Statistically based water saturations give an interpretation of the apparent fluid content of a formation independent of Rw, ‘m’ and volume of shale. An apparent total water saturation is derived as well as an apparent irreducible water saturation. This is a very powerful analysis tool but requires a good statistical sample of both wet and hydrocarbon bearing intervals to be most successful. This analysis is not generally attempted on data sets with less than 20 wells or vertical thickness less 5m. (15 feet).
With reference to the diagram above, the top left diagram shows a rock that has some pore geometry. Within the pores, there exists some bound water, some free water and some hydrocarbons. with reference to the top right diagram, this pore geometry can be filled totally with water, totally with hydrocarbons or some mixture of water and hydrocarbons. As shown the resistivity log will vary dependent upon the situation and as a result the calculated water saturation will vary.
The lower left diagram reminds us that any given reservoir has differing pore geometries and that from statistical observations from multiple wells certain resistivity profiles are observed. The lower right diagram shows us that if we can statistically observe the resistivity spectrum of a particular pore geometry, it may be possible to observe the resistivity behaviour of that pore geometry in water bearing zones as well as in hydrocarbon beraing zones.
The crossplot above is a contour crossplot of normalized density porosity vs. normalized gamma ray. This crossplot summarizes the density-gamma response for 900 wells for the Lloydminster formation, a heavy oil formation in Alberta. All depositional environments and fluid conditions traversed by each of the 900 wellbores are represented on this crossplot.
Overlaid on this crossplot are a series of cells or polygons that travel along the trend of the contoured distribution. Each one of these cells by its position on the trend and relative to the physics of the well log measurements crossplotted isolate different rock types or characteristics.
The illustration above shows the observed deep reading resistivity spectrum for each of the cells. Based on the interpretation of these spectra developed over the entire contour crossplot distrbution, apparent total, free and irreducible water saturation can be deduced. Generally. two hundred to nine hundred spectra are used depending upon the available data.
In the far right column of the well log analysis summary, the calculated porosity is displayed, increasing to the left. The contained fluids related to the porosity are shaded. Green shading represents apparent hydrocarbons. The blue shading represents apparent free water. No shading to the right of the blue shading to the edge of the column represents apparent bound water.
The interval marked by the red polygon shows an interval that is at irreducible saturation. That is no free water is present. With reference to the core marked with the red interval, it can be noted that the core is fully saturated with hydrocarbons.
The interval marked green shows an interval where free water is present. Free water would be expected upon production. Water over oil is possible in this situation as the reservoir contains heavy oil. Note the green marked interval on the core and very thin layers of a lighter color can be observed. These thin light colored layers have a pore geometry too small to allow the viscous heavy oil to enter but still large enough for water to enter. Thus, free water production would be expected to be produced above a fully oil saturated interval.
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