Laboratory Study of Lost Circulation Materials for Use in Both Oil-Based and Water-Based Drilling Muds

Abstract

Summary. Lost circulation is one of the most troublesome and costly problems encountered in drilling. This problem is magnified when expen-sive oil-based muds are used. There are many lost circulation materials (LCM's) that perform effectively in water-based muds but very few that work well in oil-based muds. Those that do work well are effective in controlling seepage loss only. This paper presents laboratory test results that compare the performance of conventional LCM's with a new high-performance material, composed of thermoset rubber, in controlling mud loss to simulated fractured formations in both oil- and water-based muds. Field case histories describe the effectiveness of the thermoset rubber in fractured formations with severe lost circulation problems. Introduction Lost circulation is both troublesome and costly. Drilling fluids are usually lost to natural or induced formation fractures. These fluids may also be lost through highly permeable formations. Lost circulation can take place while drilling is in progress or during "trips," when pressure surges occur because of the lowering of drillpipe or casing into the hole. After the lost circulation occurs, the level of the drilling, fluid in the annulus may drop and then stabilize at a particular level. depending on the formation pressure. Fractured, vugular, cavernous, or highly porous formations generally have high permeabilities and/or large openings that can accept considerable volumes of drilling fluids. If fractures and voids are large, they cannot be plugged by the solids present in drilling fluids-e.g., clays and formation Cuttings. Lost circulation, or lost returns, is defined as the loss of the drilling fluids or cement slurries into formation voids. This loss may vary from a gradual lowering of the mud level in the pits to a complete loss of returns. The loss of drilling mud or cement slurry results in increased drilling time, loss of expensive mud, plugging of productive formations, and/or loss of well control. Even with the best drilling practices, circulation losses can and will occur. Loss zones can be classified as seepage loss (when the severity of the loss is 1 to 10 bbl/hr [0.16 to 1.6 m3/h]), partial loss (when the severity of the loss is 10 to 500 bbl/hr [1.6 to 80 m3/h]), and complete loss (when the severity of the loss is greater than 500 bbl/hr [greater than 80 m3/h]). Seepage losses can occur in any type of loss zone and in any type of formation when the LCM's in the mud are not fine enough to complete the seal. Partial losses occur in gravels; small, natural, horizontal fractures; and barely opened, induced vertical fractures. Complete losses occur in long, open sections of gravel; large, natural horizontal fractures; caverns; interconnected vugs; and widely opened, induced fractures. Lost circulation problems in drilling are not confined to any one area. They may occur at any depth where the total pressure exerted against the formation exceeds the formation breakdown pressure. Previous experimental work and field experience have shown that the whole mud cannot be forced into a rock formation without fracturing it. Even gravel must have a permeability of about 300 darcies before the mud can be forced into and through the natural openings between grains. It has been established that the maximum allowable drilling-fluid loss is on the order of 1 bbl/hr [0.16 m3/h], as measured in the mud pits at the surface. Remedial measures must be taken when the mud loss exceeds 1 bbl/hr [0.16 m3/h]. In general, four types of formations are responsible for lost circulation: natural or induced fractured formations, vugular and/or cavernous formations, highly permeable formations, and unconsolidated formations. When the whole mud is lost to a formation, the cause is cracks or permeability large enough to prevent a sealing filter cake from being laid on the wellbore. If the formation is fractured mechanically, these fractures may seal themselves once the pressure is relieved. If large quantities of mud are lost to these fractures, however, they may be washed out, causing void spaces similar to naturally occurring high-porosity, high-permeability zones. In most cases, an LCM is spotted in the loss zone. Sometimes it is used as a part of the mud system. Many authors have contributed preventive techniques and materials to combat lost circulation during drilling of an oil well. A laboratory study was undertaken to compare the performance of conventional LCM's with a new material composed of thermoset rubber in both oil- and water-based drilling muds. SPEDE P. 229^