General | Input variables |
Set user Input path |
Trap description | Appraisal options | World-wide default priors | Monte Carlo procedure |
Output | Saving in- and output |
Printing input and output |
References |
Volcalc is a program to calculate reserves in a single reservoir on the basis of a number of reservoir and PVT variables. It gives the results as the conditional or "Unrisked" volumes. For a complete appraisal of an exploration opportunity, an estimate of the geological probability of success ("POSg") will be required. This is usually done by estimating a number of necessary ingredients of the Petroleum System, notably the probabilities of fulfillment of HC charge, Trap, Reservoir, Seal, and Retention. Those subjective probabilities are then multiplied to obtain the POSg.
In some cases, notably prospects in a deltaic sequence, the user has a set of separate reservoir/seal pairs in a usually large depth interval. When not much detail is known about the differences of these targets and even their number is not certain, a typical case of these reservoirs can be described by Volcalc at an average depth. Then an estimate of the uncertain number of these reservoirs can be made and the targets are summed by the AddRes program under the NIEC option.
The input/output options for units are "Metric" or "Oil Field Units", (OFU).
There are five input sheets:
In addition, there are (and explained further down):
The application of economic cutoff volumes for oil and gas will usually produce a volume expectation curve that includes zero volumes, even given a geological POS of 100%. The zeros produced in the simulation are the small volumes that are less than the cutoff. So if we have a POSg (geological, not taking economics into account) and a "conditional" POSv (the POS of the volume alone and also called "unrisked volume") , the total POSc becomes POSc = POSg * POSv (the product of POSg and POSv as fractions).
The product of these probabilities (as fractions instead of the input percent) is the POSg. It is assumed that these probabilities are fully independent. That a factor such as "recovery" is not included is because we feel that this is an economic factor, although it can be predicted by geological variables, such as reservoir and oil quality. Hence the POSg is effectively a subjective chance to find "Oil in place" (c.q. Gas , Condensate). A further refinement is to add the conditional chance of having Free Gas, given hydrocarbons P[FG|HC] and the conditional chance of having Condensate, given Free Gas: P[Cond|FG].
The above figure gives the upper part of the input window.
Metric input is the default, but a similar window for Oil Fields Units ("OFU") is provided under the menu item Options.
The condensate recovery is a two-step calculation: First gas has to be recovered, then only a fraction of the condensate in the produced gas may be recovered.
Distribution type. Five distribution types are available as listed below in more detail. For the relevant input column, click with the right button on the input cell and click on the desired distribution type in the popup menu to make sure the correct distribution code is entered.
Each variable can be have a "low" (L), a "middle" (M) and a "high" (H) value as input. These may all be equal, or in increasing order, not otherwise.
In the second column of the input grid the distribution type can be specified as follows:
The letter indications may be also lower case.
Always give three numbers as input. The meaning of the three numbers is dependent on the distribution type.
The default path to the Volalc input is in the directory of the Volcalc program. If the user wishes to use another folder, he can use the Set Path option under "Options". This allows browsing for a directory/folder. His choice will be remembered after closing the Volcalc program in the "DefPath.txt" file in the VolcalcDefaults directory.
The trap description has the following options:
The above figure shows the trap input input grid when a contoured stratigraphic trap is described. For the culmination, the top of the reservoir at the highest point, the most likely depth from the main input sheet must be chosen. The areas of the top reservoir are always greater than, or equal to those for the base at the same depth row. If OFU units are used, the depth data are in feet, but the area units of square kilometers are for both metric and OFU. Saving your input will transform the depth in feet into meters. This results in a single geometrical reprersentation of the trap, quite contrary to the varying traps generated with the "simple trap models". However the Gross Reservoir Volume is varying because of variation in the Gross Reservoir Thickness. The variation in depth is used by the program in estimation of various PVT variables. Schematically, the contoured trap models are shown below:
Anticlinal structure | Stratigraphic trap |
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Note that for the stratigraphic trap, the areas for the top reservoir are taken from a vertical plane (vertical line in the figure) through the culmination. These are areas that include the yellow and white zone in the figure. The areas for the base reservoir are taken from the same vertical plane, but include only the white area. Top reservoir areas can only be equal, or greater than the base reservoir areas.
As a check on reality, both the dip on the flank at spillpoint and the maximum dip observed between culmination and spillpoint of a trap is calculated. For this purpose a width of the trap is estimated from the area at a given depth. As for all trap models, the trap is subdivided into 1000 "slices" between culmination and spillpoint, the difference between successive half "widths" and the thickness of the slice gives a tangens of the dip. The area is in this case is approximated to be a circle.
The oil density regression has been based on a world-wide sampling from some 80 sedimentary basins. The result is a simple regression of API gravity on depth in meters and a fairly large standard error of estimate.
The condensate ratio is based on some 73 examples of condensate reservoirs, where temperature and pressure were available. The regression of the ratio on T and P, although expected to be helpful, is not significantly better than a regression on only P. This is because of the strong correlation between P and T, both largely determined by depth. The Pressure (P) explains some 45% of the variance.
The standard error of estimate is as high as 69.5 b/MMcf. The problem with the data is that samples are only available for cases with condensate. Under the same conditions of P and T it is very well possible to have no condensate at all, depending on the petroleum system elements, such as maturity, origin of the gas (bacterial gas will be very dry), etc. Therefore this default estimate is only meaningful under the conditional probability of having condensate, given free gas. That estimation of the conditional P[Condensate|Free Gas] must be based on analogons and geological insight.
The regression in terms of OFU units is:
Where CGR is in barrels /million cubic feet, Pressure in psia.
Explanation of how this regression is simulated is given in www.mhnederlof.nl.
The recovery priors are simply histograms on a world-wide basis, so having a mean and a standard deviation.
The output is given as four tables, with in the upper part the usual statistics of interest, such as the P90, P50, P10 values and the Mean ("unrisked Expectation").
The Metric and OFU list of recoverable hydrocarbons
The Metric and OFU list of In Place Hydrocarbons
In the lower part of the tables a summary list at 5% intervals of the expectation curve is given. With this data an expectation curve can be viewed by clicking on the heading of a table column.
The first three rows give the percentiles of the unrisked expectation curve: P90, P50 and P10. For some programs any addition of prospect reserve estimates are based on these numbers.
The the green highlighted row gives the Mean values ("unrisked means").
The next three rows give the minimum, mode and maximum, which can be interpreted as the parameters of a triangular distribution.
The follows a row with the standard deviation and the standard deviation of the Mean. The latter is derived from the standard deviation, divided by the square root of the number of Monte Carlo cycles.
The pink highlighted row gives the POSv, the probability of success of the volume itself. If there are no zeros generated, this POSv is 1.000, but occasionally, a distribution model creates impossible negative values. The program sets such values to zero. The POSv is then less than one.
The POSv becomes more important in the last three columns, because there the results are affected by their respective cutoffs. The standard deviation of this POSv is also calculated and displayed in the next row. This value is only relevant for the POSv, because for the POSg we have a single subjective estimate, the uncertainty of which is difficult to know, or, in any case the user is not asked for such information (to keep things simple).
The last two rows are:
The accuracy of the Mean can be estimated with the help of the standard deviation of the mean. The volumes given are supposed to be all greater than zero, as this program only estimates the conditional volume of hydrocarbons in the trap, i.e. the POSg = 1.00. However, with applying the cutoff to these unrisked volumes, the POSc can be less than one. This is visible in the three right-hand columns, where the cutoff has been applied. In addition a standard deviation of the POSc is given, using the normal distribution approximation of the binomial. With extreme high or low POSc, this standard deviation becomes unreliable for simply estimating the confidence ranges around the POSc (See the Monte Carlo Procedure). After the cutoff, the "Mean" is also the "Mean Success Volume" (MSV) for the last three columns.
The lower part of the results table contains a 21-point representation of the expectation curve for each of the categories (Columns). This summary of the total distribution of values is used for estimating the mode and to produce the "unrisked" expectation curve graph (by double clicking on the column header). Both a Metric and an OFU table of results is provided.
The input file consists of a title to identify the run, the filename and a list of all the cells of the input grid, with coordinates and cell content, in a textfile. There is no facility to file the output. Saving in and output can also be done by clicking the "Clipboard" button. Then the datagrids can be pasted into a worksheet for further refinement, display or printing.
The most convenient way to print input and results is to copy the table and paste it in Excel. Use the "Clipboard" button to copy. Then the column width may have to adjusted, but all the advantages of the Excel mechanisms for selecting and printing are available. So you have to use the mouse to select the cells of the datagrid and then copy it to the clipboard by [Ctrl] and "C".
The more direct way to print results is to use the menu Print item which allows the printing of: