Experimental study on fracture initiation and propagation law of altered gold ores by hydraulic fracturing

The initiation and propagation law of hydraulic fractures in altered gold ore hold paramount importance for the hydraulic fracturing exploitation of gold mines. This study conducted true triaxial hydraulic fracturing experiments on altered gold ore under varying water injection rates, minimum horizontal principal stresses, and angles between water injection holes and joint surfaces. Results indicate that:(1) when the difference in horizontal principal stress for altered gold ore ranges from 1 to 3 MPa, initiation stress spans 22.2 to 40.0 MPa. With increasing water injection rates(from 5 mL/min to 15 and 30 mL/min), initiation stress rises by 17.4% and 34.3%, respectively. An elevation in the minimum horizontal principal stress(from 4 MPa to 5 and 6 MPa) results in initiation stress increases by 37.4% and 80.2%, respectively. The angle between water injection holes and joint surfaces(from 0°to 45 and 90°) induces initiation stress increases by 6.3% and 24.9%, respectively. Altered gold ore, under similar in-situ stress conditions, exhibits higher initiation stresses compared to sedimentary rocks. (2) Hydraulic fractures in altered gold ore can be categorized into two modes: linear and polygonal propagation. Increasing water injection rates(from 5 mL/min to 15 and 30 mL/min) results in heightening perpendicularity between the fracture propagation direction and the minimum horizontal principal stress direction, with a reduction in the number of turns from 2 times to 0. An increase in the minimum horizontal principal stress(from 4 MPa to 5 and 6 MPa) leads to an increase in the number of turns from 0 to 1 and 3 times, with larger turning angles. With the increasing angle between water injection holes and joint surfaces(from 0°to 45 and 90°), interference from joint surfaces is intensified, causing propagation deviations towards the joint surfaces. Hydraulic fractures in altered gold ore predominantly exhibit main fractures, with fewer and simpler secondary fractures, whereas sedimentary rocks display a higher number of complexly distributed secondary fractures. (3) The fracture surfaces in altered gold ore can be classified based on morphological characteristics into smooth and rough surfaces. Increasing water injection rates(from 5 mL/min to 15 and 30 mL/min) results in a decrease in protrusion size on the fracture surfaces, with the protrusion area decreasing from 63% to 45% and 0%. Increasing the minimum horizontal principal stress(from 4 MPa to 5 and 6 MPa) leads to an increase in the protrusion area from 0% to 40% and 45%. Increasing the angle between water injection holes and joint surfaces(from 0°to 45°and 90°) all maintains joint surface features on the fracture surfaces with smooth and non-protruding characteristics. The fracture surfaces of altered gold ore are rough, while those of sedimentary rocks are comparatively smoother. © 2024 Academia Sinica. All rights reserved.