*What is
Basic Surveillance?*

The starting point for well-test analysis work consists of plotting data in a sensible way, and then using these plots to compare different sets of data in a consistent fashion. Changes in well behaviour will show up as shifts in characteristic features in the pressure transient response as shown on the analysis plots.

The main plot used to identify reservoir properties is the "derivative plot" which has two curves; a derivative curve and a "delta-p" curve. The key feature that forms the basis for most analysis work is a "derivative stabilisation" (a portion of the derivative curve that has a constant value). A stabilisation corresponds to a straight-line on a Horner (or Superposition) plot which, in turn, is related to the permeability-thickness of the reservoir where the flow geometry is radial. The other important feature on a derivative plot is the separation between the stabilisation and the "delta-p" curve which is related to the skin-factor on the well.

The main plot used to identify trends in the reservoir pressure is the "superposition plot". This is just a more elaborate version of the Horner plot in that it accounts for an arbitrary rate-history leading up to the shut-in data being analysed. The key feature on this plot is identifying a portion of the data that lies along a straight-line. This line can be extrapolated to an "infinite" shut-in time to get a value called "P*". The value of "P*" itself is meaningless because there is nothing to say the actual shut-in response will follow a straight-line to an infinite shut-in time. But keeping track of the values of P* across several sets of shut-in data will provide information about the trend in the reservoir pressure.

These two plots are linked to each other. The slope of the superposition plot straight-line must correspond to the derivative stabilisation because these are both measures of the reservoir permeability-thickness. Moreover, with a bit of rate normalisation in the plot transforms, the derivative stabilisation and superposition-plot slope will be the same for any set of shut-in data on a given well as long as the reservoir permeability-thickness doesn't change.

There is a certain degree of marketing "hype" in the petroleum industry that says well-test analysis must use type-curve models and simulation to match a transient response and derive meaningful results. At the level of an exploration or appraisal well-test, this is true. There is a huge benefit in looking at pressure transient data in great detail in order to add an additional piece to help solve the reservoir description puzzle (see SPE paper 102483 by M. Levitan). And specialised programs like PIE are directed at this sort of detailed analysis.

But when trying to keep track of the performance of a developed field, it is usually a waste of time trying to look at all the well-test data in great detail. This is because there are only two main questions involved when monitoring production data:

- has the well performance changed?
- is the reservoir pressure deviating from the expected trend?

If the answer is "yes" to either question, THEN it is time to look at the pressure-transient data in great detail. It is also probably the right time to design and carry out a closely managed test sequence in order to get pressure data that is best suited to a detailed analysis.

Which leaves the issue of deciding what type of analysis is required to answer these two reservoir monitoring questions. The best approach is to simply keep track of derivative and superposition plots for a well, and compare each new set of data against past measurements looking for distinct or unexpected changes in the derivative stabilisation and/or superposition "P*". This is particularly easy to do when the plots are rate-normalised; a well producing from a given permeability-thickness will show the same stabilisation/slope over successive sets of shut-in data. An even better trick is to simply plot each set of shut-in data over top of the previous tests (called a plot overlay). If everything is running smoothly, the derivative plots will all show a common stabilisation, the separation between the stabilisation and delta-p curve will be constant indicating no change in skin-factor, and the superposition plots will show a trend in "P*" that makes sense with the reservoir pore-volume and voidage replacement strategy.

This type of analysis typically forms the bulk of the basic surveillance work needed to keep track of well performance. Once this basic surveillance shows something has changed, then it is time to do some serious analysis using serious pressure transient analysis software (like PIE). But until then, basic surveillance is really just a question of preparing derivative and superposition plots, drawing straight-lines to set a stabilisation or slope, and overlaying plots to spot changes in behaviour.

A software sales representative will probably disagree with the following, but this basic surveillance work does not need a complicated software package. Since I am an engineer, sales is the last thing I worry about (which is probably why Well-Test Solutions is a small company). So I am providing a free "Basic Surveillance" spreadsheet which implements all of the above items. All of the equations and code in the spreadsheet are unprotected so you can see exactly how these basic plots and calculations are done.

You should be able to do most of your well-test analysis work with the spreadsheet. For some fields, you can probably do all of your well-test work using this spreadsheet. This gives you the option of only buying a well-test analysis software package when you actually need one.