Factors Affecting Well Productivity - II. Drilling Fluid Particle Invasion into Porous Media

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Abstract
Published in Petroleum Transactions, AIME, Volume 210, 1957, pages 132–139. The phenomenon of drilling mud particle invasion into consolidated porous media is discussed. The data and results are presented in two parts:a study of the ability of a mud containing sized particles to bridge and initiate filter cake formation on or in consolidated porous media, anda study of the degree and extent of reduction of the effective oil permeability of porous media by mud invasion. This study has shown thatbridging of pores by particles is required to initiate filter cake formation,a filter cake may be formed inside the pores of the invaded zone of the porous sample, andmud particles invade porous media of the type studied to a depth of at least 1 to 3 cm and cause substantial "permanent" reduction in effective oil permeability. Introduction: The first paper in this series described filtration characteristics of drilling muds and presented data on the surge phenomenon. It was shown that a reduction in permeability of the samples used resulted from exposure to the mud. The most plausible reason for a loss in permeability was the invasion of solid particles into the pores of the porous medium resulting in plugging of these pores. Several authors have pointed out that a loss of permeability of a sample occurs after exposure to muds. The particle invasion phenomenon was usually deduced, as in the first of this series of papers, from a loss in permeability of the sample after such exposure. Nowak and Krueger observed a cloudy filtrate coming from the Alundum samples used in their experiments. Electron micrographs of the solids contained in these filtrates provided good evidence of particle invasion into and through a porous medium, and certainly indicated that other mud particles probably invaded and remained in the pores. It has been suggested that mud particle bridging of pore entries (either internal or external, or both) occurred during the surge period of an experiment wherein a consolidated porous medium was exposed to a mud under a pressure differential.

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