Drilling fluids (especially inverted muds) utilize several chemicals for mud stability, borehole integrity, loss control. During drilling, invasion of components of the mud (fines, surfactants, etc) can result in skin damage and productivity impairment. The goal of the paper is to measure change in oil permeability on core plug samples, due to imbibition of mud filtrate in tight reservoirs using high frequency 3D Nuclear Magnetic Resonance spectroscopy. The depth of mud filtrate invasion across the length of core sample is also measured. This study thus provides an experimental measurement of skin factor for tight reservoirs. This learning can help in choosing between mud systems or completion fluids to minimize damage to the reservoir.
The experimental methodology starts with measuring helium porosity and permeability on core plug samples (Ratcliffe carbonate reservoir). As-Received T2, T1T2 NMR (12MHz) scans are done to understand as-received water and oil saturations and Micro-CT to identify cracks and heterogeneity within the sample. Sample is then saturated with 1% MnCl2 doped formation water solution to restore sample to 100% Sw and make it water-wet. MNCl2 being paramagnetic helps shift the NMR response to a lower T2, and aids in better separation of the water and oil signals on NMR scans. After saturation, sample is scanned again in the NMR spectrometer to confirm complete saturation and then loaded in a special core flooding vessel which does not have an NMR signal. Produced crude is injected into the sample at reservoir conditions, and initial oil permeability of the sample is measured at irreducible water saturation (Swi). NMR T2, T1T2 scans are carried out continuously during crude injection, to measure water and oil saturations and Spatial T2 scans are performed to monitor the saturation front of injected fluid through the sample. Mud filtrate is then injected in the sample from the bottom at suitable differential pressures encountered during drilling, while upstream side of the plug is kept at reservoir pressure with the crude. After 12 hours to a day of mud filtrate imbibition, crude injection is resumed to measure flow rates and permeability.
Samples show permeability degradation of 20% to 70% after mud filtrate invasion. Higher permeability samples (700-2000 nd) show higher degradation than low permeability samples (200 nd). High frequency 3D NMR scans provide a reliable and accurate way of continuously measuring oil and water saturations and monitoring the saturation front of different fluids (water, formation oil, mud filtrate) as they are being injected. The mud filtrate invasion is about a third of the sample (5cm long) during the imbibition process.
This study provides an experimental measurement of skin factor and permeability degradation measurement for tight reservoirs due to mud filtrate invasion. This study can thus help in choosing between mud and fluid systems that minimize permeability degradation.