Water Saturation SENSITIVITY ANALYSIS
Water saturation varies with many parameters, such as shale volume, porosity, rock resistivity, water resistivity, and the electrical properties of the rock. Many arguments occur because one analyst has used slightly different parameters or method than another. Most of these arguments are futile and needless as the end result, namely oil-in-place, is often relatively insensitive to the water resistivity and electrical properties, but is usually very sensitive to the shale volume and porosity. A reasonable sensitivity analysis can be run in an hour or so with a spreadsheet or computer aided log analysis program.

Even more sophisticated Monte Carlo statistical modeling can be performed in a spreadsheet. But remember, the final test is the sensitivity of hydrocarbon volume, not water saturation. A 50% error in SW, when SW = 20%, is pretty trivial in terms of oil volume, when you consider all the other parameters. A 50% error in SW, when SW = 60%, is a little more critical - now the possibility of zero hydrocarbon volume is quite real and some calibration with ground truth is appropriate.

Below are three such studies based on reasonable assumptions from three different reservoir types. Note that the comparison is based on hydrocarbon pore volume (HPV) and not on water saturation (SW).

 SHALLOW TAR SAND - SENSITIVITY ANALYSIS Hydrocarbon Pore Volume A M N RW@25C RSH Vsh PHIe Change From Pore Volume RESD=20 RESD=80 RESD=180 BASE 1.00 2.00 2.00 0.75 4.00 0.00 0.35 0% 0% 0% 2 1.00 2.00 2.00 1.00 4.00 0.00 0.35 -26% -7% -4% 3 1.00 2.00 2.00 0.50 4.00 0.00 0.35 31% 8% 5% 4 1.00 2.00 1.80 0.75 4.00 0.00 0.35 8% 6% 4% 5 0.62 2.15 2.00 0.75 4.00 0.00 0.35 25% 7% 4% 6 1.00 2.00 2.00 0.75 4.00 0.13 0.31 10% 1% -2% 7 1.00 2.00 2.00 0.75 8.00 0.13 0.31 -4% -6% -6% 8 1.00 2.00 2.00 0.75 3.00 0.13 0.31 27% 8% 3% 9 1.00 2.00 2.00 0.75 4.00 0.26 0.27 6% -10% -14% 10 1.00 2.00 2.00 0.75 8.00 0.26 0.27 -17% -18% -19% 11 1.00 2.00 2.00 0.75 3.00 0.26 0.27 18% -6% -13% TAR MASS M N RW@25C RSH Vsh PHIe Change From Base Case RESD=20 RESD=80 RESD=180 BASE 1.00 2.00 2.00 0.75 4.00 0.00 0.35 0% 0% 0% 2 1.00 2.00 2.00 1.00 4.00 0.00 0.35 -25% -6% -4% 3 1.00 2.00 2.00 0.50 4.00 0.00 0.35 28% 7% 4% 4 1.00 2.00 1.80 0.75 4.00 0.00 0.35 8% 5% 4% 5 0.62 2.15 2.00 0.75 4.00 0.00 0.35 23% 6% 3% 6 1.00 2.00 2.00 0.75 4.00 0.13 0.31 6% -3% -5% 7 1.00 2.00 2.00 0.75 8.00 0.13 0.31 -7% -8% -9% 8 1.00 2.00 2.00 0.75 3.00 0.13 0.31 21% 3% -1% 9 1.00 2.00 2.00 0.75 4.00 0.26 0.27 -3% -16% -19% 10 1.00 2.00 2.00 0.75 8.00 0.26 0.27 -23% -23% -23% 11 1.00 2.00 2.00 0.75 3.00 0.26 0.27 7% -13% -18%

The sensitivity study shown above indicates that RW has a strong effect when formation resistivity is low, but much less effect at high formation resistivity. You should only argue about water resistivity when it matters. Lowering M or N always gains some oil or gas, regardless of rock resistivity. Changing A and M matters a lot at low rock resistivity but not much is gained at high resistivity. Adding shale almost always lowers porosity so HPV usually drops unless RSH is very low AND RESD is also low, in which case HPV can rise dramatically over the base case with no shale.

Although most of these effects are fairly intuitive, the absolute value of the change is not easy to predict unless a numerical sensitivity study is run. If you are forced to argue for one set of parameters over another, always run the sensitivity to see what matters and what doesn’t.

 MEDIUM DEPTH - SENSITIVITY ANALYSIS Hydrocarbon Pore Volume A M N RW@25C RSH Vsh PHIe Change From Pore Volume RESD=20 RESD=80 RESD=180 BASE 1.00 2.00 2.00 0.27 4.00 0.00 0.250 0% 0% 0% 2 1.00 2.00 2.00 0.36 4.00 0.00 0.250 -9% -3% -2% 3 1.00 2.00 2.00 0.18 4.00 0.00 0.250 11% 4% 3% 4 1.00 2.00 1.80 0.27 4.00 0.00 0.250 6% 4% 3% 5 0.62 2.15 2.00 0.27 4.00 0.00 0.250 7% 3% 2% 6 1.00 2.00 2.00 0.27 4.00 0.13 0.225 -21% -18% -18% 7 1.00 2.00 2.00 0.27 8.00 0.13 0.225 -25% -21% -20% 8 1.00 2.00 2.00 0.27 3.00 0.13 0.225 -19% -17% -17% 9 1.00 2.00 2.00 0.27 4.00 0.26 0.200 -42% -38% -37% 10 1.00 2.00 2.00 0.27 8.00 0.26 0.200 -49% -41% -40% 11 1.00 2.00 2.00 0.27 3.00 0.26 0.200 -38% -36% -36%

In the medium depth cases where porosity and RW are lower, changing the RW or the electrical properties has very little effect unless RESD is low. In the shaly cases, RSH has only a small effect, even when RESD is relatively low. The lower porosity due to shaliness has the largest effect, so optimizing shale parameters is important.

 DEEP RESERVOIR - SENSITIVITY ANALYSIS Hydrocarbon Pore Volume A M N RW@25C RSH Vsh PHIe Change From Pore Volume RESD=20 RESD=80 RESD=180 BASE 1.00 2.00 2.00 0.03 4.00 0.00 0.150 0% 0% 0% 2 1.00 2.00 2.00 0.04 4.00 0.00 0.150 -3% -2% -1% 3 1.00 2.00 2.00 0.02 4.00 0.00 0.150 4% 2% 1% 4 1.00 2.00 1.80 0.03 4.00 0.00 0.150 4% 2% 2% 5 0.62 2.15 2.00 0.03 4.00 0.00 0.150 2% 1% 1% 6 1.00 2.00 2.00 0.03 4.00 0.13 0.115 -24% -22% -22% 7 1.00 2.00 2.00 0.03 8.00 0.13 0.115 -25% -23% -22% 8 1.00 2.00 2.00 0.03 3.00 0.13 0.115 -24% -22% -22% 9 1.00 2.00 2.00 0.03 4.00 0.26 0.083 -48% -44% -44% 10 1.00 2.00 2.00 0.03 8.00 0.26 0.083 -49% -45% -44% 11 1.00 2.00 2.00 0.03 3.00 0.26 0.083 -47% -44% -43%

In the deeper, lower porosity, lower RW case, changing RW or electrical properties is noise level stuff. Likewise, changing RSH in the shaly cases has no significant effect. Everything depends on porosity and a sensitivity study on porosity parameters would be a useful tool.

Clearly, larger changes in parameters are possible, and additive effects must be considered. Run the sensitivity with a realistic range of values and then use good judgment. Don’t argue unless it really matters and you have the backup data to support your position.

You will need good water analysis data and useful special core analysis electrical properties data to bound your study. However, remember that special core studies are not perfect and some are useless because they do not reflect all the rock types present in the reservoir.

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