Effect of pressurization rate and initial pore pressure on the magnitude of hydrofracturing breakdown pressure in tablerock sandstone

摘要

We carried out hydraulic fracturing tests in hollow cylinders of Tablerock sandstone (porosity: 26%) subjected to vertical (σ_v), and horizontal (σ_h) far-field stresses, and initial pore pressure (P_o). Pressure-time records reveal that hydraulic fracture initiation (breakdown) occurs before peak pressure is reached in the first pressure cycle. In a series of tests in which σ_h, σ_v, and P_o were kept constant throughout, breakdown pressure P_c increased significantly with wellbore pressurization rate, but appeared to asymptotically tend to an upper and a lower bound. These bounds correspond to fast and slow rates, respectively, as expected by De-tournay-Cheng criterion. However, tests conducted at a preset pressurization rate in unjacketed specimens (σ_h=P_o) revealed that (P_c -P_o) increased with confining/pore pressure, contrary to the constant (P_c -P_o) predicted by the Detournay-Cheng criterion. We modified the criterion by replacing the Terzaghi effective stress (σ~(eff)=σ-P_o) with a general effective stress (σ~(eff)=σ-βP_o, where 0 ≤β≤ 1), and determined β from the unjacketed test results. Several series of hydrofracturing tests in jacketed specimens reinforced the applicability of the modified Detournay-Cheng criterion to Tablerock sandstone in terms of correctly estimating the relationship between the unknown far-field stress and the typically known test parameters: breakdown pressure, initial pore pressure, tensile strength and pressurization rate.