Laboratory Evaluation of Drilling Mud Systems for Formation Damage Prevention in Horizontal Wells

Abstract

Abstract Drilling-induced formation damage has played a significant role in the failure of some horizontal wells. The objective of this laboratory study is to provide insights into this complex subject by investigating the main formation damage mechanism(s) and systematically quantify the damage effect caused by the newly formulated drilling mud systems, which are designed to minimize formation damage. Eleven commercial drilling mud systems were tested in this study and they produced a very wide range of permeability reduction on Berea sandstone cores with similar properties. Bentonite and cellulose based bridging agents appear to cause higher degrees of damage than ground calcium carbonate. High acid solubility of these muds is conducive to much improved permeability after acid stimulation. Results of additional dynamic mud leak-off experiments show that the most important parameters which affect regained permeability are:mud composition,core permeability,overbalance pressure,filter cake rigidity,pressure drawdown during backflow andmud particle versus rock pore-throat size distribution. The depth of mud solid invasion is limited to the first few centimetres of the injection end but the filtrate usually invades the whole core. Furthermore, repeated removal of the existing mud cake can cause much deeper invasion and more severe damage. Introduction Objectives and Scope The objective of this laboratory study is to provide insights into this complex subject by investigating the main formation damage mechanism(s) and systematically quantify the damage effect caused by these newly formulated drilling mud systems, which are designed to minimize formation damage. The first part of this laboratory study is to compare various commercial drilling mud systems, using similar cores and experimental conditions. The second part is to study the effect of various rock types and permeabilities on regain permeability of three different muds. The third part is to investigate the effect of individual drilling mud additive on the overall permeability reduction. In addition, the effect of common production, and stimulation procedures on the removal of mud cakes is also investigated in these experiments. Literature Review Horizontal well drilling has been successfully applied in large numbers of oil and gas fields worldwide. However, some of the progress has been hindered by formation damage which occurred during drilling. Although the damage mechanisms responsible for productivity loss in horizontal wells are similar to those which have been experienced in vertical wells, the damage mechanism and the extent of the damage are not well defmed in horizontal wells. The following is a list of formation damage mechanisms which may occur during drilling of horizontal wells.Mud solids plugging;Mud filtrate and formation fluids incompatibility;Polymer screening and retention;Wettability alteration;Fines migration and clay swelling;Water blockage;Geomechanical stress alteration; Formation damage prevention may not always be possible because:the drilling time of the horizontal segment in the producing zone is usually many times more than a typical vertical well. Therefore, the mud invasion is much deeper and more severe.the very low drawdown pressure that is needed to produce from a typical horizontal well reduces the viscous force available to clean up near wellbore mud damage. P. 713