Experimental Study on Rock Strength Parameters of Ultra-Deep Carbonate Reservoirs Under Multiple Working Conditions with Scratch Test

Accurate determination of rock mechanical parameters for ultra-deep carbonate reservoirs (>9000 m) is critical for wellbore stability evaluation and acid-fracturing parameter optimization, yet systematic experimental data for such depths remain scarce. This study investigates grainstone cores from the YS Section (9199.8–9624.0 m) of Well XY in the Tarim Basin using continuous scratch tests—a minimally destructive method providing continuous strength profiles—complemented by conventional uniaxial compression tests. Five working conditions are systematically compared: original (unsoaked) state, drilling-fluid instantaneous contact (0 h), fresh water immersion (24 h), drilling-fluid immersion (24 h), and 15% hydrochloric-acid immersion (24 h). In the original state, scratch-test averages yield a uniaxial compressive strength (UCS) of 179.9 MPa, internal friction angle of 34.2°, and cohesion of 47.6 MPa. None of the three water-based working fluids causes measurable strength degradation within 24 h, with UCS values of 191.6, 186.0, and 190.9 MPa, respectively, attributable to the low water sensitivity of carbonate minerals. In contrast, 24 h acid immersion reduces UCS to 109.2 MPa (−39.3%), cohesion to 29.0 MPa (−39.0%), and internal friction angle to 27.1° (−20.6%), driven by intergranular cement dissolution, with strong sample-dependent variability (individual UCS reductions ranging from 12.6% to 48.0%). Scratch depth significantly influences results: UCS at 0.20 mm depth is 30.3% lower than at 0.18 mm. Calibration against rigid-testing-machine UCS yields a ratio coefficient α = 0.47~0.61. A value of 0.56 is set for α, giving a recommended engineering UCS of 100.7 MPa. These findings provide an experimental basis for wellbore stability analysis and acid-fracturing design in ultra-deep carbonate reservoirs.