Effect of Pore-Lining Chlorite on Petrophysical Properties of Low-Resistivity Sandstone Reservoirs

Abstract

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 trimodal, 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.Concentration of clay fraction, to better characterize the clay nature, polytype, and crystallinity, was performed by sedimentation after similar cleaning. The SSA and CEC were also determined on these fractions. Fig. 1. Experiments were run in a sophisticated Hassler cell (Ergotech), allowing measurements in 2-or 4-electrode mode (with the two current electrodes at each end of the plug) on the water inlet/outlet tubes, and up to six potential electrodes every 1.25 cm along the length of the plug. No ceramic or porous plates were used. This cell was located in an oven, with a temperature regulated at 22±0.1°C. Experimental Setting. F and IR Determination. The experimental setting is shown inResistance measurements were acquired on a PC every 15 minutes on each slice (i.e., interval between the potential electrodes) of the sample, both in 2-and in 4-electrode settings, without any manipulation of the sample during the entire experiment. These records allowed us to identify possible contact problems, to check the homogeneity of the electrical values along the samples, and ensure that a plateau was reached in the electrical values.Electrical measurements were performed with a HewlettPackard 4263B impedance meter. The conditions were 1V, 1kHz. The phase angle was negligible at this frequency.For the F experiments, the brine salinities were increased from 20 to 200 g/L NaCl.The brine used for all the IR experiments was 96 g/L NaCl. The reason for this was that the field salinities are very high in all cases-sometimes at the limit of NaCl solubility at reservoir temperature, and thus impossible to reproduce at room temperature. The ...

Origin and impact of authigenic chlorite in the Upper Cretaceous sandstone reservoirs of the Santos Basin, eastern Brazil

“…The absolute permeability of the chlorite-rich sandstones commonly ranges between 0.01 and 30 mD, staying below 5 mD for most of the samples. Other potential reservoir problems caused by pore-lining chlorites include: (i) wireline log low-resistivity anomalies due to high saturation of irreducible water contained in chlorite aggregates, which may result in wrong evaluation of oil saturation (Moraes & De Ros 1992;Durand et al 2001); (ii) formation damage during well stimulation through acidification, owing to the precipitation of iron hydroxides, which makes the use of Fe-chelating agents necessary (Almon & Davies 1979;Baker et al 1993).…”

Reservoir characteristics and control mechanism of resistivity low-contrast oil pays in Chang 8 tight sandstone of Longdong West area, Ordos Basin

“…Although the pore structure of RLP is poorer and the micropores are more developed in RLP reservoir than NP reservoir, the average porosity and permeability are better. Previous researches have shown that the clay mineral is an important factor to affect reservoir physical properties (Durand et al 2001;samakinde et al 2016;Yousef et al 2018). In general, the clay minerals filling in the pores will reduce the porosity and permeability and the pore structure becomes worse.…”

An improved method for improving the calculation accuracy of marine low-resistance shale reservoir parameters

“…Further analysis of the correlation between the mass content of chlorite and the breakthrough pressure in clay minerals found that the mass content of chlorite was negatively correlated with the breakthrough pressure. As the chlorite content increased, the breakthrough pressure showed a decreasing trend (Durand et al, 2001;Wang et al, 2017). bound water.…”