Chapter Twelve - Formation Damage Challenges in Geothermal Reservoirs

Abstract

Decline of well productivity due to migration of fine particles is a well-known phenomenon occurring during exploitation of geothermal reservoirs. We performed several laboratory fines migration tests using natural reservoir cores, taken from geothermal fields. The pressure drop along the overall core and the cumulative concentration of fines in effluent streams were measured during laboratory coreflood tests with piecewise constant decreasing ionic strength of the injected fluid. Core permeability stabilized only after 100–1000 pore volumes of the flowing fluid were injected. This suggests that mobilized rock particles move significantly slower than the carrier fluid. SEM-EDX (scanning electron microscope coupled with the thin film energy dispersive X-ray analysis) analysis of the produced fine particles shows that kaolinite and illite/chlorite are the main minerals causing permeability decline. Mathematical modeling showed that decreased viscosity of water and weakened electrostatic attraction force due to temperature rise competitively affected the particles attached to a grain surface. The micro-modeling of mechanical equilibrium of a fine particle attached to the surface of a sand grain shows the domination of the electrostatic attraction of fines to the surface of sand grains over the detaching drag force. Therefore, elevated temperature leads to an increased particle mobilization and consequent permeability damage. A newly developed “velocity–ionic strength” translation procedure determines the dependency of the maximum retention function (MRF) on the fluid velocity using experimental coreflood data corresponding to piecewise decreasing fluid ionic strength. Experiment-based evaluation of velocity and temperature dependencies on the MRF is demonstrated for specific conditions of geothermal reservoirs.