You work on interpreting the information from a well in the Well Log View, which displays the drilling logs and wireline logs from the well, along with three tracks for recording your interpretation. The first two tracks are the drilling rate and drill cuttings logs. These are followed by the three wireline logs, gamma ray (API units), density (g/cc), and resistivity (Ohm-m). The next three tracks are where you enter your interpretation of the lithology, pick interval tops, and specify where to perforate the well upon completion:
(Note that the resistivity scale is not displayed properly in these figures.)
You will find that the top of the Coronado has already been picked, based on its very distinctive log signature, and the lithology track above it has already been filled in with an alternating sequence of sandstone and shale. Similarly, the top of the Precambrian has already been picked and the lithology (all granite) filled in below it. Your job is to interpret the entire sequence in between.
While you are working, you may want to click on Legend to pop up a key to various elements on the screen, including the icons representing different lithologies and oil staining in the drill-cuttings (mud) log:
You may also want to click on Well Prognosis to re-display the information on expected formation depths.
To fill in the lithology track, select a lithology from the Lithology menu at the upper right:
and then drag over the appropriate depths in the lithology track:
Note that the heading for the Lithology menu has changed to reflect the currently selected lithology. We have picked the anhydrites here based on the drill-cuttings log and the characteristic kicks on the density and resistivity logs. You can "overwrite" previously specified lithologies simply by dragging over them with a differnt lithology.
To pick an interval top, select the desired interval from the Interval menu:
And then drag in the Interval Tops track to place the top at the desired location:
Note that the list of tops in the Intervals menu is in the order that you should encounter them.
To specify that an interval should be perforated for production once the well is completed, simply click and drag in the perforation track, on the far right. Here we have perforated an interval in the Quivira Dolomite based primarily on the oil staining in the cuttings log:
To "erase" any perforations that you have entered, hold down the Control key, then click and drag over those intervals in the perforation track.
(Note that one would also use the resistivity log, which is not displaying properly here, as a source of information on oil saturation.)
Once you have worked through all the days of drilling, filling in the lithology track and picking all the tops (except possibly the Morn and Sawyer Sandstones, which may be absent from the sequence), the Finished Well Interpretation button becomes activated. At this point you may continue to revise your interpretation or click on Finished Well Interpretation to indicate that you are done. Once you do that, you will be presented with a score card for your interpretation of the well. This contains three sets of information in a scrolled window, first a comparison of your top picks with the true tops, followed by the percentage of your interpreted lithology that is correct (compared to the true lithology:
and then a comparison of your selected perforations to the "true" pay intervals:
In all cases, your interpreted results are compared to the true values extracted from the simulation of the Small County subsurface. An interval is determined to be true pay if it has a porosity greater than 8%, a water saturation less than 50%, and a bulk volume water (BVW) less than 6%. BVW is the product of porosity and water saturation and represents the proportion of the total volume occupied by water. In this example, our major errors were missing the top of the Possum Trot Shale by 173 feet, perforating 278 of non-pay -- meaning we will produce a significant quantity of water -- and missing 26 feet of pay.
You may print the score card by clicking Print.... When you click Close, you will return to the County Overview window to determine where to site your next well. The (true) top elevation information from your newly completed well will be added to the well data set, meaning any contour maps you generate will reflect the new information.