Determination of rock stress- Code of practice, Part 1: Using hydraulic fracturing technique

IS 13946 Part 1 (1994) — Scope Summary & Key Specifications

Scope (Clause 1.1)

• Covers determination of in-situ stress state underground by hydraulic fracturing through a drill hole.
• Provides magnitude and orientation of maximum and minimum horizontal stresses perpendicular to the drill hole.
• Adjunct Indian Standards:
  • IS 11315 (Part 1 & 6): Discontinuity orientation and aperture in rock mass.
  • IS 11358: Glossary of rock mechanics terms.

Key Definitions & Parameters

Measurement Equipment (Clause 5.5)

• Pressure transducers for fluid pressure at surface/packer.
• Pressure gauge/transducer for packer inflation pressure.
• Fluid flow recording instrument.

Reporting Requirements (Clause 9.1 & 10)

• Site location, geotechnical log, rock description.
• Test depth, drillhole size, test zone length.
• Graphs: injection pressure & flow rate vs time.
• Tabulated values: H, P1, Po, Pt, Pg, max/min horizontal stresses & directions.
• Fracture tracer representations (photos/acoustic logs).
• Calculation method for maximum stress (σ_max) including lab test data.

Idealized Hydraulic Fracturing Pressure Record (Fig. 2)

graph LR
A[Start Injection] --> B[Pressure Increases to P (Fracture Initiation)]
B --> C[Pressure Drops to Ps (Shut-in)]
C --> D[Pressure Rises to Pr (Reopening)]

This standard is essential for rock mechanics engineers to evaluate underground stress fields for design and safety assessments in rock engineering projects.

IS 13946 Part 1: Definitions & Symbols (Key Points)

• Depth & Pressures:

  • ( H ) = Depth at test zone below ground level
  • ( P_h ) = Static pressure head of fracture fluid
  • ( P_o ) = Initial pore water pressure (formation pore pressure)
  • ( P_t ) = Fracture initiation pressure
  • ( P_p ) = Pumping pressure
  • ( P_s ) = Instantaneous shut-in pressure (pressure after stopping injection)
  • ( P_x ) = Fracture reopening pressure

• Stresses:

  • ( \sigma_v ) = Vertical stress
  • ( \sigma_{min} ) = Minimum horizontal stress
  • ( \sigma_{max} ) = Maximum horizontal stress

• Rock Property:

  • ( T ) = Drillhole rupture strength of the rock

Important Concept: Shut-in Pressure (Clause 3.2)

• After injecting fluid to propagate fractures (~3× drillhole diameter), injection stops.
• Hydraulic system is sealed, yielding instantaneous shut-in pressure ( P_s ).

Typical Pressure Relationships in Hydraulic Fracturing:

Simplified Pressure-Time Curve (Idealized)

graph TD
    A[Start Injection] --> B[Pumping Pressure \(P_p\) rises]
    B --> C[Fracture Initiation at \(P_t\)]
    C --> D[Injection stops]
    D --> E[Shut-in Pressure \(P_s\) observed]
    E --> F[Fracture Reopening Pressure \(P_x\) if re-injected]

Reference Indian Standards for Symbols & Rock Mechanics:

• IS 11315 (Parts 1 & 6): Discontinuities in rock mass
• IS 11358: Glossary of terms and symbols applicable to rock mechanics

This concise summary covers the key definitions and symbols essential for hydraulic fracturing stress measurement per IS 13946 Part 1. For detailed

IS 13946 Part 1: Equipment Requirements Summary

1. Drilling Equipment (Clause 5.1)

• Standard drilling apparatus to create boreholes for testing.

2. Packer Equipment (Clause 5.2)

• Purpose: Isolate test section of drill hole.
• Types:
  • Double packer system: Isolates a section between two packers (preferred).
  • Single packer system: Isolates base of hole (alternative).
• Operation: Packers inflated hydraulically or by gas expansion.
• Reference: Fig. 1 (shows packer setup with flow pipe).

3. Fluid Injection Equipment (Clause 5.3)

• High-pressure pump:
  • Maintains constant flow over test pressure range.
  • Capacity to overcome friction losses and initiate hydrofractures.
• Supply rods/tubing/hoses: To lower packer and inject fluid.

4. Measuring Equipment (Clause 5.5)

• Pressure transducers: Measure fluid pressure at surface or above packer.
• Pressure gauge/transducer: For packer inflation pressure.
• Flow recorder: Records fluid flow vs. time.

Key Specifications Table

Diagram: Double Packer Setup

flowchart TB
    A[Surface] --> B[High Pressure Pump]
    B --> C[Supply Rods/Tubing]
    C --> D[Double Packers]
    D --> E[Test Section Isolated]
    D --> F[Pressure Transducer Above Packers]
    B --> G[Flow Meter]

This summary covers essential equipment and their specifications per IS 13946 Part 1 for hydraulic fracturing tests.

IS 13946 Part 1: Drilling and Inspection Procedures - Key Points

1. Drilling Equipment (Clause 5.1 & 5.1.1)

• Use drilling equipment capable of producing a stable borehole to the required test depth.
• Hole diameter must be compatible with packer equipment for effective sealing.

2. Inspection Equipment (Clause 5.4)

• To determine hydrofracture directions, use any of the following:

3. Important Specifications

• Ensure hole diameter matches packer size.
• Use orientation tools to verify borehole straightness and device alignment.
• For magnetic interference, prefer gyroscopic compasses.

Summary Diagram: Inspection Workflow

flowchart TD
    A[Drilling Stable Borehole] --> B[Check Hole Diameter with Caliper]
    B --> C{Diameter Suitable?}
    C -- Yes --> D[Insert Inspection Equipment]
    C -- No --> E[Adjust Drilling or Packer]
    D --> F[Orient Equipment using Compass/Gyroscope]
    F --> G[Visual/Acoustic Inspection]
    G --> H[Record Data & Analyze Hydrofracture Direction]

This ensures reliable hydrofracture direction detection per IS 13946 Part 1.

IS 13946 Part 1: Test Procedure and Pressure Monitoring

Key Specifications & Equipment (Clauses 5.2, 5.5)

• Packer System: Double packer system preferred; inflatable packers isolate test section.
• Measuring Instruments:
  • Pressure transducers at surface or above packer.
  • Pressure gauge/transducer for packer inflation pressure.
  • Flow meter to record fluid flow vs. time.

Pressure Monitoring & Pressurization (Clause 7.1)

• Pressure monitored at surface or within test section.
• Pressurization rate: 0.1 to 2.0 MPa/sec recommended.
• Controlled by constant flow rate; flow rate depends on:
  • Tubing elasticity and length
  • Fluid compressibility
  • Fluid volume in system
• Deep, large-diameter tubing requires higher flow rates.

Stress Calculation from Pressure Record (Clause 8.1)

From pressure-time plot (Fig. 2), principal stresses:

[ \begin{align*} \sigma_{\min} &= P_s - \frac{T}{3} - P_g - P_t - P_o \quad \text{(initial pressurization)} \ \sigma_{\max} &= 3P_s - P_r - P_o \quad \text{(subsequent repressurization)} \end{align*} ]

Where:

• (P_r, P_s, P_t) = pressures from test
• (T) = drillhole rupture strength (lab tested)
• (P_g) = gas pressure (if any)
• (P_o) = overburden pressure

Summary Table: Pressurization Rate & Flow Rate Relation

flowchart LR
    A[Test Section] -->|Pressure Monitoring| B[Pressure Transducer]
    B --> C{Location}
    C -->|Surface| D[Slow Pressurization]
    C -->|Within Test Section| E[

IS 13946 Part 1: Data Interpretation & Stress Calculation for Hydraulic Fracturing

Key Formulas (Clause 8.1)

For hydraulic fracturing tests in a drill hole, where the fracturing plane is nearly parallel to the hole axis:

• Minimum principal stress (σ_min):
  [ \sigma_{\min} = P_s - T ]

• Maximum principal stress (σ_max):
  [ \sigma_{\max} = 3P_s - P_r - P_o ]

Where:

• (P_r) = Pressure at rupture
• (P_s) = Shut-in pressure (pressure when flow stops)
• (P_o) = Pore pressure
• (T) = Drillhole tensile strength (from lab tests or Brazilian test)

Important Notes

• Tensile strength (T) must be corrected for sample size and test configuration.
• Vertical stress is assumed from overburden weight:
  [ \sigma_v = \rho \times g \times h ] where (\rho) = rock density, (g) = gravity, (h) = depth.
• Direction of (\sigma_{\max}) is in fracture plane and orthogonal to (\sigma_{\min}).

References to Related IS Codes

flowchart LR
    A[Start: Hydraulic Fracturing Test] --> B[Measure Pressure vs Time]
    B --> C{Identify Pressures}
    C -->|Pr = Rupture Pressure| D[Calculate σ_max]
    C -->|Ps = Shut-in Pressure| E[Calculate σ_min]
    D --> F[Apply formulas]
    E --> F
    F --> G[Determine Principal Stresses & Directions]

This concise approach helps determine in-situ stresses critical for underground rock engineering.

IS 13946 Part 1: Reporting of Hydraulic Fracturing Test Results

Key Formulas for Principal Stresses (Clause 8.1)

When hydrofracturing plane ≈ drillhole axis:

[ \begin{align*} \sigma_{min} &= P_s \ \sigma_{max} &= 3P_s - P_r - P_o \quad \text{(subsequent repressurization)} \ \sigma_{min} &= P_s \ \sigma_{max} &= \frac{3P_s - T - 3P_g + P_t + P_o}{3} \quad \text{(initial pressurization)} \end{align*} ]

• (P =) Fracture initiation pressure
• (P_s =) Shut-in pressure
• (P_r =) Fracture reopening pressure
• (P_o =) Formation pore pressure
• (T =) Drillhole rupture strength (from lab tests)
• (P_g, P_t =) Other pressures as defined in test

Reporting Requirements (Clause 9.1 & 10)

General Information:

• Test site location and geological description
• Geotechnical log including discontinuities and drillhole conditions
• Rock type and core availability
• Test depth, test zone length, and drillhole size

Detailed Information per Test Location:

• Graphs: Pumping rate, injection pressure vs. time
• Tabulated values: (H, P_1, P_o, P_t, P_g, P_{Imax}, \sigma_{min}, \sigma_{max}) and directions
• Fracture tracers and acoustic televiewer logs
• Methodology for calculating (\sigma_{max}) and lab test details for (T)
• Explanation of discrepancies with other data

Additional Notes

• Use consistent flow rates in repressurization cycles (Clause 7.4)
• Corrections for sample size and test configuration should be applied to (T)
• Vertical stress is usually assumed from overburden weight

flowchart LR
    A[Test Site Info] --> B[Geotechnical Log]
    B --> C[Rock Description]
    C --> D[Test Parameters]
    D --> E[Graphs & Tabulated Data]
    E --> F

Committee Composition - IS 13946 Part 1 (1994)

The Rock Mechanics Sectional Committee, CED 48 was responsible for this standard's formulation. Key details:

• Chairman: Dr. Bhawani Singh, University of Roorkee
• Member Secretary: Central Board of Irrigation & Power, New Delhi
• Members: Representatives from universities, government irrigation departments, power boards, CSIR institutes, geological survey, construction companies, and BIS.

Representative Organizations Include:

• University of Roorkee
• Irrigation Departments (Uttar Pradesh, Haryana, Maharashtra, Gujarat)
• Central Mining Research Station (CSIR)
• Geological Survey of India
• Central Water & Power Research Station
• Hindustan Construction Co Ltd
• National Thermal Power Corporation
• Indian Institute of Technology
• Central Building Research Institute (CSIR)
• Central Soil & Materials Research Station
• Central Road Research Institute
• BIS (Ex-officio members)

Summary Table of Key Roles

Notes:

• No specific formulas or tables are provided for committee composition.
• Committee details are listed in Annex A of IS 13946 Part 1.
• The committee includes experts from academia, government, research, and industry ensuring comprehensive coverage of rock mechanics.

graph TD
    A[IS 13946 Committee] --> B[Chairman: Dr. Bhawani Singh]
    A --> C[Member Secretary: Central Board of Irrigation & Power]
    A --> D[Members]
    D --> E[Universities]
    D --> F[Government Irrigation Departments]
    D --> G[CSIR Institutes]
    D --> H[Geological Survey]
    D --> I[Construction & Power Companies]
    A --> J[Ex-officio: BIS Director General]

This committee composition ensures multi-disciplinary expertise for robust standard development.