Laboratory Evaluation Of Formation Damage Caused By Drilling Fluids And Cement Slurry

A laboratory investigation was conducted to study the formation damage caused by drilling fluid and cement slurry. Investigations were performed on two different sandstone core samples typical for gas and oil reservoir in Germany. Core samples of 25 cm long and 2.5 cm in diameter were contaminated with commonly used four different drilling fluids and a typical class-G cement slurry. A 10% HCI acid was used to remove the damage of the contaminated core samples. Damage caused by drilling fluids was evaluated by means of damage ratio and sectional damage ratio. The value of sectional damage ratio was used to determine the depth of permeability impairment. Scanning electron micrograph study along the axis of the segmented core were made to determine the depth of invasion of solid particles and polymer molecules. The results have shown that the current practice of using low-solid drilling fluid weighted by ground calcium carbonate can cause about 40% reduction in permeability of the core samples extending over 20 cm. The permeability damage of the core samples caused by bentonite polymer drilling fluid was observed to be about 75%. However, this severe damage is only confined to a depth of about 5 cm which can be overcome by the perforation work. Introduction All types of porous formation act themselves on an in-depth filter. Damage is caused by drilling fluids and subsequently by cement slurry by two mechanisms:invasion of solids that plug the formation; andinvasion of filtrate that can hydrate interstitial clay, free formation fines, precipitate solids, change formation wettability, etc. (1,2). Invasion of solids and filtrate can be reduced to an acceptable level by the formation of low permeable filter cake. During the filtration process, panicles of certain sizes bridge the formation pores and establish a base on which the filter cake can form. Particles considerably smaller than the pore opening invades the formation. However, panicles of certain critical sizes stick at the neck of the pore channel and form an internal bridge(3,4). Once the primary bridge is established, fine panicles successively fill up the interstitial spaces to form a lightly packed layer of filter cake. Thereafter, only filtrate invades the formation. In order to form an effective base for the filter cake, drilling fluid must, therefore, contain panicles ranging in size from slightly less than the largest pore size of the formation to be drilled down to about one-third of that size(3,4). Polymers used in drilling fluid tie the solids in the filter film together and fill the smallest spaces of the filter cake. These results in a filter cake of very low permeability (2 to 5 × 10−1 md)(4). In fact, upon hydration polymers can acquire a chain length that may exceed 0.1 cm which is comparable to the width of a medium size clay. If these polymers deeply penetrate into the formation, they can cause considerable impairment(4). Ground calcium carbonates are acid degradable and they are often used as bridging materials.