Distinguishing Filtration Mechanisms by X-Ray Tomography: Quantification of Simultaneous Internal Deposition and External Filtercake Buildup in Sandstone

摘要

Many oil-field operations involve the injection of fluids into the formation around a well. In many cases, the fluids contain colloidal particles, either initially present or introduced during the operation through dirt or naturally-occurring particles. Therefore, all injection schemes suffer from injectivity decline. This injectivity decline is caused by clogging of the formation by particles, which form an external filter cake on the surface of the formation and block the pores inside the formation. This article reports on a set of crossflow experiments in which a suspension of particles is flowed over the surface of a sandstone core, with a portion of the suspension entering the core itself. An external filtercake is formed on the surface of the core, and internal deposition occurs inside the core. The experiments are conducted in a CT scanner, allowing measurement of the internal deposition in real time. Thus, although the time-dependent build-up of the external filtercake thickness cannot be measured directly, it can be deduced by mass balance, allowing for the crossflow and permeant effluents. This enables correlation of the external and internal mass deposition with the measured increase in injection pressure. We use the cumulative mass curves of the hematite deposited in the cake, core and in the effluent to model the dependence of the pressure drop on cake and core deposition, and compare the predicted pressures with the actual measured pressures. These results are very relevant to operational issues associated with injection or leak-off, including hydraulic fracture propagation, drilling-mud impairment and produced water matrix re-injection. The results are also relevant to water injection under fracturing conditions, since impairment of the wall of the fracture is one of the mechanisms governing fracture growth, and the implications of the results for this process are discussed.