Productivity Decline Due to Fines Migration (Modelling and Field Case)

Abstract

Abstract High formation damage, including that induced by fines migration has been reported in numerous geothermal projects. The attaching electrostatic forces acting on fines are weak at high temperatures if compared with drag and lifting forces, which detach the particles from grain surfaces. Migration of lifted fines results in their straining in thin pores preferentially near to well, causing severe permeability and productivity decline. A mathematical model has been developed, based on recently developed theory of particle detachment in suspension flows. Solution for predicting well productivity is obtained. The model is validated by comparison with the field data from geothermal well A in Australia. Productivity decline due to fines migration is more likely to happen in high temperature geothermal reservoirs if compared with conventional aquifers or oil and gas fields. Introduction Productivity decline as well as induced formation damage due to fines migration in conventional oil and gas reservoirs has been widely reported since 1960s, and is still a hot topic these days (Muecke, 1979; Lever and Dawe, 1984; Sharma and Yortsos, 1987; Sarkar and Sharma, 1990; Khilar and Fogler, 1998; Tiab and Donaldson, 2004; Civan, 2007; Geng et al., 2010; Rosenbrand et al., 2012). The phenomena including fines mobilisation, subsequent reduction of reservoir permeability and formation damage have also been observed during drilling, discharge, injection and production stages in the geothermal wells. However, the comprehensive analysis of formation damage specific for geothermal reservoirs is not available yet. Flow of suspended fine particles with particle capture, detachment and rock clogging takes place in porous media (Fig. 1), during the propagation of particles, bacteria and oil droplets in reservoirs, filtrate invasion into formation while drilling, injection and discharge of water in geothermal fields, etc. The particles at the rock surfaces are subject to the electrostatic force Fe, drag force Fd, lifting force Fl and gravitational force Fg (Fig. 1). The drag and lifting forces tend to detach the particle from the grain surface while the electrostatic and gravitational forces attach it. It is assumed that at the moment of leaving the solid surface, the particle rotates around the neighbouring attached particles or around a spike of the surface ((Fig. 1b,c). Therefore, the condition of mechanical equilibrium of a particle on the grain surface is the balance of the detaching and attaching torques exerting on the particle (Freitas and Sharma, 2001; Bedrikovetsky et al., 2012). At high flow velocity near the wellbore, the detaching torque on the particles may exceed the attaching torque. Thus, the particles can be released and brought to the suspension flow as a result. The migrating particles may be further captured at thinner pore throat, or attached to the pore wall (Fig. 2), depending on the particle and pore geometry and hydrodynamic conditions.