Page 1 of 7 Average porosity, net-to-gross (NTG) and facies determination per zone from well tops in Petrel

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Average porosity, net-to-gross (NTG) and facies determination per zone from well tops in Petrel Petrel 2013

Olawumi Owolabi highlights key steps in generating Average Petrophysical and facies attribute values from well logs. The process carefully examines an available but simple option in Petrel to calculate porosity, NTG and facies from well logs per well top zones. This useful tool allows for rapid quality check (QC) of log data and results can be used further in Geology & Geophysics workflows.

Olawumi Owolabi Geologist

Introduction Continuous and discrete attributes can be created in Petrel to represent continuous petrophysical parameters and discrete facies properties respectively. Zone attributes are very useful tools for the geologists and geomodeler. Determination of average continuous and discrete attributes per zones between two well tops provides an easy way to QC created and imported well data prior to their utilization in future exploration workflows. Petrophysical and facies logs form the core of modeling and the data should be right and ready for input for these processes. This simple workflow highlights how to create average porosity zone attributes, average NTG from NTG logs, average facies logs and how to grid these zones attributes to generate maps of average properties.

Creating average porosity attribute per zone

The simple workflow above is employed to achieve the objectives of the write-up. Well logs and well tops are imported following standard import procedure in Petrel. Created well tops (interpreted stratigraphic surfaces) can also be used if there are no well tops available for import. The zones between two well tops are then used to generate average attributes. The calculated zone attributes can then be gridding using suitable gridding algorithms to generate continuous surfaces for both petrophysical and facies properties.

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Page 2 of 7 Average porosity, net-to-gross (NTG) and facies determination per zone from well tops in Petrel

Display wells and corresponding well logs in a well section window, QC the logs and adjust the limits accordingly as shown above. Also, display interpreted and imported well tops in the same window. In this above diagram, the gamma ray log is displayed alongside the porosity log for accurate distinction between porous and non-porous lithology units.

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To access the ‘Attributes’ folder, expand the well tops folder and view Petrel pre-defined well top attributes. User defined attributes can be created using the following steps.

• Right click on the attributes folder • Select the option to insert new attribute • Select continuous attribute from the drop-down when creating petrophysical zone attributes • Rename the attribute accordingly ‘Porosity’ and apply porosity as the template • Click ‘Apply’ for changes to be updated

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Navigate to the Attribute operations tab to define how the attribute should be calculated. To defined attributes per zone (Average porosity), select the option ‘To the Zones at level’. This allows the attribute to be created between two well tops (from one well top to the next). Specify the level to be calculated by inserting numerical values (1, 2….n). Since the porosity logs are being used for the calculation (with the gamma ray as a secondary conditioning log) the option to sample attribute from well logs should be used as seen in the figure above. The right log then needs to be specified from the drop-down, select treat logs as lines (as they are continuous parameters) and the suitable averaging method for porosity is the arithmetic method. Click ‘Run’ to calculate average/zone porosity attribute.

The calculated average porosity attribute is stored in the well tops folder under the attribute object. Toggle on the average porosity attribute to display in the well section window. Notice the calculated average value between one well top and the one below it.

Generating Average porosity maps Gridded surfaces can be created from the calculated average porosity. To achieve this, the first step is to convert the active attribute (Average porosity) to points for all zones of interest. Right-click on the zone between 2 consecutive well tops and select ‘convert the active attribute to points’ as shown below.

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Using the point data and selecting Isochore interpolation as the algorithm method, gridded surfaces of the zone attributes are created in the make/edit surfaces process (See results below).

Deriving calculated Net-to-Gross (NTG) attribute NTG attribute can also be generated as an average value per well top zones. This continuous property is important in the delineation of reservoir and non-reservoir units (pay and non-pay). Same procedure as above is applied for the calculation of average NTG values. Be sure to select the appropriate template as highlighted below.

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In the attribute operations tab, select ‘to the Zone at level’ and select the right NTG log to be used as a basis for the computation of the average NTG values. Like the porosity computation, the averaging method is also selected as ‘Arithmetic’ because NTG is an additive variable. Since the calculated NTG attribute is derived from a continuous NTG log, select to treat log ‘as lines’. Click ‘Run’ for Petrel to run attribute computation

The calculated zone attribute is saved at the bottom of the attribute object in the well tops folder. As shown in the figure, the NTG log (track 2) and the derived zone NTG attribute (track 3) can be checked against the gamma ray log (track 1) as a QC process.

Computing average facies (per zones) from well top Facies are discrete properties often represented by integer codes (0, 1, 2….n) where an individual integer represents a group with similar characteristics (depositional environment) or cut-offs. To generate average facies zone attributes, the attribute type needs to be set to discrete.

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Set Facies as the template and navigate to the attributes operations tab to adjust the calculation parameters as seen in the diagram below. It is important that the correct log to be used for the computation be selected from the list of available logs. A fundamental difference between the averaging method for continuous and discrete logs is the choice of averaging method. While ‘Arithmetic Average’ is used for petrophysical properties (continuous logs), ‘Most of’ is the averaging method applied to discrete properties/logs. ‘Most of’ method compiles the highest percentage of a selected facies code at a given zone level and sets this as the average facies for that zone.

The calculated average facies attribute based on the facies with the most proportion is stored under the attributes object in the well tops folder. Display imported and derived log to QC calculated attribute. These derived average attributes are important for future modeling processes and workflows.