COLD WATER INJECTION EFFECTS IN FRACTURED RESERVOIRS

Cold water injection in a geothermal reservoir can increase injectivity through dilation of preexisting fracture flow paths or by inducing growth of fractures. To elucidate how cold water injection may affect a fractured rock reservoir, we show results of calculations of fluid flow, heat transport and geomechanical deformation for thermal stimulation of simplified models of fractured rock reservoirs. When fractures are assumed dilate freely, results demonstrate that many fold increases in injectivity can occur. For realistic flow and heat transport parameters, where the number of interconnected fractures that conduct flow away from a well is on the order of 10 to 100, a 30-day cold water injection causes increases in injectivity of a factor of 2 to 4, with larger increases associated with fewer fractures. Changes of similar magnitude were observed in the cold water injection tests at the Hellisheidi field, SW-Iceland.

The large changes in fractured rock transmissivity that can be produced by contraction of cooled rock can easily dominate the effect of increasing fluid viscosity, and dramatically decrease the pressure gradient required to accommodate a specified flow. The region of dramatically increased transmissivity effectively reproduces the effect of a very large well bore, with a flat spot in the pressure vs. distance curve that extends through the cooled region. The change in the wellhead pressure with time is thus a function of (1) the shape of the initial fluid potential drawdown curve and (2) the rate of propagation of the cooling front along that curve. As with well tests in fractured rock, the injectivity changes associated with thermal stimulation can therefore be diagnostic of those conditions.

The assumption that reservoir fractures may dilate freely is most likely to hold when propagation of the thermal front is slow and the duration of the thermal stimulation is short. In that case, the stress formerly supported by the contracting region may be entirely supported by the surrounding rock. Where that stress is transmitted to a more distal region of the fractured rock reservoir, however, permeability may decline. 2-D and 3-D simulations of coupled fluid flow, heat transport and geomechanical deformation are presented to illustrate how injectivity may be affected when such stresses are considered.