Evaluating drilling fluid infiltration in porous media – Comparing NMR, gravimetric, and X-ray CT scan methods

The infiltration of drilling fluids into underground rocks during drilling can cause adverse effects such as underground fluid loss, formation damage, and borehole collapse. An effective filter cake formed by the drilling fluid on the borehole wall can minimize such adverse effects to a degree that is dependent on the characteristics of the drilling fluid. For this reason, drilling fluids are routinely screened in the laboratory based on the sealing properties of their filter cakes. However, there appear not to be a universally adopted method for characterizing filter cakes. This study utilizes a nuclear magnetic resonance (NMR) approach to characterize filter cakes and then compares the result with some existing methods. Fluid loss tests were conducted to form filter cakes over four consolidated sandstone and carbonate rock samples. NMR measurements were conducted pre and post filter cake deposition. NMR measurements of filter cake porosity and infiltrated solids were compared with those from conventional methods such as gravimetric and X-ray computer tomography (CT) measurements. The results obtained from the three methods show significantly different values. To resolve this, cross correlations plots were made between measurements obtained from the three methods and the measured cake thickness. NMR measured porosity and infiltrated solids showed an excellent correlation with the measured cake thickness compared to the other two methods. However, the porosity values from NMR is significantly lower than the values obtained from the other methods. Through sensitivity analyses, we provide reasons why values from other methods are high and not as representative as the NMR method. We also gave a detailed explanation why NMR method is more accurate and useful compared to the other methods.