Lost circulation problems encountered in drilling are magnified when using expensive oil-base muds. There are many lost circulation materials (LCM's) that perform effectively in water-base drilling muds, but very few that work, well in oil-base muds. Those that do work well are effective in controlling seepage loss only.

A laboratory study was undertaken to compare the performance of conventional lost circulation materials with a new material composed of thermoset rubber in different oil-base drilling muds. Laboratory data are presented comparing the effectiveness of mica, modified hydrocarbons, cellulose fibers, ground walnut shells, a blend of fibers, flakes and granules, and thermoset rubber in controlling mud loss to simulated medium-size fractured formations. Included in the evaluation were tests performed in a low toxicity mineral oil-base mud, two invert emulsion oil-base muds, and two other oil-base muds (one heavily weighted).

It is shown that the LCM's specifically designed to combat lost circulation in oil-base drilling muds such as mica, modified hydrocarbons and cellulose fibers failed at all tested concentrations in all five oil-base muds. The other conventional materials such as walnut shells-coarse, a 2:1 blend of walnut shells-coarse and walnut shells-medium performed satisfactorily only at high concentrations in the five oil-base muds. The coarse blend of fibers, flakes and granules worked satisfactorily at high concentrations in one mud only. The thermoset rubber performed effectively at tested concentrations in all five oil-base muds.

Two interesting results were observed during this study. It was found that granular type LCM's, such as walnut shells-coarse and thermoset rubber-medium, sometimes exhibit a phenomenon called channeling effect. This channeling effect occurs when a high pressure differential and insufficient solids concentration in the mud (that cannot build and sustain a bridge at the formation face, or within the formation matrix) causes many channels to form within the LCM deposit, thus leading to mud or gas migration through the channels and, finally, to a test failure. It was also found in some drilling muds that high concentrations of walnut shells or thermoset rubber contribute to poorer performance than at lower LCM concentrations.

The effectiveness of the thermoset rubber LCM in oil-base drilling muds is believed to be due to: optimized particle size distribution, oil wettability, combination of particle strength and deformability, thermal stability and mud compatibility. Based on the results of this study, a solution to the problem of controlling the loss of oil-base muds to fractured formations is proposed. Comparative performance data are also presented to facilitate the use of the conventional LCM's as well as the thermoset rubber material in oil-base drilling muds.

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