Unconventional Drill Cuttings Analysis Using 23 MHZ 2-D NMR T1-T2 Mapping Techniques

Abstract

Although the ultra-low permeability of unconventional tight rocks negatively influences production, such a petrophysical property leads to reduced probabilities that drilling cuttings are invaded by drilling fluids before drill cuttings are collected at wellsite. In this study, higher frequency (HF, 23MHz) NMR 2-D T1-T2 mapping techniques are used for the first time to study fresh drill cuttings to evaluate influences of the drilling mud, and changes of the liquids inside drill cuttings. Results are compared to that of core samples to ensure the validity of the drill cuttings analysis. Selected fresh drilling cuttings of four different depths of a Permian basin well were analyzed using a 23MHz NMR instrument. The 2-D NMR T1-T2 mapping techniques with an echo spacing time of 0.07 ms and 61 T1 wait steps were used. NMR maps were taken on the “as received” drilling cuttings samples weighing in broad range of 17 to 50g to cover practically available ranges of sample volumes. The sizes of the cuttings are between 5 to 10 mm of any dimension. The initial samples were surface cleaned by wiping off visible mud, then mapped several times during a period of 5 weeks. Each NMR measurement took 15 to 30 minutes. NMR 2-D Maps of unconventional cuttings can provide total liquid porosity and saturations reliably when performed in conjunction with a Boyle’s law gas-filled porosity. The results are comparable to that of core samples. No significant differences were observed between “as received” and surface cleaned samples suggesting liquids in the drilling mud evaporated when samples arrived in the lab. It further suggests that the analysis at wellsite would enable us to work with cutting samples that are less contaminated or altered. All T1-T2 maps of drilling cuttings samples over the period of 5 weeks show liquid losses but at a relatively slow rate. The results indicate that the speeds of the oil and water evaporation from the samples depend on pore sizes of samples from different depths.