cap coned into the perforations. A fourth well, drilled later, hit the gas-oil contact during drilling and provided the pressure measurements that corrected the contact depth.

Remediation required perforation plugbacks, new perforations below the corrected contact, and in one well a workover with a mechanical bridge plug to isolate the gassed-out upper perforations. Total remediation cost was CAD 4.2 million. The larger cost was in deferred production: the three wells produced at 60 to 75 percent of expected rates for 18 months while gas-oil ratios were excessive, representing approximately CAD 81 million in deferred net revenue at the then-prevailing oil price.

The lessons applied across the operator's portfolio: gas-oil contact determination in exploratory and appraisal wells now requires at least three confirmed pressure points in the gas zone and three in the oil zone before the contact depth is certified. Extrapolating the contact from mud log indicators without pressure confirmation is disallowed for wells where the contact will be used to design completions.