Dynamic fluid loss through low permeability formations with natural fractures was experimentally investigated in a high-pressure, high-temperature dynamic fluid-loss cell to understand the effectiveness of particulate fluid-loss control additives. The study focused on understanding the relationship between the size distribution and shape of the fluid loss control material on leakoff control and on the cleanup potential.

Sandstone cores of 1-in. diameter and 3-in. length and of approximately 0.05 md were cracked on a press to simulate the naturally fractured low permeability formation. The effective permeability of the fractured cores varied from approximately 0. 05 to 2 md. The effective permeabilities of the fractured cores were measured in situ before and after the leakoff test to quantify damage caused by the external and internal filter cake formation. All tests were conducted at 150°F with linear and borate- or zirconium-crosslinked guar solutions and gels. The wall shear rate across the face of the core was varied by changing the flow rate to simulate a typical fracture shear history at the leakoff face. The core and cell were left overnight at the confining pressure (1500 psi) and temperature before flowing back with brine.

It was found that low-permeability formations with natural fractures have characteristics very similar to those of high-permeability formations. Crosslinked fluids not only are good in reducing fluid leakoff, but also provide higher matrix retained permeability than those obtained with linear polymer solutions. The concept of leakoff control effectiveness, which depends on the leakoff control characteristics and the final cleanup potential, is introduced to quantify the depth of invasion of the polymer (internal filter cake) in the core. The leakoff control effectiveness was used as a tool to compare the relative performances of various fluid-loss additives. The results show that the shape as well as the size distribution of the fluid-loss additive can have a significant impact not only on leakoff control but also on cleanup potential.

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