Previously we presented the macroscopic material balance equation. It includes many terms that can bog you down if you do not understand what they imply. Here we will make things a little bit clearer by associating the terms with reservoir drive indices.
One of the most powerful applications of the material balance equation is it can be used to infer the reservoir drive mechanisms (also known as reservoir drive indices). Reservoir drive mechanisms is the natural energy used to move oil and gas to the wellbore. The reservoir drive mechanisms associated with primary recovery include the following:
- Gas Cap Drive
- Solution Gas Drive
- Connate Water Expansion
- Rock Compaction Drive
- Water Drive
Each one of the reservoir drive mechanisms is accounted for in the macroscopic material balance equations. This is illustrated below:
The primary drive mechanisms appear on the right hand side of the equation. In many cases, the material balance equation can be reduced to have predominantly one significant drive mechanism for a reservoir. In this instance, all other drive mechanisms ban be neglected. If two or more drive mechanisms are dominant and need to be included in the material balance equation, we call it a combination drive reservoir. It all depends on the reservoir data. The left hand side of the equation includes the surface produced fluids and Secondary drive indices. Secondary drive indices are used to maintain or enhance reservoir pressure. Secondary drive indices include gas or water injection. The illustration below shows how all the primary reservoir drive indices respond as reservoir pressure decreases:
From the figure above it is clear that as reservoir pressure declines, the rocks, connate water, and gas cap expand in the reservoir space. When the pressure falls below the bubble point, solution gas evolves out of the oil and helps maintain reservoir pressure. The water influx enters the system under external energy also assists in maintaining reservoir pressure. The more pressure support we receive from different drive mechanisms theoretically leads to a higher recovery factor. In the future we will discuss how the equation can be reduced and applied to different reservoirs types. I hope you learned something.