A Pre-Flush for Lost Circulation Control Based on Expandable Fiber-Grafted Resin

With the exploration of oil and gas into deep and complex formations, lost circulation in fractured formations has become a complicated problem affecting the quality of well cementing. The pre-flushes for lost circulation control presently in use exhibit poor adaptability to fractured and high-temperature formations, making it difficult to ensure well cementing safety and return height of cement slurry. To solve this problem, a novel pre-flush for lost circulation control system was presented based on expandable fiber-grafted resin bridged with compounded rigid calcite and sepiolite fibers. Using Fourier transform infrared spectroscopy, the structure of the fiber-grafted resin was characterized, and the rheology, plugging capacity, compatibility and acid solubility were evaluated. The results show that the pre-flush can seal fractures of 1 – 4 mm in width, and has pressure bearing capacity exceeding 5.0 MPa, demonstrating that the pre-flush has favorable adaptive filling characteristics and improves significantly the plugging effect in fractured formations. The pre-flush exhibits stable rheology, the high-pressure filtration rate of which is less than 30 mL, and it prevents solid particles from invading fractures. The pre-flush is well compatible with cement slurries, and it basically shows no adverse effect on the development of the compressive strength of a cement slurry. For fractured reservoirs, the pre-flush can be removed in late stage with low concentration hydrochloric acid, a hydrochloric acid of 5% dissolve 92.97% of the plugging zone. This pre-flush for lost circulation control can significantly reduce the risk of lost circulation and mitigate damage to fractured reservoirs, providing a new technical approach for lost circulation control in cementing wells penetrating fractured formations.