Cretaceous Shaly Sand
There is no density neutron cross over in the clean sand, so this zone is oil bearing. We cannot tell about the upper shaly sand because the shale effect masks any possible gas effect. After shale corrections, the density and neutron still do not cross over, so oil is most likely.
The water zone at the base of the clean sand provides water resistivity information for use throughout the rest of the zone. Core data was available to calibrate porosity and permeability results. The answer plot shows the results of the lithology, porosity, and hydrocarbon analysis.
The raw data plot shows two interesting features: the flat SP compared to GR in tight zones and the SP excess at 3400 feet, indicating better permeability than the rest of the shaly sand. The lithology track on the answer plot shows this interval to be more sandy and less limey than the rest of the shaly sand.
Raw logs for Shaly Sand Example
The following crossplots were made:
Porosity vs Resistivity - shows
water saturation lines (shale data falls below 100% Sw line).
Porosity vs Saturation - shows
constant water volume lines. Data streaming above and to the
right indicate transition and water zones. Shale data falls to
the bottom of the graph.
Density vs Neutron - shows all data
below limestone line, indicating either no perfectly clean sand
or mixed lithology sand (GR suggests clean sand). Shale data
falls towards bottom and right.
4. Core porosity vs core permeability - shows a data cluster which cannot be used to derive a regression line mathematically. A line drawn thru the lower left corner will work fine.
Basic crossplots for Shaly Sand Example - Part 1
5. Matrix density vs matrix cross section - confirms that sand is not pure quartz, but the plot does not tell us which minerals to expect. Sample description suggests quartz, calcite, and glauconite (plots past anhydrite at top right).
Apparent water resistivity vs
density porosity - similar to above but uses effective porosity.
Shale plots near origin, water zone at top left, oil at right.
Apparent water resistivity vs gamma
ray - shows where to pick GR0 and GR100 (also can be picked from
raw logs). Best oil zone is off scale to the right.
Basic crossplots for Shaly Sand Example - Part 2
Just to illustrate that you don't need a $5000 to $75000 log analysis package to do good work, all the calculations and crossplots shown here were made with a Lotus 1-2-3 spreadsheet program, called META/LOG, written by the author, and available for a mere $50. The depth plot shown below was made with a $50.00 shareware plot utility called LAS/PLOT. This presentation is the bare minimum that would be given; more complete plots are shown in the next two case histories. Most log analysis packages can make similar or more elaborate plots.
Basic depth plot for Shaly Sand Example
Reports and data listings are an essential part of log analysis. The next illustration shows the answer report prepared automatically by META/LOG after the analyst has finalized the job. The hydrocarbon summary page shows a comparison with core. The match between porosity and permeability are extremely good, as they should be.
Basic answers and raw data listings for Shaly Sand Example
A scan of the Rwa column shows the RW @ FT in the water zone to be 0.17 ohm-m. Reserves and productivity are useful by-products of this analysis. For example, estimated productivity for the upper shaly sand is only 0.6 barrels per day compared to 231 for the clean sand. The shaly sand would be uneconomic anywhere, but an exploration play may be developed to find cleaner sands nearby.
The META/LOG cash flow analysis for this well was given earlier in Section 11.02.
Summary log and core data listings for Shaly Sand Example
When you analyze the logs on any well, or group of wells, you must learn all there is to know about the wells or offset wells before you start the job. You also must check your work against ground truth before you finish the job. Below is a copy of the well history printout for this case history. Formation tops, cored and perforated intervals, and test information are the first clues that help narrow the zones of interest.
CORE ANALYSIS FOR 02-18-18-15W4
Core data listing for Shaly Sand Example (but see next illustration)
The average porosity is 25.3% and average permeability is 624 md. The porosity is higher than the log analysis shown earlier and the log results could be made to match the core by reducing shale volume or shifting the density porosity to a higher value. This would be an arbitrary calibration shift as there is no evidence that the log is mis-calibrated. Some one may have noticed this problem at some stage because a second core analysis listing is available, dated several years after the first one. The re-analysis is shown below.
REVISED CORE ANALYSIS FOR 02-18-18-15W4
Re-analyzed core data listings for Shaly Sand Example
Notice that the average porosity is 22.7% instead of 25.3%, much closer to the original log analysis. Permeability has changed only slightly from the first core analysis. The moral of the story is that core analysis is not perfect and some errors should be expected. Checking log analysis in several cored wells is the only way to find the odd bad core or bad log.
Core data crossplots for Shaly Sand Example
The crossplots of the original and revised core analysis data are shown above. The revised analysis gives considerably higher permeability for any given porosity.
Production history for Shaly Sand Example
Production history shows excessive drawdown in
Year 4 probably hastened water breakthrough in Year 5 which
remained constant while oil declined gracefully to economic
limit. Initial production rate normalized for hours of operation
was nearly 100 bbl/day. Cumulative oil was 187 000 bbl (29 700
m3) and water was 1.2 million barrels (190 000 m3). The log
analysis predictions were quite optimistic (300+ bbl/day and 700
000+ bbl recoverable). This knowledge would allow us to adjust
parameters on future wells to obtain better agreement.
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