A calibration method for estimating mudcake thickness and porosity using NMR data

Mudcake thickness affects wellbore operations such as differential sticking, well completion, and wellbore clean-up operations. One of the properties for which a drilling fluid is screened during laboratory tests is the thickness of the mudcake it produces. Such laboratory tests are routinely conducted on ceramic discs or rock samples, at a specified temperature and pressure. Mathematical models have also been reported for predicting cake thickness and mud induced formation damage. The problem here is that the conditions upon which both laboratory tests and mathematical models are run do not match all conditions in the wellbore during drilling. Calliper tools can be used to directly quantify mudcake thickness in wellbore, however, they cannot differentiate between a sloughing hole and a mudcake. This study therefore explores the potentiality of nuclear magnetic resonance (NMR) logging tools to measure the thickness and porosity of mudcake, and mud induced formation damage in wellbores. This proposed approach utilizes measurements routinely taken from a multi-frequency NMR logging tool, namely T₂ relaxation times at three depth of investigations (DOI). For this specific application, the first DOI captures the mudcake and the near wellbore region, the second DOI captures an inner radius (invaded zone), while the third DOI measures the virgin zone. NMR measurements in these three DOIs were simulated in our laboratory experiments. From the acquired NMR measurements, we identified characteristic T₂ – curves for mudcakes and solid infiltration and then defined a workflow to obtain them. The areas under these characteristic T₂-curves showed good correlations with the mudcake porosity and thickness. Based on these, several calibration curves were developed for mudcake thicknesses, porosity and formation damage. The calibration curves appear to be robust since they were obtained from a variety of rock types with a wide range of porosity and permeability.