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How can I obtain volumetric reserve calculations from contouring?
The problem is these variables change from one well to the next in most reservoirs, and it forces us to either model the reservoir or make basic assumptions of average porosity, water saturation, and pay thickness, even if we are able to planimeter the aerial extent of the reservoir. With this article, I will take you through the process of how easy it is to perform these fairly complex calculations that can be modeled fairly painlessly, yet quite accurately, in industry software packages such as Petra.
Our first step is to create an acreage position overlay and a net pay contour grid, which are fairly straightforward principles with every available software product on the market. While you would also want to create a porosity and water saturation grid, for instructional purposes, we will simply deal with a net pay grid. But one can easily perform the following functions to include a porosity and water saturation grid as well.
In Petra we will first need to select the menu that allows us to compute volumetrics from a grid.
You have several selections, the first of which is to determine which grid to use, whether to compute the volumes for overlay polygons or for the entire grid, and the selection of the minimum or maximum thickness to use. Polygons can delineate different areas of interest within the grid for separate area and volume totals. Either single or horizontal "slice" volumes can be generated.
We will then need to choose the isopach you wish to use and generate volumes for each overlay polygon. The Reserves tab allows you to choose several options on the type of reserves to be computed. This option determines how the volume data is scaled and labeled. In the following equations, "Volume" is in feet or meters cubed.
Type of Reserves Computed
Basic - No reservoir parameter scaling is applied.The "Show Results As" option causes the area values in the report to the scaled to acres and the volumes to be converted to US barrels.
Oil In Place: OIP = Volume x Ø x (1-Sw) / RVF
Recoverable Oil In Place: ROIP = Volume x Ø x (1-Sw) x Rf /
RVF Gas In Place: GIP = Volume x Ø x (1-Sw) / Bg
where Bg = 0.02827 x (460+Tr) x (Z / Pr)
CBM Gas In Place: CBMGIP = Volume x Cf x Gc x Sg
We then need to select "Oil in Place", and review the reservoir parameters under "Oil & Gas Reservoir Factors."
In this example we are using a porosity of 10%, water saturation of 60%, reservoir volume factor of 1.5, and a recovery factor of 40%. The output data will be in MBO for the Oil in Place, and will be listed for each of the overlay polygons.
Here is a partial example of the Volumetrics Report that will be generated:
Reservoir Parameters Used To Compute Oil In Place (OIP)
Porosity (%)....................: 10.00
Water Saturation (%)............: 60.00
Formation Volume Factor (FVF)...: 1.5000
Polygon: All Polygons Combined
Polygon Area: 1,562.63 (ACRES)
Data Area: 1,400.42 (ACRES)
OIP: 3,016.18 (MBO)
Polygon: FINA
Total Area: 162.21 (ACRES)
Data Area: 0 (ACRES) OIP : 0 (MBO)
Polygon: LASER
Total Area: 319.43 (ACRES)
Data Area: 319.43 (ACRES)
OIP : 847.67 (MBO)
Polygon: PALACE EXPL
Total Area: 39.5 (ACRES)
Data Area: 39.5 (ACRES)
OIP : 63.37 (MBO)
The Volumetrics Report is written to a CSV file and stored in a grid folder under the project directory.
Grid User Models
To obtain a more complex and sophisticated modeling of the reservoir, a user model can be used, which is a text file containing a user-written program that allows somewhat sophisticated manipulation of grids. These models allow multiple grids to be input, processed with "if-then-else" logic to produce one or more grids as output. After writing the model, the user places the model into the grid folder to be used, or modified, for multiple grid operations.
The following is a user-model that takes a structure grid and net pay grid as input, along with a subsea depth of the gas-oil contact (GOC), and produces a "Net Gas" grid that has been truncated below the GOC.
! NETGAS.TXT
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
! INPUT GRIDS USED IN THE MODEL !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
GRID STRUCT IN;
GRID NETPAY IN;
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
! OUTPUT GRIDS USED IN THE MODEL !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
GRID NETGAS OUT;
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
! CONSTANTS USED IN THE MODEL !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
CONST NULL;
CONST GOC -1560.0;
NETGAS = NULL;
IF ( (NETPAY .NE. NULL) AND. (STRUCT .NE. NULL) ) THEN BEGIN;
NETGAS = (STRUCT - GOC);
NETGAS = MIN (NETGAS, NETPAY);
END;
ENDMOD;
At this point one may view and modify, and compile the model. The grids used must be re-sampled if they are not the same grid size prior to executing the model.
Once the assignments have been made we can then select Execute the Model and redisplay the new grid for the Net Gas:
We can then add a second contour of the structure on top of the net pay map to see the structure with highlighted gas water contact.
Thus, we have performed a volumetric reserve calculation of a reservoir whose basic properties are varying across the field. If the geology has already been evaluated, it is a very painless effort to extract volumetric reserve volumes. Even if the geology has been performed in another software package, it is still quite simple to import into Petra the contour layer files from another package and perform the volumetric calculations within Petra. If the contour map is a paper map it is still a simple process of digitizing the contour map and then performing the calculations.
If CEC Energy Consultants can be of any assistance in helping your staff get up to speed to accomplish these tasks or to perform them for you on an outsourcing basis, please do not hesitate to contact us.
Back to March 2004 Newsletter
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