BUOYANCY-INDUCED FLOW IN POROUS-MEDIA GENERATED NEAR A DRILLED OIL-WELL .1. THE ACCUMULATION OF FILTRATE AT A HORIZONTAL IMPERMEABLE BOUNDARY

A substantial amount of drilling fluid can invade a permeable bed duri ng the drilling of an oil well. The presence of this fluid, often refe rred to as filtrate, can greatly influence the performance of instrume nts lowered into the wellbore for the purpose of locating these permea ble beds. The invaded filtrate can also substantially alter the physic al properties of the porous rock. For these reasons, it is of great in terest to known where the filtrate goes upon entering the bed. The obj ective of this study is to quantify the influence of the difference in density between the filtrate and the naturally occurring formation fl uid on the shape of the filtrate front as the filtrate invades the for mation. This type of phenomenon is often referred to as buoyancy or gr avity segregation. In this study, Part 1, we determine the behaviour o f the filtrate as it accumulates (and spreads out) at a horizontal imp ermeable barrier within the formation. This is a combined theoretical and experimental study in which an X-ray CT scanner is extensively use d to determine the appropriateness and limitations of the simplifying assumptions used in the theory. In Part 2, the flow of the invading fi ltrate within the entire bed will be presented. The problem addressed in Part 1 may be viewed from the broader, more fundamental, perspectiv e, as a well-defined model fluid mechanics problem for flow in porous media. One fundamental issue infrequently addressed concerns the conse quence on the dynamics of the fluids of heterogeneities, always presen t to some degree, in consolidated porous solids. The X-ray CT scanner enables the assessment of the appropriateness of modelling such porous solids as spatially homogeneous, a very popular assumption. This stud y also addresses the limitation of the small-slope approximation when a fluid-fluid interface occurs in a porous solid, an approximation whi ch has enjoyed great success in free-surface fluid mechanics problems when no porous media is present.