ion of water from aquifers has the effect of modifying the effective stress born by minerals as a result of pore pressure drawdown, as well as altering the composition of the fluid in contact with aquifer minerals. Such perturbations in stress state and chemical equilibrium initiate a series of fluid-rock reactions, including stress corrosion, mineral dissolution and precipitation. In a nonproducing aquifer, these are diagenetic processes which take thousands to millions of years to occur. Here, we describe laboratory experiments which demonstrate that in an active aquifer, these processes are extremely fast and can modify the fluid flow behaviour within the aquifer in a time dependent manner. Experimental methods and results. Experiments were conducted in a new recirculating flow rig (Elphick et al, 1992; Ngwenya et al, 1993) rated to 700 bars confining pressure, 345 bars pore pressure and 100*C designed for simulating various improved oil recovery injection strategies in the North Sea. Analogue core samples of various mineralogies have been flooded with different fluid compositions at varying stress conditions. During flooding, the fluid was regularly sampled for analysis of various analytes, either by HPLC or by ICP-MS. Differential pressures across the core were recorded at regular intervals and used to calculate permeability. The cores were characterised after flooding using SEM, XRD, probe permeametry, ion and electron microprobe imaging. Together, these measured parameters were used to deduce possible fluid-rock interactions induced during the flooding process. The role of fluid chemistry. The role of fluid chemistry was investigated by comparing the response of cores to injection of distilled water as a representative of compositions close to rain water, and seawater which may be taken to represent seawater intrusion into an aquifer. In both cases, the physical conditions were kept constant throughout the duration of the experiment, these being 80~ 345 bars confining pressure, 207 bars pore pressure and injection rates of 0.3 ml per minute. Fluid composition and subsequent SEM studies of core during pure water flooding of an arkosie sandstone showed that this results in dissolution of K-feldspar (Ngwenya et al., 1993). The permeability of the core increased, although some of these increases are offset by pore-throat blockage due to mobilisation of interstitial clay particles. In contrast, if the recirculating fluid is seawater, the permeability of the core invariably decreases. The fluid lost K +, kinetic modelling of which yielded first order apparent constant (Fig. 1). When corrected for surface area this apparent rate constant yielded a true rate constant close to that for the precipitation of K-feldspar. Often, Mg and Ca were also lost from the fluid, consistent with precipitation of smectite and anhydrite respectively (Hajash & Bloom, 1991), but only in small amounts. However, with arkosic sandstones containing dolomite cements, large amounts of Ca were removed from the fluid. All these changes occurred over timescales of about 150 hours. The role of effective stress. Here, we describe pore fluid chemistry and permeability changes in ~ 0 ' t ~ . y=-0 .471.5e-6t