PrintThe typical life-cycle of most oil or gas fields can be classified into five stages:
- Exploration Stage
- Appraisal/Delineation Stage
- Development Stage
- Plateau Stage
- Decline Stage
- Abandonment Stage
Life Cycle of an Oil or Gas Field or Reservoir
The life-cycle is illustrated in Figure 8.01. Drilling engineers are essential during all stages of this life-cycle.
The life-cycle of an oil or gas field starts with Exploration Stage where undrilled acreage is evaluated to determine its potential for future commercial development. During the exploration stage of field development, Exploration Geologists and Geophysicists (amongst others) develop prospects which look promising for future evaluation. Once these prospects are generated, exploration wells are drilled to prove whether hydrocarbons actually exist at the locations identified by the geologists.
The statistical success rate of exploration wells (exploration wells finding commercially viable hydrocarbon reservoirs) is between 25 – 45 percent[1]. Once a well encounters a crude oil or natural gas reservoir, that well is designated the Discovery Well for the reservoir. This starts the beginning of the Appraisal and Delineation Stage of the field development.
During the Appraisal and Delineation Stage of the field development, wells are drilled to provide data for the Development Geologists to evaluate the newly discovered reservoir. Wells drilled in the interior of the reservoir are referred to as Appraisal Wells and are used to gather information from Core Analysis, Well Logging, and Well Tests that can be used to analyze important reservoir properties and create trend maps of these properties, such as, depth, thickness, porosity, water saturation, permeability, etc. Wells drilled towards the periphery of the reservoir are referred to as Delineation Wells and are used determine the areal extent and size of the reservoir. During this stage in the life-cycle, the appraisal and delineation wells produce limited amounts of hydrocarbons. Oil or gas production, when it occurs during the appraisal and delineation stage, is typically limited to the volumes produced during well tests where the wells are allowed to flow for limited periods of time (several days) to determine the production rates that can be anticipated from the reservoir. Additional fluids may be produced if an Extended (long-term) Well Test or if a Pilot Test is run during the appraisal and delineation period.
Once the development geologists and reservoir engineers feel that the reservoir has been adequately appraised and delineated, the Development Stage begins. In the development stage, all of the wells required for initial Field Development Plan are drilled, completed, and tied back to the production facilities. During this period, multiple drilling rigs may be running simultaneously in order to drill and complete all of production wells, injection wells, and, possibly, observation wells required by the development plan in a timely manner.
After the field has been put onto production, the wells produce into production facilities which have a fixed capacity. If the production potential from all of the development wells exceeds the production capacity of the surface facilities, then production will be limited by the capacity of the production facilities. (In other words, the facilities act as a bottleneck in the system, and the wells must be produced below their potentials). As reservoir pressure is depleted, the potential production capacity of the wells declines (due to the reduced drawdown), and they can no longer produce oil at the full capacity of the facilities. (In other words, at some time, the reservoir pressure declines to the point where the wells become the bottleneck in the system.)
The early period of production, when the facility capacity is the bottleneck, is often referred to as Plateau Stage of production because the facility capacity is relative constant over time (barring any upgrades or additions to the facilities). The later period of production, when the well capacity is the bottleneck is referred to as the Decline Stage of production because the well capacity is tied to the reservoir pressure which is continually decreasing with time. Both of these stages are illustrated in Figure 8.01.
While being the more common name, I personally do not like the term “Plateau Stage” because this has somewhat of a passive sound to it. In order to keep the facilities producing at capacity, there is a lot of activity that needs to be performed behind the scenes. From the start of production, the reservoir pressure will decline due to fluid withdrawals. In addition, as time increases, unwanted water or gas production may increase and Well Interventions (Workovers). To keep oil production at the facilities capacity, Infill Wells may need to be drilled; completion designs may need to be altered; wells may need to be stimulated; water or gas shut-off workovers may become necessary; artificial lift may be required, secondary production techniques may be used; etc. So, during this plateau stage, a lot of reservoir management activities may be required to keep the facilities fully utilized. Therefore, I prefer to call this period the Reservoir Management Stage or Managed Plateau Stage of production.
At some point in time, the reservoir management options can no longer keep production at the facilities capacity, and the field production rate will eventually go into decline. During this stage, reservoir management activities are still performed, but with the objective of arresting the decline rate, not keeping the facilities fully utilized.
Finally, at some finite, non-zero production rate, the revenues generated by the oil and gas sales can no longer support the costs of the operations, and the field must be abandoned. This stage is referred to as the Abandonment Stage of the field.
As stated above, drilling engineers are required during all stages of this life-cycle.
[1] Energy Information Administration / OGJ.