Borehole measurements are subject to uncertainty resulting from the effects of mud-filtrate invasion. Accurate interpretation of these measurements relies on properly understanding and correcting for mud-filtrate invasion effects. Although attempts to experimentally investigate mud-filtrate invasion and mudcake deposition have been numerous, the majority of published laboratory data are from experiments performed using linear, rather than radial geometry, homogeneous rock properties, and water-base, rather than oil- or synthetic oil-base drilling mud.
This paper introduces and applies a new experimental method developed to more accurately represent conditions in the borehole and near-wellbore region during, and shortly after, the drilling process, when the majority of wellbore measurements are acquired. Rather than using a linear flow apparatus, these experiments are performed using cylindrical rock cores with a hole drilled axially through the center. Radial mud-filtrate invasion is induced by injecting pressurized drilling mud into the hole at the center of the core while the outside of the core is maintained at a lower pressure. During the experiments, the core is rapidly and repeatedly scanned using high-resolution X-ray micro-computed tomography (micro-CT), allowing for the spatial distribution of mud-filtrate and mudcake thickness to be visualized and quantified as a function of time. Therefore, using these experiments we are able to accurately evaluate the influence of various rock properties, such as the presence of heterogeneity, and fluid properties, including water-versus oil-base mud, on mud-filtrate invasion and mudcake deposition.
Costs for offshore drill ships exploring and appraising oil and natural gas prospects in the Gulf of Mexico often exceed $500,000 per day (Transocean, 2019). Borehole measurements acquired during, and shortly after drilling of these wells are subsequently used to support long-term investment decisions that can extend into the billions of dollars. Rapidly obtaining reliable data that accurately describes the true reservoir properties is essential for evaluating new prospects, quantifying reserves, and predicting future production performance.
Borehole measurements, including electrical resistivity, neutron porosity, density, sonic slowness, formation pressure, and reservoir fluid sampling operations, are subject to uncertainty resulting from the effects of mudfiltrate invasion. Additional uncertainty arises when these measurements are acquired in heterogeneous formations, across capillary transition zones where the effects of mud-filtrate invasion vary with depth, and when they are performed in the while-drilling environment where dynamic conditions persist in the borehole and near-wellbore region while measurements are being acquired. Accurate interpretation of these measurements requires properly understanding and correcting for the complicated behavior of mud-filtrate invasion and mudcake deposition.
