Injectivity decline in ultra-filtered water flooding of high permeability sandstone reservoir

Abstract

Water injection is a core element of geothermal, petroleum and waste water management industries where water is injected into the subsurface after treatment and filtration to comply with environmental regulations and water quality requirements. Some impurities still remain in the water even after the filtration process. It has been widely reported that water injection operations encounter severe injectivity decline which is of substantial concern for field management. Total dissolved and suspended solids in injection water are one of the significant reasons for injectivity decline. These solids are then filtered by the porous media and cause formation damage which can result in significant injectivity decline.
Major past studies have assumed typical filtration techniques (micro-filtration) where the size of remaining dispersed particles in the water are reduced to ~2 microns. Hence, the majority of the experimental studies have used suspended particles of ~1-5 microns to investigate injectivity decline. These studies show that initially internal filter cake starts to develop and after some transition time external filter cake is formed, after which injection face is almost completely plugged. However, not many studies have been performed with ultrafiltration where remaining dispersed particle size in injection water is reduced to nano size range.
In this study, injectivity decline by ultra-filtered water injection was investigated experimentally. To mimic ultra-filtered water, spherical silica nanoparticles of 120 nm diameter were used as dispersed particles in the injected water. First, stability study of nanoparticle colloid was carried out by varying nanoparticle concentration, brine compositions and pH. Hydrodynamic size and zeta potential measurements showed that there exists a salinity and pH range in which nanoparticle colloid remain within the expected size range.
Core flood experiments were conducted on Bentheimer sandstone core plugs. Pressure measurements along the core and influent/effluent analysis were used to study the transport and retention of nanoparticles in porous media. Experimental results showed about 50 to 70 percent less injectivity decline compared to micron size suspended particles. Furthermore, results showed that external filter cake does not form by nanoparticle flow through porous media if the injection fluid’s pH and salinity are kept within a defined range obtained from stability study. Only deep bed filtration takes place where three main retention mechanisms dominate i.e. surface deposition, plugging and entrainment.
Finally, a numerical model is presented in this study that describes deep bed filtration taking into account observed retention mechanisms. Model results are found to be in good agreement with experimental results.