Lost circulation is a major contributor to nonproductive time (NPT). It occurs during drilling as well as while cementing the well. To cure the losses, lost circulation materials (LCMs) are widely used. In severe cases when the fracture size is larger than the LCMs, sealing is not effective and does not sustain high differential pressures. The focus of this study is to provide a solution to plug wide natural fractures at high differential pressures using LCM in cement.
Evaluation of LCMs was based on performance tests conducted using a permeability plugging apparatus (PPA). The individual LCMs and combinations of them were used to perform the study. High differential pressures of 2,500 psi and slot widths of 1, 3, and 5 mm were used during testing. Observations were made in terms of time to form the seal, cumulative volume of cement slurry lost before a seal was formed, and the efficiency of the seal. The cement slurry loaded with LCM was tested for rheology, suspension, and compressive strength development.
LCMs having different geometry, such as granular, lamellar, fibers, and resilient graphite, were studied either individually or in combination. For a 5-mm slot and differential pressure of 2,500 psi, it was observed that the combination of LCMs performed superiorly compared to the individual LCMs in terms of loading required, time to plug, and slurry lost before an effective seal was formed. Fibers are bridging material that form an interlocking net over the pores or fracture and prevent other particles and fluids from passing through. This physical interaction of LCMs with the cement slurry forms an effective seal that demonstrates resiliency at high differential pressures and wide fracture widths. Inclusion of these LCMs has minimal effect on other slurry properties, such as compressive strength and stability. This paper also presents observations for various potential fracture widths vs. LCM combinations based on these results.
A combination of LCMs was developed that can be used in extreme loss circulation zones to seal wide fractures at high differential pressures.