Average Reservoir Pressure (p or pR)

Subtopics:

The average reservoir pressure in a reservoir at a given time is an indication of how much fluid (gas, oil, or water) is remaining in the reservoir. It represents the amount of driving force available to drive the remaining fluid out of the reservoir during a production sequence. When dealing with oil, the average reservoir pressure is only calculated when it is undersaturated (flowing pressure above the bubble point). 

Average reservoir pressure can be estimated in two different ways:

1. By measuring the long-term buildup pressure in a bounded reservoir. The buildup pressure eventually builds up to the average reservoir pressure over a long enough period of time as shown below.

Note:    This time depends on the reservoir size and permeability (k) (i.e., hydraulic diffusivity).

2. Calculating average reservoir pressure from the material balance equation (MBE) – as described below.

Material Balance Equation (MBE)

Gas

For gas, the MBE is defined as the relationship between the original gas in place, initial pressure (pi), cumulative gas production, and the current average reservoir pressure. The basis of the MBE for gas flow is the volumetric balance of all the fluids at a given time. The following equation proposed by Ramagost and Farshad (1981) is used to calculate the average reservoir pressure for gas systems. This equation considers that gas is the only mobile phase in the presence of residual fluid saturations (oil and water) in a compressible formation.

Note that this equation is only valid when the term ce(pi– p) < 1.

Oil and Water

For oil and water, the MBE is defined as the relationship between the original fluid in place, initial pressure (pi), cumulative fluid production, total system compressibility (ct), and current average reservoir pressure. The following equation for liquid flow is based on the definition of total compressibility (ct) at a given time. This equation considers the selected fluid (oil or water) as the only mobile phase in the presence of residual fluid saturations, if present, in a compressible formation.

For oil:

For water:

where:

References

"p/Z Abnormally Pressured Gas Reservoirs", Ramagost, B.P. and Farshad, F.F., Paper SPE 10125 presented at the AFTCE, San Antonio, TX, October 5 - 7, 1981.