Objective, compliance-corrected hydraulic-fracturing stress determination and stress–hydraulic coupling in fractured crystalline rock: open questions from the Xinjiang HLW candidate-area campaign (Zhao et al., 2015)

Hydraulic fracturing (HF) is the standard borehole method for measuring in-situ stress at depth, yet in fractured crystalline rock its derived stress magnitudes remain contingent on how the reopening pressure and shut-in pressure are picked from pressure–time records and on biases from measurement-system compliance and fluid penetration into natural fractures. A 180-test HF campaign across the Yamansu, Tianhu and Aqishan granite sub-areas of the Xinjiang candidate area for China's high-level radioactive waste (HLW) disposal (Zhao et al., 2015) crystallized four unresolved asks: (1) objective protocols and corrective treatments for reopening/shut-in pressure determination that reduce system-compliance and fracture-penetration bias in the maximum horizontal stress; (2) quantified applicability limits of HF in fractured crystalline rock, including near-borehole fracture-network characterization and threshold criteria for declaring a test interval unreliable; (3) establishment of how stress magnitude and orientation control the hydraulic properties (permeability, connectivity) of the fractured granites; and (4) systematic assessment of the robustness of individual shut-in-pressure extraction algorithms where their outputs scatter.

Background

In the hydraulic fracturing (HF) method of in-situ stress measurement, a borehole interval (typically about 1 m, sealed by inflatable packers) is pressurized with fluid until the rock tensile-fails (breakdown pressure $P_b$), after which the well is shut in and the pressure decays; later repressurization cycles re-open the induced vertical fracture (reopening pressure $P_r$), and the pressure at which the fracture closes (shut-in pressure $P_s$) is read from the decay record. Under the classical interpretation, valid for a homogeneous, isotropic, linearly elastic medium with the induced fracture confined to the interval, the minimum horizontal stress equals the shut-in pressure, $\sigma_h = P_s$, and the maximum horizontal stress is $\sigma_H = 3\sigma_h - P_o - P_r$, where $P_o$ is the pore pressure. Both formulas assume that $P_s$ and $P_r$ are identified objectively on the pressure–time record. Decades of work show they often are not: comparisons of shut-in-pressure identification methods (tangent/inflection, $dP/dt$–$P$, $dt/dP$–$P$, bilinear-pressure-ratio, etc.) find systematic offsets between algorithms, and the reopening pressure is theoretically contested (poroelastic and fracture-mechanics effects), prompting proposals to avoid $P_r$ altogether. A further, instrument-related bias arises from measurement-system compliance — the elasticity and stored fluid volume of the surface pump, tubing and packer string, which couples to the fracture during closure and re-opening and can shift both $P_s$ and $P_r$; formulations that account for this compliance exist but have not been widely adopted. In naturally fractured crystalline rock the interpretation assumptions themselves can fail: pre-existing fractures intersecting the isolated interval can hydraulically connect with the induced fracture, so the recorded pressures no longer reflect the near-field elastic stress state. The suggested methods of the International Society for Rock Mechanics (ISRM) (Haimson and Cornet, 2003) codify good practice, including the alternative of hydraulic testing of pre-existing fractures (HTPF, in which existing fractures are jacked and pressurized instead of creating new ones), but prescribe no quantitative thresholds for declaring a test interval invalid. These issues came to a head in the site-characterization campaign for China's deep geological repository of high-level radioactive waste (HLW, spent-fuel waste requiring isolation for hundreds of thousands of years): Zhao et al. (2015) reported 180 HF tests in eight boreholes (about 17–647 m depth) in the Yamansu, Tianhu and Aqishan granite sub-areas of the Xinjiang candidate area, averaging three $P_s$ extraction methods (inflection point, $dP/dt$–$P$, $dt/dP$–$P$; within-cycle differences below 0.6 MPa, coefficient of variation below 8%) and discarding 17 tests as invalid. In one borehole (TH01), tests below about 300 m showed anomalously low $P_r$ and $P_s$ that double-packer constant-head injection tests (pressure sensors above, inside and below the interval) traced to hydraulically connected natural fractures forming a through-going conduit. Their discussion raises the interpretation-uncertainty and applicability-limit questions, mentioning borehole radar (single-hole georadar imaging of fractures around the borehole, a mature technique — Olsson et al., 1992) as a candidate means of near-borehole fracture-network characterization, and calling for field and numerical study of the stress dependence of the granites' hydraulic properties.

Problem Statement

As raised by the Xinjiang candidate-area campaign (Zhao et al., 2015), establish an objective, bias-controlled basis for hydraulic-fracturing stress determination in fractured crystalline rock, and quantify how the measured stress field controls the hydraulic behavior of the candidate granites. Concretely, the campaign's discussion raises four linked open items: (1) objective protocols and corrective treatments for determining the reopening pressure $P_r$ and shut-in pressure $P_s$ that reduce bias from measurement-system compliance and from fluid penetration into natural fractures, so that the maximum horizontal stress $\sigma_H = 3\sigma_h - P_o - P_r$ derived from HF records carries quantified rather than assumed uncertainty; because compliance and fluid penetration act on the same pressure–time record, the required bias reduction must be demonstrated independently of the record used to derive the correction; (2) quantified limits of applicability of HF in fractured crystalline rock, comprising methods to characterize the near-borehole fracture network and validated threshold criteria for declaring a HF test interval unreliable; (3) establishment of the influence of the magnitude and orientation of the in-situ stress field on the hydraulic properties — permeability and connectivity — of the fractured granites at the site; and (4) systematic assessment of the suitability and robustness of the individual $P_s$ extraction algorithms where their outputs scatter. These four items are separately answerable but jointly required: each can be addressed on its own, yet the task is resolved only when all four are addressed, and progress on any subset of them leaves the remaining items open.

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