Most subterranean rocks contain natural fractures. Fluid loss severity in reservoir sections increases with increasing width of the fractures. Apart from the economics of lost drilling fluids, preventing formation damage in the reservoir section is of paramount importance. The application of non-damaging lost circulation material (LCM) has been the traditional solution to control losses in these scenarios. The performance of such LCM mixtures is usually determined with a Particle Plugging Apparatus (PPA), using aloxide discs for evaluating fluid loss. These discs more closely emulate a permeable sandstone, with varying pore throat sizes, than a natural fracture. The formation of an impermeable plug which can sustain higher wellbore pressure is desirable when treating a fracture, but in a producing formation the plug is removed by the produced fluid or by a remedial treatment.
To better emulate both natural and induced fracture geometry, slotted discs of constant width were introduced; however these slots represent only the face of the fracture where LCM forms the bridge. The bridge may become eroded due to shear stress of the drilling fluid, leading to reestablishment of lost circulation. To remove these limitations, a new tapered slot was designed in which the fracture opening size varies from its face to its endpoint over a certain length, more closely resembling a fracture.
Different combinations of ground marble (acid-soluble) were tested using the tapered slot. Many times it was observed that although the slot was completely filled with LCM, there was continued fluid loss. Considering this, components of high fluid loss pills for such application need to be redesigned. Cellulosic and polylactide (PLA) fibres (5-15% V/V of LCM) were used in combination with particulates resulting in significant improvement in fluid loss control. Other mechanical properties of particulate material were also studied and their importance in controlling lost circulation control established. This paper reviews the application of this testing equipment and methodology with non-damaging fibers for lost circulation control to minimize formation damage in reservoir sections.