Preparation and Application of a Controlled-Setting Lost-Circulation Material for Severe Lost Circulation

To address the challenges associated with severe lost-circulation formations, including large loss-channel dimensions, rapid fluid losses, and the tendency of conventional bridging or physical-filling materials to accumulate at fracture entrances rather than penetrate deeply into fractures and form stable pressure-bearing plugs, a slag–fly ash–gypsum–sodium ethylenediamine tetramethylene phosphonate (EDTMPS) controlled-setting lost-circulation system was developed in this study. A 4 wt% bentonite base slurry was used as the dispersion medium, while a slag–fly ash blend served as the principal cementitious component. Gypsum was used to regulate the setting reaction, and EDTMPS was employed to control the slurry-thickening process, thereby coordinating slurry pumping, fracture filling, and in situ setting. The operational feasibility, consolidation capability, and short-term pressure-bearing performance of the system were evaluated through formulation screening, tests of slurry placement and hardened-material properties, water-based drilling-fluid contamination evaluation, and pressure-bearing tests in regular fractures with apertures of 1–5 mm. The results showed that, at a slag-to-fly-ash mass ratio of 6:4, the hardened material exhibited a compressive strength of 12.91 MPa. Within the investigated dosage range, the highest observed 1 d and 3 d compressive strengths were both obtained at a gypsum dosage of 2.0%. Under conditions of 150 °C and 50 MPa, the addition of 1.5 g of EDTMPS extended the slurry thickening time from 2.6 h to 7.3 h, while the compressive strength of the hardened material after 24 h of curing reached 17.18 MPa. At a water-based drilling-fluid contamination ratio of 30%, the compressive strength of the hardened material was 13.86 MPa, corresponding to a strength-retention ratio of 80.7%. In straight, constant-aperture fractures with apertures of 1.0, 2.0, 3.0, 4.0, and 5.0 mm, the maximum pressures sustained by the plugs before breakthrough were 16, 15, 14, 13, and 12 MPa, respectively, and decreased with increasing fracture aperture. These results indicate that, within the formulation range, water-based drilling-fluid contamination conditions, and short-term pressure-bearing conditions in regular fractures investigated in this study, the system can coordinate slurry-thickening control, 24 h hardened strength, contamination tolerance, and pressure-bearing performance in regular fractures. The findings provide a verifiable formulation-design approach for reconciling the placement-time window of lost-circulation materials for severe lost circulation with their post-placement pressure-bearing capacity.