Editor’s comment: This article is the first in a series of short “tutorial-like” notes styled to mentor users of digital well log data in becoming confident practitioners of Petrophysics.
This first tutorial is aimed at those analysts of digital well logs who are lulled by slick pictures of computed answer products into believing the task is easy and quickly done. One is led to believe that all one needs to do is to buy a seat on a fancy digital well-log application, take a short course in what all the buttons do, then crank out porosity, water saturation and net-pay values that are correct and verifiable. (Pardon me while I chuckle!) One of my mentors (Lou McPherson) often repeated the phrase, “all logs are damned lies,” and of course he is correct. No wireline log recorded at the wellsite actually reads the true reservoir property printed on the well-log heading (or file name of a digitally recorded well log). So, I hope to impress upon the reader that there is much to be done before using recorded data “as is” and much to be understood about the algorithms and their limitations in the fancy black box. It is far from being an “easy” task and manifestly is never a “quick” one. The well logs are but one of at least five data streams that one must analyze and synergize with well logs to hope to approach an acceptable result for a reservoir property within a 12-in. radius around the borehole; those other data streams that must be analyzed and simultaneously considered with wireline well logs are (1) mud logs, (2) cores and core analyses (looking at the rocks is so very important), (3) possibly logging-while-drilling (LWD) logs, (4) wireline formation test (WFT) data, and perhaps (5) drillstem test (DST) observations. Not all wells will have a full complement of data streams, but the missing data must be considered when placing error limits on any calculated values and predictions. In addition to the aforementioned difficulties, and before using a digital well-log processing (DWLP) program (commercial or proprietary), one needs to consider some often overlooked yet critical “points or steps or truisms.”
