The production log analysis flowmeter: FLT1 shows that most of the production comes from a single very thin layer. At this layer, the drilling hole diameter, as shown by the caliper CAL1 , is larger than the nominal value caving in a highly brittle rock and the porosity log analyses NPLE and RHOBE are disturbed by this caving phenomenon. We may consider, however, that the layer porosity is very high 0.
The highly brittle rock forming the super K was flushed away during the coring operation. This is an example of the principle according to which, even during an excellent coring operation, some extremely important details of the reservoir may be overlooked. The samples were prepared using several resins It has only very poorly connected vugs, hence the very low permeability. On photograph b, we can see extensive white areas corresponding to large dolomite crystals. These large crystals take the. FLT1: flowmeter as a percentage of the total production.
Log KH : air permeability on plugs with horizontal axis logarithmic scale in mD. They are the result of cementation after the vacuolarisation phase of photograph a. This recrystallisation has been extremely erratic, which explains the petrophysical variations between samples located several tens of centimetres apart in the geological series. The samples were prepared using 3 resins Pores of access radius less than 0.
The values are extracted from the database of Figure Oolitic limestones clearly illustrate the variability of the -K relations. These rocks, whose microtexture is clearly defined for the petrographer oolitic grainstone , are dispersed throughout the -K space Fig. On the 0. There is a simple explanation for this diversity: in oolitic limestones, the distribution of the pore access radii is always bimodal apart from the rare cases of exclusive microporosity.
The microporosity inside oolites, always present but in variable proportions, plays virtually no role in the permeability. The proportion of macroconnected porosity between the oolites is highly variable.
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It determines the permeability value. Limestones d and e reveal only intraoolitic microporosity, their permeability is therefore very low. The other limestones exhibit macroconnected and microconnected porosities in variable proportions. Limestone a exhibits exceptional intergranular macroporosity, which explains its high permeability in spite of the fact that its porosity is relatively low.
Limestone f exhibits only moderate intergranular macroporosity and significant intraoolitic microporosity. What makes this limestone so unusual, however, is the very high moldic porosity inside the oolites. This abundant in terms of pore radius macroporosity can only be accessed through the intraoolitic microporosity. It is therefore known as microconnected porosity. This explains the relatively low permeability of this exceptionally porous oolitic limestone.
Epoxy pore casts Same scale for all six photographs Limestones a, b, d: core samples. Limestone c: oolite miliaire outcrop, Normandy France. Micrites Fig. The -K relation, however, is quite different from those of the oolitic limestones. This relative simplicity of the -K relation in micrites is explained by the fact that they only have a single type of porosity.
The porosimetry spectra Fig. So far, we have emphasised the extreme diversity of -K relations.
However we can make a few observations which will prove useful when studying limestone reservoirs:. Limits of the -K space. The various -K relations described in this section respect the matrix value criterion They come from plugs and the minimum homogenisation volume is millimetric. When studying reservoirs, special attention must be paid to values outside this area. They generally indicate measurement errors or faulty samples e.
In the other cases, however, a special study could prove well worthwhile. Main trends Some main trends in the -K relation may also be observed according to the petrographic texture of the limestones Fig. Some oolitic grainstones lie within the same area of the -K space, for the same reason: proportionally very low microporosity. If power 3, 5, 7 law graphs are plotted from this point, we observe that the mudstones are grouped on the power 3 law see above , the wackestone-pelstones around the power 5 line and the bioclastic grain-packstones around the power 7 line.
Although these are obviously very general observations except for the mudstones , they may prove useful when looking for orders of magnitude, during modelling for example. In limestone rocks apart from mudstones , at least two types of porosity affect the petrophysical characteristics, in quite different ways. The microporosity microconnected porosity always present in the allochems and matrix mud. The macroconnected porosity sometimes found between the allochems intergranular , and also rarely in the dissolution vugs if they are present in sufficient numbers to be interconnected.
Only macroconnected porosity plays a significant role in permeability. We would therefore need to plot the macroconnected vs. K relations to obtain a rough estimation.
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This approach would, however, be both costly in terms of porosimetric measurement and of little practical application, since the -K relations are of most use when there are no samples drilling. The porosimetric diversity of carbonates is represented on Figure Although a basic point, we must reiterate the fact that the most important value required to understand -K relations is the equivalent pore access radius. The dimension of the pore itself has virtually no impact on permeability.
The accessible porosity between two pore access radius values is therefore equal to the area of the portion of histogram. The scale is the same for all spectra, but the y-axis of micrite 3 chalk had to be cut off at the top due to the extremely high modal value.
Note the clearly bimodal distributions for the bioclastic and oolitic facies with the microporosity accessible through access radii of less than 1 m. Micrites exhibit only microporosity. One extreme example is that of the limestones and especially the dolomites with large moldic pores Fig.
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This remark partly explains the disappointment often felt when interpreting NMR log analyses in terms of permeability. The geometric data deduced from NMR is much closer to the pore radius than the access radius Despite saturation under vacuum, the red epoxy resin did not have time to invade all the vugs due to the low permeability. C Porosity-Permeability relations in sandstones comparison with carbonate rocks The -K relation in Fontainebleau sandstones is an exception in the natural environment. In the general case of clastic rocks, petrological variations of various origins granulometry, cementation, etc.
After providing an overview, we will briefly examine the main petrological parameters affecting this relation. The data concern exclusively measurements on core samples from oil or gas reservoirs. Most are routine measurements taken on 1 inch The results of Figure Note, however, that different sampling principles were used for the databases corresponding to these two figures.
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Since the figure concerning the limestone rocks includes numerous outcrop samples, it may be slightly more representative of the. In nature, this favourable situation is only found in very specific cases, such as that of a rock which has never been in contact with anything but water or brine and which is invaded by liquid or gaseous hydrocarbons secondary migration This is also true near the surface, in vadose zone, where the porous space contains both water and air.
But even in this case, it is not uncommon to observe states at least transient of nonwater wettability see Fig. Petroleum is a mixture of a large number of chemical compounds, including molecules exhibiting a special affinity for solids.
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This affinity may be more or less marked for certain minerals. We can therefore see that when a porous rock is invaded by oil, deposition of these molecules may have a significant impact on the nature of the relations between the solid and the fluids, in other words, by definition, the wettability. The example we give for the oil reservoirs may be almost identical to the situation in surface geology hydrogeology, pedology , for example in the event of deliberate or accidental injection of chemical products. Consequently, the capillary behaviour described in Section If these phenomena led to a clear wettability reversal, i.
This type of wettability reversal can be carried out in the laboratory by depositing silane molecules, for example, on the entire porous wall. In nature, however, this is very rarely the case. This explains why the definition of wettability when studying geological materials can only be based on a phenomenological approach. To make it easy for non-specialists to understand the wettability phenomena observed in rocks, a few elementary but useful remarks must be made. The first, and most important, is that the diagnosis and quantification of petrophysical wettability is not the result of a rigorous physical analysis based, for example, on the interactions of electrostatic forces, but the result of a particular experiment.
Several types of experiment see below giving different results can be performed. It is therefore not surprising that the wettability state of a rock varies depending on the test conducted. The second remark is that, unlike porosity or single-phase permeability, wettability is not an intrinsic characteristic of the rock, but a variable state of the interface between the mineral solid and the fluids contained in the porosity. When defining this state, the fluids and the history of their relation with the solid are as important as the solid itself. Lastly, it should be added that, in everyday language, oil wettability may not be the reciprocal of water wettability.
Oil wettability is often the observation deduced from a particular experiment discussed below. Although the same may be true of water wettability, the result is often actually more conclusive, being based on a number of observations.
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It would often be more appropriate to speak of a trend towards oil wettability. The debate between innate and acquired is a recurrent feature in some fields of biology. The same applies in petrophysics with the problem of wettability since it is never easy to distinguish clearly between the causes intrinsically related to the nature of the porous space: geometry, mineralogy innate and those related to the nature of the fluids and the particular history of their coexistence acquired. The debate is of much more practical importance than it seems, since only the innate parameters can be practically extrapolated by using geology methods.
For detailed information on wettability, readers can refer to the bibliographic work of Anderson .