A Novel Method of Assessment of Spurt and Filtrate Related Formation Damage Potential of Drilling and Drilling-in Fluids

摘要

Solids as well as the liquid phases of drilling muds invade andinteract with reservoir formations that contain oil and gas.These invasion and interactions change the formationpermeability and in extreme cases may stop the fluid flowfrom the reservoir to the wellbore. The damage caused bymud spurt and drilling mud filtrate can influence the logresponse significantly due to the change in near boreholepermeability and displacement of formation fluids by the mudfiltrate. This dictates the assessment of the spurt and filtraterelated formation damage potential of drilling muds and drillinfluids to screen and select the best fluid for drilling thereservoir section of the borehole. This paper describes a novelmethod to highlight the formation damage potential of drillingmuds by assessing the erosional characteristics of mudcakesformed by different mud systems.A unique test procedure and a novel test facility have beendeveloped for experimental determination of mud spurt andfiltrate related formation damage potential of different mudsand drill-in fluids. The test set up consists of a wooden bridgeto fix the mud agitator, an erosion cell for closed loopcirculation of eroding fluid, a mud agitator to simulate thehydrodynamic condition of the borehole-drill collar annularsection, a lab jack to elevate the erosion cell to a desiredheight to maintain a constant clearance between the boreholewall (filter paper) and the extreme end of the rotating shaft.Experimental results indicate significant variations in theerosion coefficients of mudcakes for an identicalhydrodynamic condition and clearance between the boreholewall and the end point of a rotating shaft. Cakes formed byseveral water-based muds were evaluated using the newlydeveloped technique. The results indicate that the presence ofelectrolyte increases the erosion coefficient of bentonitemudcake and thus significantly increases the damagingpotential of bentonite mud. Ionic fluid loss additives such as PAC and CMC reduced the erosion behaviour of salt waterbasedbentonite mudcake and thus indicate their ability toreduce the spurt and filtrate related formation damage. Thepresence of non-ionic fluid loss additive ‘modified starch’shows little improvement of erosional behaviour of salt waterbasedmudcake and thus indicates severe formation damagepotential if the reservoir section is drilled using this type ofdrilling muds.