Previous studies indicated that the filter cake was heterogeneous and contained two layers with different properties such as thickness, porosity, and permeability. The difference in properties will affect the filtration process and the filter cake buildup for each layer. Previous models of filter cake buildup assumed that the filter cake was homogenous and contained one layer, which undergoes compression followed by buildup.
The objective of this study is to measure the thickness of the two layers of the filter cake and its effect on the filtration process by using a new approach. Our objective also is to develop a valid method to determine local filtration properties, porosity and specific resistance, for each layer in the filter cake. A HPHT filter press was used to perform the filtration process under static conditions (225 °F and 300 psi). CT (computed tomography) scanner was used to measure the thickness and porosity of the filter cake.
The results obtained from the CT scan showed that there was an inflection time at 7.5 min of filtration at which the process changed from the compression to buildup. Compression region before 7.5 min, in which the thickness of the external and internal layer decreased and there is a great change in the average porosity in both layers. Buildup region after 7.5 min, in which the thickness of the internal layer increased from 0.03 to 0.07 in. and the average porosity decreased from 35 to 15%. The thickness of the external layer also increased from 0.04 to 0.09 in. and the average porosity decreased from 25 to almost 0.0% due to poor sorting in this layer, which prevented more filtration thereof. The new approach better explained the change in thickness and porosity of each layer of the filter cake with time.