Clay Minerals, (2014) 49, 147–164 OPEN ACCESS The influence of individual clay minerals on formation damage of reservoir sandstones: a critical review with some new insights 1, 2 2 M. J. WILSON *, L. WILSON AND I. PATEY 1 James Hutton Institute, Aberdeen, UK, and 2 Corex(UK)Ltd,HoweMossDrive,Aberdeen,UK (Received 29 June 2012; revised 26 July 2012; Editor: Harry Shaw) ABSTRACT: The influence of individual clay minerals on formation damage of reservoir sandstones is reviewed, mainly through the mechanism of fine particle dispersion and migration leading to the accumulation and blockage of pore throats and significant reduction of permeability. The minerals discussed belong to the smectite, kaolinite, illite and chlorite groups respectively. These minerals usually occur in an aggregate form in reservoir sandstones and the physicochemical properties of these aggregates are reviewed in order to reach a better understanding of the factors that lead to their dispersion in aqueous pore fluids. Particularly significant properties include the surface charge on both basal and edge faces of the clay minerals and how this varies with pH, external surface area of both swelling and non-swelling clays, porosity and pore size distribution in the micro- and meso-pore size range and overall aggregate morphology. For non-swelling clays, and perhaps even for swelling clays, dispersion is thought to be initiated at the micro- or meso-pore level, where the interaction between the pore solution and the charged clay surfaces exposed on adjacent sides of slit- or wedge-shaped pores brings about expansion of the diffuse double electric layer (DDL) and an increase in hydration pressure. Such expansion occurs only in dilute electrolyte solutions in contrast to the effect of concentrated solutions which would shrink the thickness of the DDL and so inhibit dispersion. Stable dispersions are formed, particularly where the solution pH exceeds the isoelectric pH of the mineral, which is often at alkali pH values, so that both basal face and edge surfaces are negatively charged and the particles repel each other. The osmotic swelling of smectitic clays to a gel-like form, so effectively blocking pores in situ, is often invoked as an explanation of formation damage in reservoir sandstones. Such swelling certainly occurs in dilute aqueous solutions under earth surface conditions but it is uncertain that stable smectitic gels could form at the temperatures and pressures associated with deeply buried reservoir sandstones. KEYWORDS: formation damage, reservoir sandstones, kaolinite, smectite, illite, chlorite, mixed-layer minerals, glauconite. Formation damage may be defined as ‘‘a decline in be irreversible and which may have a serious the initial permeability of the reservoir rock economic impact upon the productivity of the following various wellbore operations, which may reservoir’’. An alternative definition is ‘‘a reduction in the initial permeability of the reservoir rock around the well bore following various operations such as drilling, completion, injection, attempted * E-mail: [email protected] stimulation or production of the well’’.These DOI: 10.1180/claymin.2014.049.2.02 definitions relate to formation damage as a # 2014 The Mineralogical Society 148 M. J. Wilson et al. dynamic and progressive process and not to damage MECHANISMS INVOLVED IN described as ‘‘incipient’’ where the poroperm FORMATION DAMAGE properties of a reservoir rock are adversely affected by the nature and distribution of authigenic clay Various processes involving clay minerals are minerals found in the original rock. thought to be responsible for formation damage in Formation damage is a problem of increasing sandstones. In brief, these include (a) swelling of significance as the cost of oil extraction rises, it smectitic clays, (b) dispersion and migration of a being more important than ever to maximize variety of clay minerals, including smectites but productivity both in conventional well productivity principally involving kaolinite and illite, and and during enhanced oil recovery operations. (c) transformation of clay minerals to other Krueger (1986) gave a broad and comprehensive mineral phases. Each of these processes will be overview of the many factors that cause formation reviewed in turn with emphasis on the basic damage, as well as the operational procedures that mechanisms involved. may be used to control or mitigate these deleterious effects. He observed that, although there may be Swelling of smectitic clays any number of reasons for the decline of well productivity, in most instances the transport of fine The well known ability of smectitic clays to particles, usually clay minerals, and the chemical adsorb water between their constituent silicate reactions of these particles with formation fluids are layers and to swell to many times their original most often involved. It is of course well known that volume suggests that it is highly likely that similar the physicochemical properties of the clay minerals responses must occur in smectitic sandstones under are extremely variable, particularly with regard to appropriate chemical conditions. Smectite swelling basic attributes like particle size, morphology, can be directly related to the negative layer charge swelling characteristics, hydration, cation exchange, on the 2:1 layer which varies between 0.2 to 0.6 dispersion, aggregation properties, etc., and there is charges per O10(OH)2 of the formula unit and also much diversity in the ways in which these fine which is compensated for by various interlayer particles interact with fluids, either native to the cations. In the natural environment, these interlayer formation or introduced during exploration and cations are normally hydrated and freely exchange- production operations. able with cations in external solutions, so that the An early and prescient paper on formation distance between the silicate sheets is variable damage caused by clay minerals in sandstones is depending on the nature of the interlayer cation that of Gray & Rex (1966). This paper, which pre- and on its hydration status. The seminal paper of dated the development of scanning electron Norrish (1954) showed that montmorillonite microscopy, showed that in non-smectitic sand- exhibited two types of swelling, depending on the stones, loss of permeability was associated with the nature of the exchangeable cation and on the salt migrating and pore-plugging characteristics of concentration of the external solution. The first euhedral illite and kaolinite particles following type, sometimes referred to as ‘‘crystalline swel- water movement within the formation. Similar ling’’, may be illustrated where the interlayer space observations have been made in reservoir sand- of montmorillonite is occupied by divalent cations stones many times since, but at the time it was such as Ca2+ or Mg2+. In these circumstances, the widely believed that the main mechanism for smectite maintains a basal spacing of ~15 A˚ formation damage was to be found in the swelling through a wide range of relative humidities, of smectitic clays. A particularly interesting indicative of a two-layer water structure in the observation was that analysis of the effluent from interlayer space, but swells to a limit of 19À20 A˚ the cores of the sandstones examined showed that when immersed in water (MacEwan & Wilson, illite was more abundant than kaolinite, whereas in 1980). The second type of swelling is usually the untreated sandstones kaolinite was described as described as ‘‘osmotic’’ and is best illustrated an abundant component and illite as a minor one. A where the interlayer space of montmorillonite is possible implication of this finding was that the occupied by a monovalent cation such as Na+ or illitic clays were more mobile than kaolinite and Li+. Norrish (1954) showed that at low salt preferentially migrated within the rock in response concentrations of an external solution the indivi- to water movement. dual silicate layers of a Na-montmorillonite will Formation damage of reservoir sandstones 149 delaminate and will be separated by distances of micro-aggregate structure, morphology, surface tens or even by hundreds of A˚ ngstroms. This area, porosity and particle size distribution. These phenomenon finds a general explanation on the factors are all highly variable and will be reviewed basis of the electrical double-layer theory, where on an individual clay mineral basis later. It may be the diffuse double layers exert a repulsive force noted, however, that generally the clay minerals of which is far greater than the attractive van der most interest with regard to fines migration have Waals forces between the unit layers. Osmotic formed during diagenesis. Detrital clays could also swelling may lead to the formation of a gel which be involved in dispersion and migration, although it is thought could lead to severe formation damage they are unlikely to be significant in clean, well through the reduction of permeability, either sorted sandstones typical of many reservoir rocks. directly through in situ swelling within pores or Such clays occur largely in shales and if shaly indirectly through the breakdown of the gel thus intercalations occur in reservoir sandstones then leading to the release and migration of fine they could be significant in forming intra-forma- particles (Zhou et al.,1996).Theformationofa tional permeability barriers. gel through the osmotic swelling of a highly Dispersion of clay minerals will also relate