Presented at IADC/SPE Drilling Conference and Exhibition, San Diego, California, USA, March 2012.
Filter Cake Properties of Water-Based Drilling Fluids under Static and Dynamic Conditions Using CT Scan
Heterogeneity of the filter cake plays a key role in the design of chemical treatments needed to remove the filter cake. Filtrate invasion into the formation can substantially reduce the permeability of the near wellbore region and in turn, damage the formation. In addition, the nature and the thickness of the filter cake will increase the potential of differential pipe sticking. Extensive lab studies were conducted to measure the filter cake thickness and porosity of 69 pcf water-based drilling fluid using a CT (computed tomography) scanner and develop a relationship between the porosity and the permeability of the filter cake. An HPHT filter press was used to perform the filtration process under dynamic conditions at 250 °F and 300 psi differential pressure using 10-micron ceramic disks. Experiments were done at 1, 5, 10, 15, 20, 25, and 30 min. After each experiment, the filter cake was scanned using CT. The results obtained from the CT scan showed that the filter cake contained two layers with different properties. No compression period was observed during the filtration operation. The ratio of the layer closer to drilling fluid thickness was 70% of the filter cake, which will affect the amount of enzymes and acid, which are used to remove the filter cake. The layer closer to rock surface had a near constant thickness (0.04 in.), a decrease in porosity from 30 to 10% during the buildup of the filter cake, and an average permeability of 2.4 µd which was obtained from different correlations (Elkatatny et al. 2011a). The porosity of the layer closer to drilling fluid decreased from 0.25 to 0.0 %, due to poor sorting, which resulted in zero permeability. Using CT for the first time allowed us to evaluate existing models to predict porosity-permeability relationships of the filter cake.