CHALLENGES with FINES MIGRATION
(see also text on ‘solution’)
Fines migration may result from movement of fine clay particles or similar materials within the reservoir formation. Fines migration causes particles suspended in the formation water to bridge the pore throats near the wellbore, reducing well productivity or injectivity (Figure 1 right). Damage created by fines usually is located within a radius of 1 to 2 m of the wellbore but can also occur in gravel-pack completions.
Mobilization of fines in geothermal reservoirs
Production and injectivity challenges may be due to reduced permeability in the geothermal reservoir caused by migration of fine-grained minerals, also called fines (Khilar and Fogler 1984, Rosenbrand et al. 2015, Bedrikovetsky and Caruso 2014). Fines are typically defined as mobile particles with an equivalent diameter smaller than 40 μm and in sandstone formations, fines are primarily mobilized clay particles present on the surface of matrix grains. Under certain conditions, these fines become mobile to move with the formation water to the pore throats where they may plug the pore throats and thereby reduce the permeability.
The mobility of the fines is controlled by the sum of the forces acting on the particles, typically the attractive van der Waals forces, the repulsive electrostatic forces and the drag forces. The fines are released when the sum of the electrostatic forces and the drag forces exceed the van der Waals forces. Generally, both the sandstone grains and the fine particles are negatively charged and therefore a repulsive electrostatic force acts on the particles. At “high” salt concentrations, the high content of ions in the formation water will to some degree shield this negative charge on the surface of the sandstone grains and the fines, and the van der Waals forces are sufficiently large to keep the fines attached to the grain surfaces. With decreasing salinity, the repulsive electrostatic forces increase, due to the fewer ions present to shield the negative charge. At a critical salt concentration, the repulsive electrostatic forces exceed the attractive van der Waals forces and the fines are mobilized (Khilar and Fogler 1984). Refer to Figure 1 left.
Thus, if a geothermal sandstone reservoir is exposed to brine with a salinity below a critical salt concentration, there is a risk that fines may be released causing a measurable reduction in the permeability. The drag forces increase with increasing flow velocity and fines migration can therefore also be induced when the fluid velocity is above a critical velocity (Sharma et al. 1992, Das et al. 1995, Freitas and Sharma 1997). Furthermore, at high pH values fines may be generated in the presence of alkali hydroxide by the alteration of kaolinite to dickite, nacrite and halloysite through chemical oxidation (Hayatdavoudi 1998).
Examples of operations at geothermal plants that may cause fines migration include loss of freshwater-mud filtrate or completion fluid to the formation during drilling and completion of the wells, and high well production or injection rates (rates above the critical velocity). During operation stand-still, the pressure gradient in the reservoir may be reversed and geological related fines may be transported to the injection well, where it may plug the pores of the sand control thereby reducing the fluid conductivity.
Typically, injection of clear, cooled formation water during geothermal production is unlikely to cause injectivity problems due to fines migration (Schembre and Kovscek 2005). Introduction of particles formed by processes in the geothermal surface loop, e.g. scaling due to degassing, corrosion, oxygen contamination, or addition of incompatible chemical additives and inhibitors (Seibt and Kellner 2003, Ungemach 2003) may, however, lead to reduced injectivity since the generated particles may form filter cakes or plug the pores in the reservoir.
Permeability reduction may not be reversible (Porter 1989, Civan 2007) and therefore great care should be taken to prevent reduction of the permeability.
Hayatdavoudi, A. 1998: Controlling formation damage caused by kaolinite clay minerals: Part II. SPE39464 paper, SPE International Symposium on formation damage control, Lafayette, Louisiana, February 18-19, 1998, 421–429.
Rosenbrand, E., Kjøller, C., Riis, J.F., Kets, F. & Fabricius, I.L. 2015: Different effects of temperature and salinity on permeability reduction by fines migration in Berea sandstone. Geothermics 53, 225–235.