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Department of Geology and Geophysics, Texas A&M University, College Station, Texas 77843-3115
* E.mail: wenwu{at}geo.tamu.edu
Creep compaction experiments were conducted in flow-through reactors at 150°C and 34.5 MPa of effective pressure to investigate the effects of pore-fluid flow on creep compaction rate. The starting materials were St. Peter quartz sand (124-180 µm and 250-350 µm) and disaggregated novaculite (10-60 µm). Theoretical analysis was carried out to investigate the relationships between flow velocity (residence time), solute concentration in pore fluid, the removal rate of dissolved material, the grain-convergence rate due to grain-contact dissolution, and compaction rate. Creep-compaction experiments indicate that the compaction rate of quartz aggregates is related to fluid flow rates. Compaction rates increase by factors up to 6 as flow rate increases from zero to between 0.13 and 0.15 ml/h. The modeling results are similar to the experiments, and indicates that pore-fluid flow affects compaction through its control of solute concentration. When the input-fluid concentration is lower than the pore-fluid equilibrium concentration, an increase in flow velocity (a decrease in residence time) leads to higher removal rate of dissolved materials from the system, lower solute concentration, and more rapid grain convergence and compaction. The grain-convergence rate would even become lower than that in a closed system if the input-fluid concentration is higher than the pore-fluid equilibrium concentration. The effects of fluid flow on grain convergence rate also vary with grain size, strain, and grain-boundary properties. In reservoir sandstones, descending fluids that heat up would become undersaturated and then enhance compaction, whereas ascending fluids that cool would become supersaturated and then slow or inhibit further compaction.
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