Understanding of the possible range of hydrocarbon volumes in an exploration lead or prospect is fundamental to evaluation of the economics of a project, yet there is seldom sufficient data (or time) to perform detailed volumetric calculations, such as in a 3D geologic or simulation model. Simple screening methods may provide sufficient information to make decisions – for example, to bid on a block, rank prospects for drilling, or participate in a farmout. Other useful options in the volumetrics operations include creation of area or volume/depth functions and polygons around closure areas.
Petrel offers several methods to generate quick volumetric estimates using maps which should already exist for a prospect – at the minimum, a depth surface and a contact.The method selected will depend on the amount of additional data available and the project’s requirements for more or less detail and accuracy.
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SCA Instructor:
Laurie Green, MSc, PG
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Advance Your Petrel© Mapping Skills
Taught by Laurie Green, SCA’s Principles of Mapping with Petrel© provides the knowledge and techniques needed to make accurate and geologically correct maps. Geologic principles taught in traditional mapping courses are tied to the advanced tools available in the workstation interpretation platform. This course bridges the gap between the “tried and true” geologic principles taught in traditional pencil and paper mapping courses, and the advanced computational tools available from the workstation interpretation platform.
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Petrel’s© Single-surface and Two-surface methods are simple operations which calculate deterministic values for gross rock volume (GRV) only. To estimate in-place resources, additional operations would need to be performed using Petrel or a spreadsheet to apply zone-average net percentage, porosity and water saturation factors.
The Map-based method is significantly more advanced. Besides calculating a variety of volumetric quantities such as recoverable hydrocarbons for multiple zones, it can create property height maps and even perform rudimentary uncertainty analysis.
Single-surface and Two-surface Volume Calculations
The single-surface and two-surface methods are accessed through the Settings \ Calculations \ Volume operations for a surface. The operations are named “Volume below surface (constant level)” and “Volume below surface (surface)” respectively:
The single-surface method calculates the volume between structural surface and a flat contact. It is suitable for order-of-magnitude, quick-look estimates within a boundary such as a lease outline. It can also exclude volumes within closed fault polygons.
This method has significant limitations. For example, if a second surface is positioned between the original surface and the contact, the calculated volume will be too large:
Also, the calculated volume is reported only as unformatted text in Petrel's Message pane.
The two-surface method calculates the Gross Rock Volume (GRV) between two surfaces and can produce reasonable GRV estimates if the surfaces are prepared properly. For example, if a fluid contact is present both surfaces (or at least the lower surface) must be “clipped” against the contact, as shown below:
This method reports the results of a volume calculation in the same way as the single-surface method, as unformatted text in the Message pane.
Map-based Volume Calculations
This method is functionally similar to Petrel's© 3D volumetric calculations and includes may enhancements compared to the simple methods:
- Multiple zones within specified boundaries
- For each zone:
- Input of constant values or maps for contacts of different types (O/W, G/O, etc.)
- N/G and porosity maps or constants as input
- Oil, gas and water saturation maps or constants
- Surface conditions (e.g., Bo) and recovery factors
- Validation of surface geometry (checks for crossovers)
- Uncertainty workflows for contact depth, N/G and porosity
The output options are also enhanced, generating formatted spreadsheet reports, volume height maps and area/volume vs. depth functions. Its uncertainty workflows can be used to evaluate the effects of variability in porosity, N/G and structure, or for a range of contact depths.
The Map-based volumetric calculations are most easily accessed from its icon on the mini toolbar for a selected surface: 
It also appears in the Seismic Interpretation ribbon and in the Process pane.
The dialog box has four panels – Input, Output, Structure adjustments and Settings. The Input panel is where the user can specify multiple zones, contacts and other properties. Each zone may have a different set of contacts and different properties. Properties are specified as either constants or maps – for example, porosity or N/G may have been mapped from log values.
Structural surfaces are checked for crossovers and can be adjusted to tie well depths.
Properties including Bulk, net or pore volume, STOIIP or GIIP and recoverable oil or gas, can be output as a formatted report which can be cut and pasted into a spreadsheet. Results can be grouped by contact sets, zones or boundaries. Reporting units can also be selected, such as barrels or cubic meters.
Output options include maps of selected volumes which can be displayed individually or draped over structure maps, such as these displays of
HCPV:
This type of display can be extremely helpful to visualize “sweet spots” in structural or stratigraphic plays.
Perhaps the most powerful feature of Map-based volumetrics is its uncertainty workflow. By activating options to evaluate the effects of incremental contact depths or variability in N/G or porosity for each zone, multiple cases can be generated for further analysis, including histograms to display the range of potential outcomes:
To evaluate contacts over a depth range, the user specifies the upper and lower contact depths and the number of increments. The results are reported as formatted text and can also be displayed as depth/volume functions:
Variability in N/G and porosity properties are based on their mean and standard deviations, which can be estimated from well log calculations for each zone. A different case is generated each time the volumetric calculation is run. The calculation can also be embedded in a looping workflow, which will generate as many cases as desired.
The results of individual uncertainty cases are reported as formatted text in the same way as deterministic calculations.
To compare the results of multiple cases, selected values can be displayed in table format without having to export them to a spreadsheet. Color-coding, sorting and percentile displays make it easy to identify cases of interest, such as those which are close to P10/P50/P90 rankings:
Individual cases can be selected for further review. In the tables shown above, Cases 11, 22 and 17 are all close to the P50 ranking based on Pore volume. However, Case 4 is actually closest to the P50 ranking for Net volume, so it would be important to create maps for key properties from these cases to understand the areal distribution of the oil or gas.
Summary
Decision-makers are often forced to decide between different options quickly and with limited data. Even the simplest screening volumetrics provide visual and quantitative results which will enhance understanding of a prospect. Methods which incorporate uncertainty ranges can provide input for detailed economic evaluations, and all of the methods can be re-calculated in automated workflows throughout the project’s live cycle. When applied consistently to an entire portfolio, consistent screening methods should lead to better-informed decisions and outcomes.