Subsidence

What is Subsidence?

Subsidence occurs when the removal of pore fluids from a reservoir rock causes the rock to compact. This causes all the overlying rocks to subside, which ultimately results in a surface depression above the area of extraction.

The surface depression is typically somewhat larger in area than the zone of compaction, but shallower. The volume of the depression is normally close to the volume of fluids removed.

It might seem like the extraction of oil from a reservoir involves such small volumes that any sinking of the land surface would be inconsequential. However, the volumes add up when producing from a reservoir over a period of years or decades, and subsidence has caused a lot of damage to both production infrastructure, such as pipelines, and unrelated infrastructure such as roads and structures.

Take the example of the Wilmington Oilfield, in Long Beach, California. Between 1926 and 1968, the land above the oil field subsided 9 meters (28 feet), with a maximum subsidence rate of 0.75 meters per year. Since the field was located next to a major port and near sea level, extensive rebuilding was needed, both to repair structures and prevent them from being inundated by the sea. This reconstruction process cost $100 million dollars.

One of the biggest hazards of subsidence from a production standpoint is casing failure. The casing is cemented to the formation along much of its length. That means that it will move along with the formation – if the formation is compacted, the pipe will need to shorten. Small amounts of shortening can be taken up by the flexibility of the metal, but beyond a certain point, the pipe will start to buckle and split. Failure of well casing can obviously lead to problems such as groundwater contamination and loss of well control.

In addition to damaging surface infrastructure, formation compaction reduces the porosity and permeability of the compacted formation, which could have an impact on its future production capabilities.

Predicting subsidence during formation evaluation

Subsidence doesn’t always occur when fluids are removed – it depends on how much of a role the pore pressure is playing in supporting the structure of the rock. Compaction is most likely to occur under the following conditions:

  • Thick producing intervals: A thick producing interval will compact more than a thinner one simply because the same percentage reduction in thickness will result in a greater absolute change in thickness.
  • Poorly consolidated producing units: Poorly consolidated producing units will compact more than hard formations because, in softer formations, pore pressure plays a larger role in supporting the overlying rocks.
  • Large changes in reservoir pressure: The bigger the change in reservoir pressure, the higher the probability that compaction will occur.
  • Depth of reservoir: For a given thickness and change in reservoir pressure, a deeper formation will result in a smaller amount of surface subsidence over a larger area. A shallow formation will cause more sever subsidence, but the subsidence will be limited to a smaller area of the surface. Widespread, slight subsidence is likely to cause less damage than sever, localized subsidence, but this depends on what kinds of infrastructure and resources are located at the surface.

These factors can all be evaluated during the formation evaluation stage of production. If severe subsidence is expected, injection of fluids to maintain pore pressure can be made part of the drilling plan.

Measuring subsidence

There are a variety of methods for measuring subsidence. These include traditional surveying, the use of tiltmeters to detect minute changes in the slope of the ground surface, and satellite interferometry, which detects minute changes in surface elevation.

A worker in reflective orange vest holds a surveying and level rod

However, GPS is now the primary mechanism for detecting subsidence. Under ideal conditions, GPS devices can resolve subsidence within an accuracy of +/- 2 millimeters. By monitoring the movement of a benchmark through time, GPS devices can provide detailed data on the shape and size of a subsiding region, as well as the rate of subsidence.

Mitigating subsidence

Once subsidence is detected, there are several options. The first is to do nothing. This would make sense in a remote area encountering a small amount of subsidence. The economic cost of subsidence is small because there’s little human infrastructure to damage beyond the immediate drill site. More intensive subsidence may damage production equipment and should be dealt with. And, in regions of dense human settlement, doing nothing is not likely to be an option.

The most common course of action to reduce or even reverse the rate of subsidence is to inject water into the reservoir through a neighboring well. The water takes the place of the hydrocarbons in the pore spaces, preventing compaction. As an added benefit, the water will help push hydrocarbons to the well. In the Wilmington Oilfield, injection was able to slow, stop, and eventually even slightly reverse subsidence.

Images: “Crack in the Street” by olaser via iStock; “Level Rod” by Klubovy via iStock