Filtration From Mud During Drilling

Abstract

Abstract This paper describes experimental investigations conducted at the California Research Corporation's model oil well. The first part describes filter loss from several drilling muds through bore hole walls during mud circulation and drill string rotation. The effect of mud properties and drill string rotation with mud circulation is described. Filtration from a bore hole full of drilling mud, not circulating, but under pressure for extended time periods, is also discussed. The second part describes filter loss to permeable formations from beneath a drilling bit where filter cake has no opportunity to develop appreciable thickness. If potential flow theory is applied to predict filtrate invasion from this hole bottom, large invasions are calculated. But model well experimentation shows that filtrate invasion from the hole bottom is controlled by mud particles that penetrate the formation ahead of the bit. This experimentation provides data to estimate the effect of mud particle penetration on fluid loss to formation. The third part presents an estimate of mud filtration during drilling of a well. Introduction As oil wells are drilled by the rotary method, drilling mud circulates up the annulus between drill pipe and well wall. Drillers adjust mud density so that the pressure at the bottom of the mud column is several hundred psi greater than the pressure of reservoir fluid. As a result of this pressure difference, the liquid part of the mud filters into the rock around the bore and mud solids deposit as a filter cake on the well wall. For many reasons the petroleum industry, within the past 20 years, has spent much money and energy to determine the volume of mud filtrate that enters rock around the well bore and dedicated much research effort to reduce this volume. Among the reasons for attempts to determine the volume of mud filtrate and to reduce this volume are the following:If filtrate damages the permeability of oil sand, the resultant damage to oil well productivity will depend on the distance that the filtrate invades the oil sand; reduction of filtrate volume may, therefore, increase well productivity.Filtrate that penetrates shale sections may cause the shale to swell and to slough into the well bore. Uncontrolled sloughing may stick drill pipe. Reduction in filtrate volume may reduce drilling trouble.Electric log resistivity curves are changed by invasion of a mud filtrate; the change depends on the depth to which filtrate invades. Knowledge of this depth is necessary before resistivity logs can be interpreted accurately.