Particulate invasion from drilling fluids

We investigate the invasion of solids and their mobility during cleanup. We study the effect of weighting agent particle size on rock substrates of va rying permeabilities. We find permeability damage increases hut flow initia tion pressures decrease with increasing substrate permeability. We obtain quantitative profiles of solids invasion by scanning electron mic roscopy/x-ray mapping and synchrotron energy-dispersive x-ray diffraction t omography. We compare these profiles to core sectioning data. We find invas ion profiles drop steeply but fines are observed deep within the core. We e xamine the effect of backflooding on the invasion profile. Near-surface dam age is reduced but deeply invaded fines are unaffected by backflow. We develop a deep bed filtration model for solids invasion and consequent p ermeability reduction. This model is compared to the profiles obtained in t he invasion experiments. We find that we can tit the experimental invasion profile for monomodal particles using a single trapping coefficient. Backfl ow is modeled by reversing the flow rate. We postulate a phenomenological r ate of erosion to untrap particles in line with experimental observations ( 30%). When erosion is included in the model, a peak in the backflow pressur e is found. This peak may be correlated with the experimentally observed fl ow initiation pressure.