Major Steps in Statistical Well Log Studies

Summary

 

            The following is a brief sketch of the major tasks involved in computer assisted geologic and reservoir studies using digital well log data as the main data source.  This summary is an outline of how Hitchner Exploration Services Limited (HESL) views the steps of a multi-well analysis project and how HESL in its consulting role would address the project analysis.

 

            The comments provided are based on HESL using LOGMAP, a proprietary well log analysis and mapping software package.  Other software packages can provide help with some of the same tasks mentioned below but may not be as well integrated for multi-well project work as LOGMAP.

 

Project Definition

In consultation with the client a project area is defined and the scope of the analysis expected is agreed upon based on known available data.  Preparing a list of the wells to be analyzed is the next step.  Then representative base maps are ordered or downloaded for the project area. 

Data Collection

All available LAS, raster, core analysis and relevant geological and reservoir data for the wells in the project area are collected.  Any required data that is missing or deficient is purchased or retrieved from commercial data sources if available.  Well location data is checked against a master list to confirm accurate well locations.

Print Well Logs

It is always handy to have paper copies or raster images for reference.  Now, with most of the well log data available in raster format, it can be an easy task to refer to the raster images for any questions related to the digital well log data without specifically having a specific hard copy for reference.

Headers

Build well header forms, collect and enter the required data.  Header forms have all well information, location, latitude and longitude as well as what curves are digitized, the types and properties of the drilling mud used when the well logs were generated.  General information as to the well logging company and vintage of the data is built into the header as well.  Available DST (drill stem test) and production information specific to each well location is added into the analysis database at this time as well.

Digitization

For wells without existing digital data, the data will have to be digitized.  If the data has to be digitized, a decision has to be made as to whether the digitization will be done by hand or by getting a commercial digitizing company to generate the digital data.  Hand digitizing is viable for single zone projects with less than 30 wells to analyze.  For multi-well, multi-zone projects, HESL recommends getting a commercial digitizing company involved.

 

For hand digitizing, the well log and core analysis data will be retrieved from microfiche, paper copies of the data or prints of the raster or TIF images of the data where available. 

 

For projects greater than 30 wells, use commercial digitizing services as they can provide reasonable turnarounds on the digitization.  They also generally digitize the available well log data from surface to total depth.  So that, even if only a couple of zones are of immediate interest, the digits will have been captured over the total well interval, ready future use (highly recommended). 

 

With hand digitizing, what generally happens is that only enough of the stratigraphic interval required for normalization procedures and analysis is digitized. So, if other intervals of interest surface later, then more time must be spent digitizing those intervals.

 

The added benefit of using the commercial digitizing companies is that the client does not have to copy any logs from microfiche or raster images and that the commercial digitizing companies do all the set up related to digitizing.  Plus, the client gets CDs or DVDs with files stored in LAS format, which are easy to handle within LOGMAP, or for that matter any well log analysis software. 

 

The digitizing aspect of the project can be a formidable and costly task.  Fortunately the digitization program within LOGMAP is very easy to use and can provide quality digits in a reasonable time frame especially for small numbers of wells (<50) with small intervals of interest (less than 200' of well).

 

Word of warning, even if commercial data services are used, check plot all the data they provide against existing paper copies of the well logs or prints of the raster images and make sure the proper digital data has been received.  Often there can be mis-scaling of the data due to misreading of the well log data headers or capture of data from multiple well log runs where the data has not been ‘stitched’ together properly.

 

It will always be of great advantage if all the data from all wells have the same digital increment.  The choice of digital increment can vary depending upon the type of project but a good rule of thumb would be to use a 0.1 m or 0.25 foot digital increment, which gives good depth resolution and allows for reasonable data storage and computational time.

 

Always, always ensure that the digital well log data is depth correlated prior to proceeding with analysis.

Stratigraphic Correlation

Input any supplied stratigraphic tops data.  Confirm major tops picks and quality control as necessary.

Cross Sections and Strip Logs

Plot well log data in strip log form or as cross sections for correlation purposes.  This process is used to determine the overall framework correlation as well as define stratigraphic units that will be used in the normalization process. 

 

That is, choose a stratigraphic interval or intervals that display a good range of well log responses related to all of the depositional intervals penetrated by the well bores within the target horizons of the project area.

Normalization or Optimization

With regard to multi-well projects, digital well log data from varying well logging companies will always be encountered.  The data will be of varying vintages.  Some of the data will have been captured from well bores that were in poor condition whereby the data capture has suffered and in some wells the well log data will be mis-calibrated or missing altogether.

 

Normalization or optimization of the data to accentuate the geologic responses while minimizing the effects of well log vintage, bore hole conditions, well logging company and well log calibration must be done prior to any serious attempt at analysis.  For instance, our experience in the Western Canada Sedimentary Basin suggests that in any digital well log project over 50 wells, the gamma ray log will be miscalibrated in at least 15 of the wells.

 

With regard to multi-well digital well log analysis projects proceed at your peril if you do not plan to normalize your data set.  From experience in dealing with digital well log data from multiple wells, almost all interpretation busts or failures, aside from using the wrong interpretation procedures, can be linked back to poorly calibrated data and not doing a proper and consistent normalization prior to analysis.

 

The normalization process is iterative.  Basically, appropriate statistical measures and crossplots of various combinations of the well data are generated on a single well and multiple well basis for selected stratigraphic intervals.  On a well by well based these measures and cross plots are compared and interpreted with appropriate adjustments made to well log curves to establish consistency in the well log data.  These procedures are iterative and use cross sections, contoured frequency cross plots and mapping to confirm consistency.

 

The normalization processes and techniques within LOGMAP are one of the major strengths of the program suite.  All of these techniques and processes are discussed more fully with several sets of examples for reference in the normalization module.

 

Develop Preliminary Analysis Parameters

Once the digital well log data has been normalized a first pass computer assisted well log analysis is generated for each stratigraphic interval targeted for analysis.  Crossplots of different combinations of well log data are generated for each stratigraphic interval. 

 

Based on interpretations of the cross plots, the analyst determines various end point parameters required for interpretations of the basic reservoir parameters such as, porosity, water saturation and volume of shale. Using these parameters, a first pass analysis of the well log data is generated.

 

At this point, depending upon the complexity of the project and if core analysis data is available the core data is brought in to compare with the first pass analysis.

Core Data

Collect all core analyses data, enter data into computer, plot and depth correlate to well logs.  Merge core data into main digital well log database.  As core data is provided in unequal increments, merging core data with log data requires two steps. The first step is to get the uneven increment core data depth correlated to the well log data.  The second step is to resample the uneven increment core data to match the sample increment of the digital well log database. These process stages are discussed more fully in the core analysis module.

 

Generally, HESL would like to depth correlate the core data to the well log data using the core gamma ray to compare to the well log gamma ray.  In HESL’s experience as consultants, HESL is very rarely provided with the core gamma data so HESL has found that it is easier to depth correlate core data to the first pass analysis data such as the analysis porosity data rather than to raw well log data.

Core to Log Correlation

This highly iterative process compares core analyses versus calculated analyses through cross plots, rationalizes any anomalous results and corrects analysis parameters where justified to finalize the well analyses.

 

In addition, relationships between the core permeability and the well log analysis are determined.

Calculate Well Log Analyses

Based on core to log core comparisons HESL, adjusts the appropriate well log analysis end point parameters to compare favorably with the core analysis data.  This is not a forced fit of the data; the well log analysis parameters selected will be based on rationalized experience based on previous projects of a similar nature. 

 

As a first step, this comparison will be between core porosity and well log analysis porosity.  With the well log analysis porosity supported or validated by the core porosity data, the parameters necessary to calculate water saturation must be developed. 

 

In most cases, this involves the use of a Pickett plot, which is a plot of deep reading resistivity vs. porosity (more full explained in the analysis module).  This is done separately for each of the target stratigraphic intervals.

 

Once the parameters for calculating water saturation are determined, a finalized well log analysis can be generated.  Pseudo-permeability is also generated based on the core to well log relationships.

 

Detailed Geologic Correlation

With the analyses more-ore-less finalized, time must be taken to review the original geological tops correlations and verify that they have been picked consistently and correct the tops as necessary. 

 

Major changes in tops correlations at this stage in the analysis process would require re-analysis of all stratigraphic intervals above and below the tops change.

Report

Generate a formal written report of the analysis procedures and results.  Included in the report will be relevant net pay calculations, analysis summary plots, cross sections and maps specific to the project.

Net Pay Calculations

With the geologic tops finalized and the analysis finalized, the stratigraphic intervals used to determine net pay can be determined.  Standard net pay calculations for each target stratigraphic horizon are generally based on the analysis of a cross plot of analysis porosity versus analysis water saturation but can include the use of other cross plot combinations. 

 

From inspection of the cross plot for a given stratigraphic interval, the threshold porosity over which production of hydrocarbons would be expected is determined.  Next, the threshold water saturation below which hydrocarbon production would be expected is determined.

 

Using these values as cutoff parameters, net pay can be determined for each stratigraphic interval for each well in the project.  That is, for stratigraphic interval A in well number 5, when the porosity is greater than 10% and the water saturation is less than 50%, the net interval thickness is 5 m. (16.4 ft.) with an average porosity of 20%, an average water saturation of 34% and an average permeability of 20 mD.

 

Calculate and Contour Maps

Calculate various map sets and prepare plot files for posting and rough computer generated contouring.  A wide variety of maps are possible based on the well log analysis or derivatives of it.  Most popular are facies flag maps where by well log data is plotted to scale at the well location and then other data is shaded relative to the log data to show various information.

 

Dependent upon the scope of the project, if mapping is required then HESL always favors hand contouring all geologic maps, such as structure and isopach maps.  The hand contoured maps can be based on computer contoured maps but ultimately the input of a qualified individual is required to check results against other maps to confirm consistency and to provide a proper interpretation of the data presented.

 

Again, depending upon the scope of the project, the resultant hand contoured maps will need to be digitized.  The digitized data will then need to be sent to a data display package such as SURFER and dressed up for final output.

Calculate and Plot Analysis Summary Plots and Cross Sections

HESL generally provides the client with a customized analysis summary plot for each well.  In addition, depending on the scope of the project, representative cross sections are prepared and plotted at this time.

Prepare Report Figures

Prepare and plot appropriate display figures to be included in report.

Prepare for Final Delivery

Make the number of copies of each map set, cross plot and analysis summary plot as required.  Photo copy and bind report. Check, sort and package for delivery.  Write up transmittal forms and ship project results to client with final billing invoice.

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hitchner@nucleus.com