Fines mobilisation by low-salinity water injection: 3-point-pressure tests

Fines mobilisation and straining by changing the ionic strength of the reservoir water results in a significant permeability reduction, and is a well known mechanism of formation damage. The phenomenon occurs during drilling and completion, where the fluid invasion with fines lifting may cause significant productivity damage. The paper experimentally investigates the permeability damage of natural sandstone rocks in order to select ionic compositions for drilling and completion fluids. Two sandstone rock cores from the same production well from the Birkhead Formation in the area of underlying Patchawarra Trough (Eromanga Basin, Australia) were chosen to study formation damage during successively decreasing (piece-wise) the ionic strength of injected fluid using a three point pressure measurement method along the core. Effluent particle concentration and electrolytic conductivity were measured for effluent streams. It was observed that formation damage in the studied rock cores after their contact with low-ionic strength fluid is caused by fines mobilisation, their migration in the porous medium and straining in thin pores. Each coreflood is characterised by decreased rock permeability with stabilisation times significantly exceeding one pore volume, suggesting that mobilised particle’s drift velocity is significantly lower than that for a carrier fluid. The electrolytic conductivity profile shows that the fluid front in porous medium is not piston-like, rather diffusion driven, suggesting that there is an additional reason behind long permeability stabilisation time – a delayed contact of rock with low ionic strength fluid.