In-situ visualization and characterization of mud-filtrate invasion and filter cake deposition using time-lapse X-ray micro-computed tomography (micro-CT)

Borehole measurements acquired during and shortly after drilling of oil and natural gas wells are often subject to uncertainty resulting from the effects of mud-filtrate invasion. Accurate interpretation of these measurements requires a thorough understanding of the invasion process, including the deposition of filter cake, or mudcake, on the borehole wall and the flow of mud filtrate around the wellbore. Failure to properly account for these effects can lead to inaccurate estimation of petrophysical properties, errors in the quantification of hydrocarbon reserves, and misinformed long-term investment decisions.

In this work, we designed and performed novel laboratory experiments that involved water- and synthetic oil-based mud-filtrate invasion in cylindrical rock core samples of varying petrophysical properties. Using high-resolution X-ray micro-computed tomography (micro-CT), we were able to visualize and characterize mudcake deposited on the borehole wall and the distribution of mud filtrate in the core samples as a function of both space and time during the experiments. Additionally, with the aid of a new set of generalized filtration equations, we were able to reliably estimate average in-situ mudcake properties, including mudcake porosity and permeability, during the experiments.

Experiments were performed to study the influence of drilling fluid properties and rock petrophysical properties on the process of mud-filtrate invasion. Results for the rate of mud-filtrate invasion and the time evolution of mudcake thickness were consistent with behavior predicted by the derived generalized filtration equations. For rock core samples with permeability exceeding 8.9 mD, aside from a brief initial period of spurt loss, the mud-filtrate invasion rate was primarily controlled by the properties of the deposited mudcake rather than the distinct petrophysical properties of the core samples. In-situ mudcake properties, including porosity and permeability, generally remained constant while mudcake was thin, justifying the use of average in-situ values with models derived assuming incompressible mudcake. Filtrate flow through the core samples, including the movement of the invasion front and the redistribution of mud filtrate within the invaded zone, was strongly influenced by capillary forces, which caused deeper, more-uniform invasion than would be expected in the absence of capillary-driven flow, both in homogeneous and spatially heterogeneous rocks.