This article will demonstrate a material balance equation in gas cap drive mechanism. First, we start with a full material balance equation.
Figure 1 – Full Material Balance Equation
Assumptions
Material balance equation can be applied for any drive mechanism and this article demonstrates how to apply the material balance equation in a solution drive mechanism. For a solution drive mechanism, there are 2 cases. The first case is when reservoir pressure is above a bubble point and the second case is when reservoir pressure is below a bubble point.
Solution Gas above Bubble Point
Start with a full material balance equation.
Figure 1 – Full Material Balance Equation
Assumptions Continue reading
Material balance is a mathematic way to express mass conservation in a reservoir and a simple key principle is “what reservoir is produced must be replaced by other mass.”
Volume Produced = Volume Replaced
Volume Produced comes from Gas Production, Oil Production, and Water Production.
Volume Replaced comes from volume expansion, water in flux and water/gas injection.
Figure 1 shows the relationship of the material balance.
Figure 1 – Concept of Material Balance
Let’s take a look at each component of equation. Continue reading
Some reservoirs have communication with a water zone (aquifer) underneath. When reservoir pressure drops due to production, the compressed water in an aquifer expands into a reservoir and it helps pressure maintenance. This mechanism is called “water drive”.
Water drive mechanism will be effective if an aquifer contacting reservoir is very large because water compressibility is very low. For example, an anticline structure with extensive water zone (aquifer) will have the most advantage from the use of a water drive mechanism. Conversely, stratigraphic reservoirs or highly-faulted reservoirs will have limited aquifer volume so water drive is insignificant.
Figure 1 – Water Drive Mechanism
Some reservoirs have a gas cap, which provides energy from gas expansion to help production from a wellbore; therefore, this is called “gas cap drive”. When oil is being produced, the gas cap expands and pushes oil downwards to a producing well (Figure 1).
Figure 1 – Gas Cap Drive
For this type of drive mechanism, it is imperative to keep gas within a reservoir as long as possible since it is an excellent energy source of the reservoir. Wells which are drilled into a high structure area where the gas cap is located must be closely monitored because this well will have more of a chance to produce gas. Continue reading
