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Abstract

Terrigenous clastic alteration during thermal heavy oil recovery processes has been documented in whole core and laboratory tests from four California fields. Sediments are subjected to either steam/hot water processes (up to 200°C) or in-situ combustion processes (up to 500°C) or both. Consideration of original and altered mineral compositions do not fully describe reservoir changes. Fabric and permeability changes can be understood in a predictive sense by comparison to original grain size and sorting.

Sand-size constituents are plagioclase feldspar and quartz, with lesser amounts of mica, volcanic rock fragments and volcanic glass. Original matrix is dominated by smectite, biogenic opal, zeolites and mica/illite. During hot water/steam alteration, primarily the fine-grained matrix is altered. Sediments with either finer grain sizes or increased original matrix have the highest magnitude of permeability decrease as compared to coarser, better sorted material. This pronounced permeability decrease is due to: (1) additional smectite and zeolite growth as the expense of original matrix components and (2) the dispersion, migration and pore-throat blockage by the matrix. The finer-grained sediments, characlerized by smAller pore throats, are subjected to extensive pore blockage as unstable silt- and clay-size particles move (fines migration).

Sediment reaction during in-situ combustion follows the alteration pattern above. Fine-grained matrix continues to alter, with some smectite transforming to illite. Calcite and oil reaction rims form in the burn stage. The overAll fabric is a lightly-consolidated sandstone. Only in the near-wellbore area of an combustion injector have partiAlly-melted fabrics been recognized.

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