Summary

Thermal‐recovery methods are commonly used for production of viscous crude oil. Injected hot fluids react with reservoir rock, and some of these reactions might result in a change to the reservoir mineralogy. Depending on the physicochemical conditions and initial mineralogy, some such alterations can lead to formation damage and flow‐assurance issues. This paper investigates these mineral reactions, the conditions in which they occur, and the effects of these processes on porosity and permeability of sandstone rocks with various initial mineralogy.

Quartz, calcite, dolomite, kaolinite, and montmorillonite were used for preparation of 12 rock samples of various mineralogical contents. These mixtures were packed and used for steam‐injection experiments. Porosity and permeability of the initial and steamed sandpacks were determined using computed‐tomography (CT) scans and coreflood experiments, respectively. Composition of the collected condensate samples was analyzed using inductively coupled plasma optical emission spectroscopy (ICP‐OES). Rock mixtures were aged in high‐pressure/high‐temperature (HP/HT) cells. Rock morphology and pore‐space configuration were studied using scanning‐electron‐microscope energy dispersive X‐ray spectroscopy (SEM‐EDS). Mineralogy of the samples was analyzed using X‐ray‐diffraction (XRD) analysis.

Mixtures of quartz with calcite were found to be the least prone to formation damage associated with steam injection. Silica dissolution/precipitation reaction resulted in 5% initial permeability reduction and 1 to 2% initial porosity reduction. Mixtures of quartz, kaolinite, and carbonate minerals (calcite or dolomite) after steam injection lost 11 to 22% of initial permeability and 2 to 7% of initial porosity. Kaolinite fines were shown to be mobilized during steam treatment. Aging of these rock mixtures for 10 days at 400°F and 1,000 psi led to formation of swelling smectite clay (Ca montmorillonite). Growth of montmorillonite was demonstrated to be possible only at low carbon dioxide (CO2) partial pressures. Mixtures with dolomite are shown to produce more montmorillonite than mixtures with calcite. Steam injection in montmorillonite‐rich sandstone caused up to 84% loss in the initial permeability and up to 8% loss in initial porosity. The principal formation‐damage mechanism proved to be clay swelling, which led to filling of pores with montmorillonite. The microporous network that filled the pores significantly restricted the flow. Aging of montmorillonite‐rich mixtures did not reveal mineral alterations but did allow visualization of the morphological reorganization of montmorillonite and its pore‐bridging effect.

This paper describes interactions between superheated steam and rock samples. This study further characterizes formation‐damage mechanisms caused by hydrothermal alterations and their effects on petrophysical properties of reservoir rocks. Data about conditions of mineral reactions can be used to shift the physicochemical or operational conditions to prevent growth of swelling clays such as montmorillonite.

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