At borehole Blanche-1, drilled to ~2 km depth for the Olympic Dam hot-dry-rock (HDR) geothermal project in South Australia, three independent stress-determination methods (borehole breakout analysis, core discing analysis, and hydraulic fracturing) agree on the stress regime (Sv≤Sh<SHS_v \leq S_h < S_H) and on an East-West orientation of maximum horizontal compression, but return systematically different absolute stress magnitudes, with breakout- and discing-based estimates exceeding hydraulic-fracturing magnitudes. The causes of this discrepancy - rock-strength and failure-criterion inputs, sampled-volume differences, core-barrel geometry, neglected post-failure behavior, and hydraulic-fracturing pressure-record interpretation - remain unquantified, and the extrapolation of the measured stress-depth trends to the 4-5 km target reservoir depth remains untested. The problem is to identify and quantify these method-specific biases, validate the core-discing inversion for the actual coring geometry, and thereby establish a reconciled, uncertainty-quantified stress profile credible for reservoir-depth HDR stimulation design. Case studies document method-dependent results and partial bias analyses, but none has produced a validated general reconciliation, a re-analysis of Blanche-1, or a deeper measurement at the site: the problem remains open.

Background

The in-situ stress field at depth is described by the magnitudes and orientations of three principal stresses: the vertical overburden stress SvS_v, and the maximum and minimum horizontal stresses SHS_H and ShS_h. It is the controlling input for designing hot-dry-rock (HDR, now more commonly termed enhanced geothermal system, EGS) reservoirs, because it governs the orientation of stimulated fractures and the pressures needed to shear-open natural fractures at reservoir depth. Because direct access is limited, several indirect determination methods are used, each with its own sampling volume and assumptions. (1) Borehole breakouts: stress-concentrations at the borehole wall cause compressive failure on opposite sides of the hole, forming elongated zones whose azimuth gives the SHS_H direction and whose width, combined with a rock-failure criterion and rock-strength parameters, constrains stress magnitudes. (2) Core discing: in high-stress rock, the core retrieved during drilling splits into thin discs; the disc geometry and spacing can be inverted, using a model of tensile failure at the bottom of the advancing borehole, into stress-magnitude estimates. Bunger (2010) developed a stochastic discing analysis, treating stresses and tensile strength as random variables, using a failure model calibrated to HQ-size coring. (3) Hydraulic fracturing: a borehole interval is sealed and pressurized until the rock fractures; the shut-in or closure pressure of the induced fracture approximates ShS_h, and, with fracture orientation from acoustic borehole televiewer imaging (ultrasonic images of the borehole wall), SHS_H can be inferred from breakdown and re-opening pressures, with interpretations formalized in the suggested methods of the International Society for Rock Mechanics (ISRM). Klee, Bunger, Meyer, Rummel and Shen (2011) applied all three methods to the granitic 800-1,740 m section of borehole Blanche-1 at the Olympic Dam HDR project, South Australia. The methods agreed on a compressional regime Sv≤Sh<SHS_v \leq S_h < S_H with East-West SHS_H, consistent with the Australian regional stress field, and the hydraulic-fracturing-derived ShS_h (about equal to the overburden) was regarded as most reliable; but the absolute magnitudes from the three methods deviated significantly, with breakout and discing estimates exceeding hydraulic-fracturing magnitudes. The authors identified the unresolved causes as potentially including inappropriate rock-strength inputs, differences in the rock volume sampled by each method, core-barrel geometry effects (the discing model assumed HQ coring while Blanche-1 used the smaller NQ2 — HQ and NQ2 being standard wireline-coring sizes, with HQ retrieving a larger-diameter core than NQ2), neglect of post-failure rock behavior in the elastic models used, and interpretation ambiguity of the hydraulic-fracturing pressure-time records (breakdown, re-opening, shut-in event identification, and the assumption of negligible pore pressure in the low-permeability granite). They further noted that extrapolating the measured stress-depth trends down to the 4-5 km depth of the planned HDR reservoir was conjectural, and that faults and mining-induced perturbations near Olympic Dam could locally reorient stresses. The general issue - different stress-measurement methods returning systematically different absolute magnitudes at the same site, without a validated way to attribute and correct the differences - recurs across the later literature (e.g., Schmitt, Currie and Zhang 2012; Yaméogo, Corthésy and Leite 2013; Kim et al. 2017), making this borehole a concrete instance of a standing methodological problem in rock mechanics.

Problem Statement

For the granitic section (800-1,740 m) of borehole Blanche-1 at the Olympic Dam hot-dry-rock site, South Australia - and for the general methodological problem it instantiates - determine why independent in-situ stress determination methods return systematically different absolute stress magnitudes at the same borehole, and use that understanding to produce a reconciled, uncertainty-quantified stress profile credible to reservoir depth. By a reconciled, uncertainty-quantified stress profile is meant a depth-resolved set of the three principal stress magnitudes in which each magnitude carries an explicit uncertainty interval and the set is mutually consistent with the three measurement methods within their stated uncertainties over the measured 800-1,740 m section; below that interval the profile is an extrapolation whose values carry propagated uncertainties and must be labelled as such. Concretely: (1) identify and quantify the physical causes of the discrepancy whereby breakout-based and core-discing-based magnitudes exceed hydraulic-fracturing-based magnitudes, including the roles of rock-strength and failure-criterion inputs, the difference in rock volume sampled by each method, core-barrel geometry effects (HQ-calibrated discing model versus the NQ2 coring actually used), and the neglect of post-failure rock behavior in the elastic interpretation models; (2) validate and adapt the stochastic core-discing inversion - including its assumed distributions of stress and tensile strength and its elastic failure criterion - to the actual coring geometry and to post-failure processes, and test it against the Blanche-1 discing record; (3) quantify the uncertainty in hydraulic-fracturing-derived ShS_h and SHS_H arising from pressure-time event identification (breakdown, re-opening, shut-in/closure) and from the assumed negligible pore pressure in low-permeability granite; and (4) establish, by reconciled measurement, targeted new deeper measurements, or other independent evidence, whether the inferred compressional East-West high-horizontal-stress regime (Sv≤Sh<SHS_v \leq S_h < S_H) persists, and how magnitudes, stimulated-fracture orientation, and required operating pressures scale, at the 4-5 km depth of the planned HDR reservoir - including how faults and mining-induced perturbations near Olympic Dam modify the local stress field relative to the regional South Australian pattern. These four strands share one objective: reducing the uncertainty in absolute in-situ stress magnitudes at the site to the level required for reliable HDR stimulation design and risk assessment. All four strands are required: the Blanche-1 site is the concrete target, the general reconciliation method it instantiates is a required, transferable part of the result, and no subset of the strands resolves the task.

References

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