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Geological mapping of existing and redundant kaolin workings within the St Austell Granite has identified a suite of granitic rocks which show evidence of complex late-stage magmatic and hydrothermal processes. Coarse porphyritic biotite granites, like those which predominate in southwest England, occur much more widely than previously acknowledged, and are intruded by an apparently cogenetic suite of lithium-mica granites and tourmaline granites. The tourmaline granites characteristically exhibit very variable textures, with coarse quartz grains set within a fine grained, tourmaline-rich matrix. A highly evolved fine-grained tourmaline granite represents the most evolved of this suite. Topaz granites intrude the earlier granite varieties, and all are intruded by rhyolite porphyry dykes (elvans). Major and trace element chemical data suggest that the biotite granite–lithium-mica granite–tourmaline granite suite represents the product of crystallization of a granitic magma within which B (but not F) became progressively enriched until water saturation was achieved. Water exsolution effectively quenched any remaining granitic melt, resulting in the very variable textures shown by the tourmaline granites. The topaz granites are chemically distinct from their predecessors, showing marked enrichment in F, Li and P 2 O 5 (but not B). Instead of being products of differentiation of biotite granite magma, the topaz granite melts may have been derived separately in a later episode of partial melting of the same source. Kaolinization is widespread throughout the western part of the St Austell Granite, and deposits worked at present tend to be located in granite varieties other than biotite granite. The geochemical parameters used to distinguish the primary granite types (particularly Nb v. Zr and Ga–Nb–Zr plots) are sufficiently robust to permit the parent granitic rock type to be identified for heavily kaolinized material.

High levels of dissolved iron present in produced waters have caused concern in a number of wells in North Alaskan Reservoirs. In this paper, two dynamic "reservoir condition" scale inhibitor core flooding experiments have been conducted to simulate scale inhibitor treatments using a pre-selected commercial phosphonate based scale inhibitor. In these floods very high levels of iron were released during the chemical injection and shut-in stages (> 450ppm in solution). Evidence from pre and post treatment SEM analysis revealed that the major cause for the excessive iron release was extensive dissolution of siderite, inducing considerable mechanical weakness of the core, which was particularly evident for core 1, and much less so in core 2. However, although both cores have similar percentages of siderite, it is the textural arrangement, which is of prime importance in relation to formation damage. In core systems where siderite supports many grain contacts any fluid/mineral dissolution reactions will significantly impair mechanical strength (core 1), potentially leading to sand production. Where siderite is enclosed within finer carbonate mud lenses, or is not grain supporting, the mechanical damage is much less severe (core 2). Reactions of siderite with the core are quite complex. In this paper we demonstrate the formation of an oxidised rim of hematite which surrounds remnant siderite, with voids present between it and the hematite rim. The high levels of iron release recorded in such systems have considerable implications with regard to the effectiveness of the pre-selected scale inhibitor. Inhibitor performance tests for a range of generically different inhibitor clearly indicate that the performance of certain types of scale inhibitor products is significantly reduced in the presence of dissolved iron. These results concur with field experience where phosphonate based inhibitors (initially selected based upon performance in the absence of iron) have proven less successful in the field. In summary it is evident that potential dissolution of iron bearing minerals, and the subsequent influence of dissolved iron on performance, has a significant bearing on scale inhibitor selection. Background & Introduction High levels of dissolved iron (>100ppm) recorded from produced waters in a number of wells in a North Alaskan reservoir (denoted field A) have caused concern with regard to potential interference with the working of certain production chemicals i.e. scale inhibitors. Although calcium and magnesium have an influence on scale inhibitor performance1 the case for iron is less well understood2,3,4. Natural dissolved iron in formation waters is normally around 10 ppm and rarely more than 100 ppm5. This can be increased significantly by the dissolution of Fe bearing minerals such as siderite, pyrite and chlorite, especially at the low pH values often associated with the application of acid phosphonate based scale inhibitors. This paper presents results of two reservoir condition scale inhibitor (SI) core floods on core originating from the same well, but different depths, of the North Alaskan Reservoir of concern. It focuses on the derivation of the dissolved iron, makes an assessment of potential formation damage following treatment and comments on the affect on scale inhibitor performance. Field Experience The North Alsakan field (field A) of concern is a sandstone reservoir which contains significant amounts of siderite and glauconite, both iron bearing minerals and is typical of other neighboring fields in the area. Squeeze treatments had been carried out on these fields with a variety of inhibitors with no adverse affects6. Indeed, one of the reservoirs was considered to be quite similar in terms of reservoir characteristics and formation water composition.