Introduction |
  
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Introduction |
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Welcome to REP, the resource evaluation program. For more information on what can be accomplished using REP, read our section below on 'What REP Does'.
The two major components to REP are the Prospect/Field and Consolidations modules. If you want help with running either module, refer to 'Prospect/Fields Summary' or 'Consolidations Summary' sections, or browse for a specific topic in the contents.
For information on Maps and Set-up issues, browse beneath the relevant headings in Contents.
The Tools menu item contains some useful facilities:
Calculator allows you to define your own probability distributions and manipulate them with you own equations.
PDF fit is a way to fit normal, log-normal and beta distributions to entered data
Fluid correlations are a way to estimate formation volume factors for oil and gas using standard correlations.
In Unit conversions you can convert values from one unit to another.
For notes on program changes by version see Version notes.
Below is a very brief overview of what the REP program does.
The basic calculation in REP determines the in-place and recoverable reserves in a field or prospect.
The equation is: Reserves = GRV x Porosity x Net-to-Gross x (1-Sw) x FVF x RF, where:
GRV is the gross reservoir volume;
Porosity is the field average porosity;
Net-to-gross is the field average net-to-gross ratio;
Sw is the field average water saturation - (1-Sw) is the hydrocarbon saturation;
FVF is the reciprocal oil or gas formation volume factor (FVF for oil is 1/Bo, for gas it is the gas expansion factor);
RF is the recovery factor.
In the reservoir, there may be oil, gas, condensate or combinations of these and each hydrocarbon type (phase) is calculated separately.
Oil reservoirs may have associated gas. Gas reservoirs may have associated NGL's and may also contain inert elements such as nitrogen, carbon dioxide or hydrogen sulphide. Condensate reservoirs always produce liquids and may also have inerts. It is, therefore, clear that the situation can become complicated.
It is usually the case that the biggest uncertainty in reserves comes from the GRV, and there are a number of ways you can enter GRV into REP, incorporating maps, spill points, fluid contacts, etc.
If you solve the equation with single values for all the parameters, you get a single answer for reserves. This is a "deterministic" calculation. The power of REP is that, rather than using single values (though this is an option) you enter all the parameters as probability distributions. A probability distribution describes your uncertainty in the value of the parameter. For example, you may think that the field average porosity is 14%, but there is no possibility at all that you could say that it is exactly 14%, or any other value. So you can say that it is most likely around 14%, but could range from 11 - 18%. The probability distribution quantifies this sort of statement.
Using probability distributions gives a result that is itself a probability distribution. Using this you can make statements such as "The mean reserve is 246mmstb; and I am 90% confident that the reserve is greater than 110mmstb, and there is a 10% chance that the reserve is greater than 490mmstb."
If you have a prospect which has not been drilled, then there is a possibility that hydrocarbons will not be present. The chance that they are is called the "Chance of Success" and an important part of the REP process is to estimate this chance.
See 'Prospect/Fields Summary' for more information on the Prospect/Field module.
The reserves distribution for a single field or prospect is important and interesting. Based on it, you decide whether to drill a well, or another well. It will also be the basis of the decision to develop a discovery or whether to bid for a licence.
However, it is frequently the case that there is more than one prospect or more than one discovered reservoir to be considered in the decision. Often, the prospects are linked, in the sense that if one of them contains hydrocarbons the chance that the others also contain hydrocarbons is increased - and vice-versa.
So REP allows you to "add up" the various prospects, and presents a reserve distribution reflecting the likely outcomes of several fields or prospects. This is consolidation, and it is an extremely important part of the reserve evaluation process. It is not a straightforward addition, because (a) the things being added up are probability distributions and (b) if one or more of the things being added up has a chance of success of less than 100% - and additionally that COS is linked to other chances of success - the process is not straightforward.
See 'Consolidations Summary' for more information on the Consolidation module.
REP allows you to digitise maps. It is a quick and simple module and does not aim to replace the very sophisticated mapping in specialist packages. You need a scanned image of the map; the digitisation is done on-screen, using the mouse. The result of the digitisation is a set of contour/area pairs, which are used by the prospect calculations.
See 'Map Digitisation' for more information.
There are a set of environment variables and defaults which control the way REP works and how the results are presented. One is the way you (or, at any rate, your company) prefers to risk prospects.
See 'Set-up and Defaults' for more information on environment variables and defaults.