Effect of Pore-Lining Chlorite on Petrophysical Properties of Low-Resistivity Sandstone Reservoirs
- Claudine Durand (IFP) | Etienne Brosse (IFP) | Adrian Cerepi (EGID)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Evaluation & Engineering
- Publication Date
- June 2001
- Document Type
- Journal Paper
- 231 - 239
- 2001. Society of Petroleum Engineers
- 4.1.5 Processing Equipment, 5.1 Reservoir Characterisation, 1.2.3 Rock properties, 4.2.3 Materials and Corrosion, 5.5.2 Core Analysis, 1.14 Casing and Cementing, 5.6.1 Open hole/cased hole log analysis, 4.1.2 Separation and Treating, 5.8.7 Carbonate Reservoir, 4.3.4 Scale, 1.6 Drilling Operations, 1.6.9 Coring, Fishing
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Chlorite-bearing sandstones usually give low resistivity signals and are thus erroneously identified as nonpay zones, even if they exhibit good preserved porosities at depth. The purpose of this paper is to provide petrophysical and mineralogical laboratory measurements that help improve the log interpretation of these sandstones.
The main results were obtained from a selection of reservoir cores. For sandstones having an amount of chlorite between 3 and 11 wt%, they show that the cementation index and saturation exponents (m and n) have values lower than 2, with n frequently around 1.5. These low values have been checked carefully to avoid experimental artifacts. Cationic Exchange Capacities (CEC) and Specific Surface Areas (SSA) both have low values, as expected from the clay structure. The distribution of pore throats is bi- or tri-modal, with a large contribution of microporosity.
The interpretation suggested by these results is that the amount and distribution of microporosity associated with pore-lining clay is the key to the chlorite sandstone electrical behavior. The values of CEC or SSA are too low to fully explain the low values of the saturation index n.
Use of these low values in log interpretation has the effect of increasing the interpreted oil in place. Advantages of a multidisciplinary approach for better evaluation of such complex argillaceous sandstone reservoirs is highlighted.
Pore-lining chlorite is a variety of authigenic clay that commonly coats the surface of detrital quartz grains in sandstones. As it prevents the subsequent diagenetic precipitation of quartz overgrowths, it can be responsible for the preservation of favorable reservoir properties in deeply buried sediments. On the other hand, its electrical signature is such that the host rock is commonly considered a low-resistivity sandstone. In these sediments, the classical methods used for appraising the water saturation from wireline logs provide overestimated values and can lead to erroneously discarding the sediments as a nonpay zone.
Possible causes of the manifestation of low resistivity have been recently summarized1: laminated sands and shales, fresh waters, electronic conduction, fine grains, and internal and superficial microporosity. Pore-lining, chlorite-bearing sandstones belong to the latter group and can be studied at the laboratory scale.
The purpose of this paper is to provide petrophysical and mineralogical laboratory measurements that help improve log interpretation. Unraveling the possible causes of such an electrical signature is likely to provide guidelines for interpreting the behavior in other wells.
Determining the relationship between the formation factor F (defined as F=R0/Rw), the resistivity index IR (defined as IR=Rt/R0, and the brine saturation Sw in the formation is at the heart of the interpretation of resistivity measurements that provides access to the amount of hydrocarbons in place.
Hence, based on the fact that laboratory data acquired on well-situated samples increases the consistency of the log-data interpretation, F and IR are the most useful petrophysical parameters to acquire in the laboratory for the purpose of Sw appraisal. Details on a dedicated experimental setup will be provided. However, the amount of hydrocarbon calculated depends heavily on the model that is applied to describe the links between electrical measurements and saturation. Several models may be able to explain the behavior induced by pore-lining chlorite in sandstones.
Because chlorite is a clay mineral, the models for shaly sands may be applicable. It is thus useful to determine the CEC of the rock as well as of the clay mineral.
The texture of the pore lining induces both roughness and microporosity, so that parameters defining the SSA, the pore size, and the pore thresholds radii distribution, are also useful. They can be determined from mercury intrusion porosimetry and Scanning Electron Microscopy (SEM).
Because a geochemical study was performed at the same time, the composition and amount of chlorite and other clays in the samples will be used in the discussion to propose an interpretation of the electrical behavior vs. the mineral composition and the rock texture.
The samples come from four reservoirs, with different locations and ages. The wells will be referred to as follows: LS1, for Lower Silurian from Libya; S1, for Strunian (Upper Devonian) from Algeria; D1, for Lower Devonian from Algeria; and C1-C5, for Cretaceous from Argentina.
More details on the locations cannot be given for proprietary reasons.
The first three reservoirs, which will be referred to as Saharian cases, come from a marine depositional environment, the last from a lacustrine basin. The water zones have low Rw (0.01 to 0.04 O·m), indicating salty brines (from 60,000 to 300,000 ppm equivalent NaCl). The resistivities of the formations are in the range of 0.3 to 20 O·m.
Preparation of Samples.
Among more than 100 samples, 11 plugs from four reservoirs were chosen for resistivity-index measurements. The plugs (38.1 mm diameter, 40 to 60 mm length) were drilled horizontally, parallel to the bedding. In addition to the geological selection criteria, homogeneity (checked by CT scan) and permeability were also considered. Samples with permeabilities lower than 1 md were discarded. The samples retained have porosities between 15 and 28%, and permeabilities between 1.6 and 648 md.
Because the plugs came from very salty reservoirs, they were first cleaned with a relatively low-salinity brine (10 to 20 g/L NaCl), then with ethyl alcohol, isopropyl alcohol, and ethylic alcohol again; they were dried under a primary vacuum and dried in an oven at 60°C. Some of the cores that had not been processed previously for k and f measurements were cleaned with dichloromethane in between water and alcohols. They were then vacuum-saturated with brine, and the brine was flushed at high flow to ensure a good saturation.
Additional characterizations (SEM, mercury intrusion porosimetry) were performed on chips located as close to the plugs as possible. These chips were cleaned with fresh water and Soxhlet extraction with dichloromethane.
CEC and SSA determinations were performed on gently disaggregated chips close to the former ones to avoid the creation of extra surfaces.
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