Clay induced formation damage in the vicinity of injection and disposal wells is a worldwide problem in the petroleum industry. The physical chemistry and dynamics of the internal clay transport are still poorly understood issues. In this paper we give a comprehensive theoretical overview over effect of the main physico-chemical factors on the stability and transport of clay particles in sandstone porous media and their influence on the rock permeability. First we give a short "reservoir oriented" introduction to the DLVO theory (Derjagin - Landau - Verwey - Overbeek) which makes it possible to quantify the interactions between clay particles and the rock matrix in terms of electrostatic repulsive and Van der Waals attractive forces. Secondly, we explain the mechanisms of clay surface charge formation. Than we describe the structure of the electrical double layer around clays. Subsequently we consider in detail the influence of hydrodynamic and physico-chemical factors on coagulation - dispersion of clay particles in sandstone. In particular we describe the effect of interstitial velocity, chemical composition, pH, and temperature of the permeating fluid, as well as clay mineralogy, micromorphology, and composition of the exchangeable cations. Furthermore we describe the effect of structural forces on clay stability, which are often ignored in the petroleum literature. Finally we consider the influence of rock microstructure and clay micromorphology on sandstone water sensitivity. Introduction Environmentally safe production water re-injection is an important issue in the petroleum industry as well as in the geothermal energy exploitation. The injectivity of a disposal well often decreases as a result of formation damage caused by in-situ clay particles, released from the rock matrix1. The rate of permeability decline, besides intrinsic fluid velocity, depends on the physico-chemical factors controlling clay and matrix surface properties. The most important of them are clay and rock matrix mineralogy, chemistry of the permeating fluid, and micromorphology of the porous medium. The mineralogy of clay particles controls their size, shape, and surface charge distribution. Physico-chemical properties of the fluid, such as ionic strength, pH, temperature, together with clay mineralogy determine coagulation - dispersion processes and consequently the rate of clay particle release. The porous medium parameters such as porosity, permeability, pore size distribution, tortuousity control intrinsic fluid velocity microdistribution and therefore determine the hydrodynamic forces effecting the clay particle. It is difficult to control all these parameters during laboratory experiments and to keep them at the pre assigned values in the reservoir. This often makes the percolation experiments mutually incomparable and complicates quantification of the effect of the physico-chemical factors on the reservoir permeability. Hence in-situ clay and colloidal particle transport is still one of the issues in both hydrocarbon and geothermal reservoirs exploitation that are poorly understood. We believe that better understanding of the fundamental principles of clay particle stability and transport in porous media will help the petroleum specialists to apply more effective existing methods and to develop new techniques for preventing in-situ clay induced formation damage. Clay—matrix interaction and mechanism of the in-situ clay-induced formation damage The porous medium of sandstone can be represented as a network, consisting of pores connected with each other by narrower pore channels (or throats). The widely accepted model of in-situ clay induced formation damage supposes that the permeability reduction occurs due to release of clay particles from pore walls and their subsequent re-deposition downstream in pore throats, which presumably have smaller diameters than the pores.

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This study provides a new rock-typing approach for low-resistive and low-permeable clastic rocks. The approach includes integrated interpretation of routine core analysis data with microstructural characteristics, acquired from computed tomography (CT) and nuclear-magnetic resonance (NMR) data. The studied formation comprises siltstones in its bottom, which are replaced by sandstones in its top. Sandstones form the main part of the oil reservoir, whereas siltstones were originally considered as water-saturated. The reserves calculation was performed based on a single Archie equation for the whole formation. Despite on apparent water saturation and low permeability of the siltstones, incidental perforation showed considerable oil inflow from them as well. In order to delineate missed productive intervals within the low-resistive siltstones, we had to develop a new rock-typing approach, acknowledging rock multimineral composition, diversity of microstructures, a wide range of porosity, permeability, and residual water saturation values. Designed laboratory program included porosity, permeability, electrical resistivity measurements, capillary, NMR and CT tests. The experiments were performed on the same core samples that enabled reliable correlation between measured parameters. The joint interpretation of flow zone indicator, calculated as a function of porosity and residual water saturation, together with the results of petrophysical and microstructural measurements allowed reliable rock-typing of the clastic formation. It will serve as a petrophysical basis for identification of the missed productive intervals. The developed laboratory program and rock-typing algorithm can be implemented in other oilfields.

The proposed method of predicting spatial distribution of leaching zones in carbonate reservoirs includes integrated interpretation of well logging data, including electrical microimages (Formation MicroImager – FMI), together with analysis of lithofacies within depositional cycles. Based on the comparison of FMI data with results of lithological and petrophysical studies of the core, an atlas of diagenetic porosity images for the studied formation was developed. Choquette and Pray international classification was used for pore typing. Applying the developed atlas to FMI logs, karstification zones were identified and classified in reference wells. In wells, where only standard well loggings was performed, karstification zones were identified using a developed decision tree. Sedimentological and sequence stratigraphic analysis made it possible to identify sedimentation cycles and to correlate the main lithofacies within the target reservoir. It was established that microbial and shallow facies form the most productive part of the reservoir. These facies with initially high primary porosity were subjected to the most intensive karstification along sequence stratigraphic boundaries due to subaerial exposure. The established relationships between leaching zones, lithofacies, and sequence-stratigraphic boundaries made it possible to carry out inter-well correlation of karstified intervals. The correlation will allow construction of leaching zones maps, that will be further applied for geological modeling.