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D4645-08 – Standard Test Method Technical Guide
🔬 Scope and Application of D4645-08
This standard test method, designated D4645-08, covers the determination of the in-situ state of stress in rock using the hydraulic fracturing method. It is the widely accepted field method available for in situ stress measurements at depths greater than 50 m and can be used in drill holes of any diameter.
The method yields stresses averaged over a few square metres—the size of the induced hydraulic fracture—rather than over grain size areas as seen in overcoring techniques. All values must be stated in SI units, and data handling must comply with Practice D6026 for significant digits and rounding.
⚠️ Critical Scope Note: The standard explicitly states that the method used to specify how data are collected, calculated, or recorded is not directly related to the accuracy to which the data can be applied in design or other uses. The application of results is strictly beyond the scope of this test method.
📐 Test Parameters and Terminology
The procedure involves isolating a section of the borehole by pressurizing two inflatable rubber packers. The fluid pressure in the sealed-off interval is increased until the rock fractures. Specific pressure thresholds are recorded, including the breakdown pressure (fracture initiation), secondary breakdown or refrac pressure (fracture reopening), and the shut-in pressure (ISIP) (fracture closure). These values are critical for calculating the vertical and horizontal principal stresses, which are assumed to act in the vertical plane and the horizontal plane, respectively.
💡 Advantage Over Overcoring: While hydraulic fracturing can be used in short holes where overcoring is possible, it provides a distinct advantage by sampling a larger rock volume. Stresses are averaged over the induced fracture plane rather than grain-size areas, reducing the influence of local rock heterogeneities.
| 🟦 Term | 📏 Definition (from D4645-08) |
|---|---|
| Breakdown Pressure | The pressure required to induce a hydraulic fracture in a previously intact test interval. |
| Secondary Breakdown Pressure | The pressure required to reopen a closed, previously induced hydrofracture after the test interval pressure has been allowed to return to its initial condition. |
| Shut-in Pressure (ISIP) | The pressure reached when the induced hydrofracture closes back after pumping is stopped. |
| In-Situ Stress | Rock stress measured in situ (as opposed to by remote sensing). |
| 🎯 Standard / Parameter | ⚡ Specification |
|---|---|
| Standard Designation | D4645 – 08 |
| Depth Range | Greater than 50 m |
| Borehole Diameter | Any diameter |
| Unit System | SI Units (Standard) |
| Drilling Standard | Practice D2113 |
| Significant Digits | Practice D6026 |
❓ Frequently Asked Questions
🔍 What is the minimum recommended depth for applying D4645-08 as the primary field method?
Hydraulic fracturing is the widely accepted field method available for in situ stress measurements at depths greater than 50 meters.
💡 What distinguishes “breakdown pressure” from “secondary breakdown pressure” in this test?
Breakdown pressure is the force required to create a new fracture in an intact interval. Secondary breakdown (or refrac) pressure is the lower pressure required to reopen an existing, closed fracture from a previous test cycle.
⚡ What does the standard explicitly state regarding the use of test results in design?
The standard states that how one applies the results obtained using this standard is beyond its scope, and the data collection methods are not directly related to design accuracy.
📌 To which ASTM practice must the reporting of significant digits and rounding conform?
All observed and calculated values shall conform to the guidelines for significant digits and rounding established in Practice D6026.