WHAT YOU NEED TO KNOW BEFORE YOU START

This page covers a brief overview of computer aided log evaluation, sometimes erroneously called computer processes interpretation software (CPI). Computer software can analyze, process, evaluate, get answers – but it cannot interpret. That’s your job!

My original version of this webpage was written in 1983-84 as part of an AI project, and times have changed a bit since then. So this is a totally new version for 2021 and beyond. Forty years agp, there were over 50 software packages named in my original survey. None of the tradenames still exist. All have either disappeared or been absorbed by new corporate ventures. A few have evolved to keep up with current computer capabilities, gaining new tradenames along the way. You may find results from some of these gems in your well files.

Today there are fewer than a dozen commercially viable petrophysical software packages. I don’t intend to review or recommend any of them. Test drive a few, talk to other users, and above all check how responsive the support team is.

Most come suppkied wirh basic deterministic analysis algorithms and some have advanced functions like multi-mineral, neural network, or statistica / probabilistic models. A user-defined-equqtion (UDE) editor is absolutely essential. No system has all the math you will need for the vast variety of special cases you will run across in your career. Virtually all the algorithms you may need to add can be found in this Handbook.

None of the systems can choose the “Best” analysis model for a particular geological sequence, or pick any of the parameters needed. That’s your main task as a petrophysicist. To do this effectively, you need to know how the math works. Take a course or check out Chapters 11 through 17 in this Handbook.

You also get to check results against ground truth and re-compute until all available data is reconciled.

Plot results in a uniform manner and include core porosity and permeability on top of the log analysis results to show how well your results match ground truth. Be sure to annotate tops, tests, perfs, cores, and any other helpful data.

HOW IT WORKS
This section shows how the comouter aidded log analysis system works and how it integrates with other disciplines in the organization.

The computer-aided petrophysical system

The interconnecting links in the system are its most important feature. Good communication must exist, along with mutual trust and understanding, between the "user" (the engineer, geologist or geophysicist) and the "doer" (the petrophysicist). The analyst in turn, must effectively communicate with the computer hardware-software package and staff.

A good system must be built around a team concept, consisting of the lead or senior petrophysicist, a junior or trainee analyst, up to two technicians, and possibly a clerk/technologist. Some of these people are shared or "float" to projects as needed.

The senior analyst is responsible for project definition, parameter and method selection, difficult editing, work scheduling and organization, review of intermediate and final results, presentation and discussion of final results with the end-user, and training and work allocation of subordinates. He must have a thorough knowledge of log analysis methods, and be aware of all the available features on the hardware / software package. He can run the package effectively after a few days exposure to it and can modify programs to suit special cases or local requirements.

The more junior members of the team run the package under the direction of the analyst. and perform the many clerical tasks involved in organizing and filing large volumes of data. These people must be keen and adept in the use of computers.

Log analysis should be performed on a definable zone - not on an entire well at once. As many zones as needed are run to cover all potential pay sections.

The entire well may be analyzed, but as a series of discrete zones. A run control sheet is used to describe the zones to analyze, the data available, the computation method, and parameters required, as well as a brief well history to aid the analyst. The well history is also annotated on the final results to aid discussion and understanding of the log analysis by others.

On large projects, a group of 5 to 10 related zones, preferably cored and tested, will be picked, digitized, and computed as a "batch". These are reviewed, parameters adjusted as needed, recomputed, reviewed again and eventually finalized. In the earlier stages of a large project, the batches consist of those zones with the most core and DST data available. These zones are used to calibrate log analysis parameters before un-cored zones are analyzed. The organization of this procedure and the data bases required are illustrated in the block diagram below.

Data flowchart for computer-aided log analysis system

These stages may seem simple, even trivial or obvious, but clear definitions benefit the end-user and the analytical team, not to mention management, who may have no idea how petrophysics is really done. Large projects or continuous, on-going projects slow down if the job stream or data structure is unorganized or chaotic.

The two feedback loops shown above indicate that successive re-runs to optimize methods or parameters are easy, rapid, normal, and probably necessary.

This is the key to satisfying both the technician and the professional analyst, because individual zones are usually finished completely in just a few elapsed hours instead of days or weeks. A reasonable number of zones (5-20) may be interleaved, so that different functions are performed on different zones. This is a natural outcome of the variable number of times the zone has to be re-computed.

In smaller organizations, the analysis team may be one person, and in some instances, the team and the end-user may be the same person. This does not change the need to organize and review data and results.

Other organizations use a dispersed or distributed systems approach, in which the end-user, or their technical staff, do their own log analysis. This may be successful if training and standards are excellent, and specialists are available for certain jobs and for training.


SOME EXAMPLES

Example 1: Tight Oil - Silt/Sand

Here is a different well with the pyrite correction applied to the resistivity log. The before and after versions of the resistivity are shown in Track 2, along with the pyrite fraction determined from a 3-mineral model using PE-density-neutron logs. The correction raises the resistivity about 0.5 ohm-m and reduces water saturation by about 10%. Making the pyrite more conductive would raise RESD further, but as yet no one has provided any public capillary pressure data in this area to calibrate SW. The SWir from an NMR log would also help calibrate this problem.

 

Example 2: GAS Shale - dolomitic sand/silt

This example is the same well as the first image in this series, showing results based on a fixed matrix density and matrix sonic travel time, used to obtain a good match to core in the cored interval. Both porosities are shown (blue is sonic, left edge of red shading is density). The kerogen correction is buried in the false matrix values required to get the results to match the core data. There is nothing criminally wrong with this approach when mineralogy and TOC are roughly constant, but that is not the case here. TOC weight percent varies from 1 to 3%, which translates into 2 to 7% by volume. Clay, quartz, and dolomite volumes also have large ranges.

EXAMPLE 3: Oil SAND - unususal fluid distribution



Oil sand analysis with top water, bottom water, top gas, and mid zone gas. Core and log data match - but oil mass is the critical measure of success. Core porosity matches total porosity from logs, due to the nature of the summation of fluids method used in these unconsolidated sands. Minor coal streaks occur in this particular area.