Encountering unexpectedly small compartment size has been one of the leading sources of production shortfalls in the petroleum industry. It follows that traditional methods to characterize compartments are substantially inadequate. Downhole Fluid Analysis (DFA) has recently been established as a new tool to address compartmentalization. In particular, the objective of an entirely new type of log, a continuous downhole fluids log, is being realized. Here, the combination of DFA with pressure measurement is shown to be very effective for compartment characterization. Compartments of startlingly small size are shown. The ability of thin barriers to hold off large depletion pressures is established. Methods are given to optimize the use of DFA coupled with pressure for characterization of compartments.
Petroleum reservoirs can consist of flow units or compartments that are massive or quite small. This characterization does not describe the overall size of the reservoir but does enormously impact the ability to drain the reservoir. For pedagogic purposes, it is useful to relate reservoirs to objects of the mundane. A kitchen sponge is an open cell structure; the individual cells are connected so water can flow easily throughout the sponge. In reservoir lexicon, the sponge is a single compartment. A single hole or well placed in the sponge could drain the entire contents of the sponge. On the other hand a spool of bubble wrap is a closed cell system. Fluids cannot flow from one bubble or cell to another. Indeed, if a knitting needle penetrated through the spool of bubble wrap, only those cells that are penetrated would drain. The spool of bubble wrap is highly compartmentalized, again in the oil field lexicon.
Oil reservoirs can be highly compartmentalized. Wells placed in such an environment will initially flow vigorously, as the small compartments often are at high pressure. However, the volume of drainage in highly compartmentalized reservoirs is small, so the wells stop flowing sometimes in months. If the econometrics of field development anticipated 10 years of well life, then major economic losses follow. In deepwater, the risks and rewards are of large scale; corresponding losses due to such unexpected compartmentalization are unacceptable. One method to determine the size of compartments penetrated by an oil well is to do an extended well test. Pressure vs. flow data can then be used to determine overall compartment shape and size. However, extended well tests are expensive with internal costs to an operating company sometimes exceeding $100 million. With this cost structure, well tests, if initiated, are often terminated due to mounting costs prior to the well test objects being completed.
