Henderson Petrophysics

Outline for a Typical Petrophysics Field Study

Introduction

The needs of clients differ for each Petrophysics field study but most projects follow the general outline given below.

The Petrophysics field study is often part of a larger reservoir description project such as a reservoir simulation model or a geostatistical study. All of the project technical team members must be kept fully informed of the progress of the Petrophysical analysis. Any reservoir anomalies, including heterogeneity features, must be promply discussed and documented.

The reservoir surveillance activities will likely out-live the Petrophysics Field Study, so it is vital that the final study report and all material used in the study be carefully archived.

Suggested Outline for a Petrophysics Field Study

(1) Review previous studies.

(2) Conduct a literature search for work done on analagous fields

(3) Data collection phase;

Digital wireline log data,
Wireline log paper prints,
Mud logs including digital data if available,
Cuttings sample descriptions,
Sidewall core descriptions,
Core descriptions,
Core analysis results including any SCAL reports,
Petrographic reports,
Well test results,
Formation pressure data (MDT, RFT, DST etc.)
Fluid analysis (gas, oil, water) reports,
Geological correlation sections,
Depositional environment reports,
Well survey data.

(4) Database development:

Data entry;

Digital wireline log data from a service company, log vendor or client,
Digitized log data: either as the original data or as supplemental or remedial data,
Log header information - usually entered manually,
Core analysis results - usually entered manually,
Formation pressure data,
Well survey data. Be aware of order of validity (single-shot, multi-shot,Gyro)

Verify the validity of digital log data with hard copies of the original data.

Identify and document fluid contacts in all wells. Use Mud log data, well test results, formation pressure data, cores, mudlogs and well log response. Create color-coded "stick" plots (Subsea depth) to show the distribution of fluids in all reservoirs.

(5) Edit the log data;

Splice logs,
Normalize log curves, if necessary, after examination of regional trends,
Baseline SP curves,
Delete or edit invalid log data intervals (for example cycle-skipped sonic).

(6) Create hole size and "washout" curves.

(7) Create "bad hole" logic to identify zones with invalid log data,

(8) Create a set of TVD and/or subsea log curves for each well,

(9) Apply environmental corrections to log data, where appropriate.

(10) Develop and test an interpretation model using the available data (logs, core analysis, petrography, etc). The model will typically include;

A method to predict formation temperature,
Formation water resistivity (Rw) curves calculated using salinity data and formation temperature,
Shale fraction determination (often using multiple logs),
Shale properties algorithms (Qv, Rhoma, Rwb, etc.),
Reservoir parameters (matrix and fluid density, a, m, n, etc.),
Porosity algorithms. Porosity is often calculated using several logs and logic is developed to select the most appropriate value,
Water saturation models, and,
Permeability prediction algorithms.

(11) Working with other members of the study team develop facies identification models, if possible.

(12) Develop permeability transforms for each depositional facies, if possible. Calculate permeability-thickness for each well/zone.

(13) Compare the log interpretation results with available core data.

(14) Refine the interpretation with zoned analysis parameters. It may be necessary to use more than one interpretation model.

(15) Review the analysis model and reservoir zonation parameters with the project technical team (Geologists, Geophysicists, Reservoir engineers).

(16) Apply the refined interpretation model to all of the wells using the zoned analysis parameters.

(17) Review all analysis results prior to writing a final written report.

(18) Prepare a summary report for the Petrophysics field study, containing;

An executive summary of the project and the analysis results,
A detailed description of the techniques applied,
Implications of the results of all computations
Analysis results in tabular and graphical forms.
Petrophysics composite logs ("Answer" plots) with;

Raw logs, core analysis data, interpreted lithology and,
Log analysis results curves.

(19) Prepare presentation materials;

Slides or overhead projections,
Posters,
Cross sections showing petrophysical properties.

(20) Archive all hard copy and digital data. A very useful technique is to write a CD containing the project directory structure.  The directory structure should contain the raw data, results curves, interpretation programs, report files and plot files.

Notes: Organize your report with general material at the front and increasingly more detailed material towards the back. Create separate appendices for the analysis model description, and results summary tables. My recommendation is to use "net" reservoir cutoff parameters with extreme caution. Using combinations of "effective porosity", shale fraction or water saturation cutoff parameters is rarely appropriate in shaly and/or thinly bedded reservoirs and will result in a very pessimistic reservoir evaluation. Often, an interval with very high calculated shale fraction and water saturation values will be consist of thinly laminated, good quality reservoirs in a dominantly shaly section. Another common example is where the limited vertical resolution of the logs makes "clean" reservoirs with sharp boundaries with adjacent shales appear to be "shaly" close to the reservoir boundaries. Never try to correlate water saturation values with DST results. A well can have high flow rates from very thin, high permeability zones. The high "average" Sw values calculated from the logs is likely to be correct. A good example of this is very high flow rates from a single fracture, even though the host rock contains little or no hydrocarbons.

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