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In situ stresses

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presented by : Hamed Zarei …

presented by : Hamed Zarei
Hydraulic fracturing
overcoring
Core Discing

Published in: Engineering, Business, Technology

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  • 1. In The Name Of God
  • 2. outline Hydraulic Testing on Pre-existing Fractures &Hydraulic fracturing overcoring - Borre probe USBM Deformation Probe Conical Strain Cell deep doorstopper gauge system (DDGS) Core Discing
  • 3. Hydraulic Testing on Pre-existing Fractures & Hydraulic fracturing HTPF & HF
  • 4. Hydraulic Testing on Pre-existing Fractures& Hydraulic fracturing There exist two stress measurement methods that use hydraulics as an active method to stimulate the rock surrounding a borehole and hence to determine the stress field. These methods are hydraulic fracturing and HTPF. methods use the same type of equipment, including straddle packers, impression packers and high-pressure pumps to generate high-pressure water during either the formation of new fractures or reopening of pre-existing fractures HTPF & HF
  • 5. Hydraulic Fracturing Method A section, normally less than 1m in length, of a borehole is sealed off with a straddle packer. The sealed-off section is then slowly pressurised with a fluid, usually water. This generates tensile stresses at the borehole wall. Pressurisation continues until the borehole wall ruptures through tensile failure and a hydrofracture is initiated. The fracture plane is normally parallel to the borehole axis, and two fractures are initiated simultaneously in diametrically opposite positions on the borehole periphery. The hydrofracture will initiate at the point, and propagate in the direction, offering the least resistance.
  • 6. The orientation of the fracture is obtained from the fracture traces on the borehole wall – it coincides with the orientation of the maximum horizontal stress, in a vertical or sub-vertical hole where it is assumed that one principal stress is parallel to the borehole. The fracture orientation may be determined either by use of an impression packer and a compass or by use of geophysical methods such as a formation micro-scanner or a borehole televiewer. Hydraulic Fracturing Method
  • 7. The two measurements taken are the water pressure when the fracture occurs and the subsequent pressure required to hold the fracture open. These are referred to as the breakdown pressure(Pc’ or PB) and the shut-in pressure (Ps). Hydraulic Fracturing Method
  • 8. Relationships σh = Ps σH = 3σh–Pc’-Po+ σt σt = Pc’-Pr σH = 3σh–Pr-Po In calculating the in situ stresses, the shut-in pressure (Ps) is assumed to be equal to the minor horizontal stress, σh. The major horizontal stress, σH, is then found from the breakdown pressure (Pc ’ or PB). In this calculation, the breakdown pressure has to overcome the minor horizontal principal stress (concentrated three times by the presence of the borehole) and overcome the in situ tensile strength of the rock; it is assisted by the tensile component of the major horizontal principal stress. Hydraulic Fracturing Method
  • 9. The following points should be noted with respect to HF: There is no theoretical limit to the depth of measurement, provided a stable borehole can access the zone of interest and the rock is elastic and brittle. Principal stress directions are derived from the fracture delineation on the borehole wall under the assumption that fracture attitude persists away from the hole. Evaluation of the maximum principal stress in the plane perpendicular to the borehole axis assumes that the rock mass is linearly elastic, homogeneous, and isotropic Hydraulic fracturing is an efficient method for determining the 2D stress field, normally in the horizontal plane, and is therefore suitable at the early stages of projects when no underground access exists. Due to its efficiency, it is especially advantageous for measurements at great depth. . The method is also not significantly affected by the drilling processes. Hydraulic fracturing normally includes large equipment, which requires space. Furthermore, the theoretical limitations normally imply that the measurements should be done in vertical holes. Hence, the method is most suited for surface measurements in vertical or sub-vertical boreholes.
  • 10. Hydraulic Fracturing Method - HTPF The HTPF method (Hydraulic Testing on Pre- existing Fractures), consists of reopening an existing fracture of known orientation that has previously been isolated in between two packers. By using a low fluid injection rate, the fluid pressure which balances exactly the normal stress across the fracture is measured.
  • 11. The following points should be noted with respect to HTPF: There is no theoretical limit to the depth of measurement, provided a stable borehole can access the zone of interest. The method assumes that isolated pre-existing fractures, or weakness planes, are present in the rock mass, that they are not all aligned within a narrow range of directions and inclinations, and that they can be mechanically opened by hydraulic tests. Fractures used in stress computations are delineated on the borehole wall under the assumption that their orientation persists away from the hole. The method is valid for all borehole orientations. It is independent of pore pressure effects and does not require any material property determination. It assumes that the rock mass is homogeneous within the volume of interest. The method is more time consuming than hydraulic fracturing as the down-hole equipment must be positioned at the exact location of each discrete fracture to be tested.
  • 12. Hydraulic Fracturing Method - Example Q. A hydraulic fracture test in a granite rock mass yield the following results: Given that the tensile strength of the rock is 10 MPa, estimate the values of σ1, σ2 and σ3 assuming that one principal stress is vertical and that the pressure values were adjusted to account for the formation pressures (i.e. Po=0 for calculation purposes). A. Assuming that the rock mass was behaving as an elastic material, The minimum horizontal stress can be calculated from the expression: σh = Ps σh = 8 MPa Relationships σh = Ps σH = 3σh–Pc’-Po+ σt σt = Pc’-Pr σH = 3σh–Pr-Po
  • 13. Hydraulic Fracturing Method - Example Q. A hydraulic fracture test in a granite rock mass yield the following results: Given that the tensile strength of the rock is 10 MPa, estimate the values of σ1, σ2 and σ3 assuming that one principal stress is vertical and that the pressure values were adjusted to account for the formation pressures (i.e. Po=0 for calculation purposes). A. The maximum horizontal stress can be calculated from the expression: σH = 3σh–Pc’-Po+ σt σH = 3(8 MPa) – 14 MPa + 10 MPa σH = 20 MPa The vertical stress can be estimated from the vertical overburden (assuming a unit weight of 27 kN/m3 for granite): σV = 500 m * 0.0027 MN/m3 σV = 13.5 MPa σ1 = σH = 20 MPa σ2 = σv = 13.5 MPa σ3 = σh = 8 MPa
  • 14. )Overcoring(
  • 15. overcoring - Borre probe
  • 16. The Borre probe with Logger connected to a portable computer for activation and data retrieval. overcoring - Borre probe
  • 17. Principle of soft, 3D pilot hole Overcoring measurements: (1)Advance φ 76 mm main borehole to measurement depth; (2) Drill φ 36 mm pilot hole and recover core for appraisal; (3) Lower Borre Probe in installation tool down-hole; overcoring - Borre probe
  • 18. (4) Release Probe from installation tool. Strain gauges bond to pilothole wall under pressure from the cone; (5) Raise installation tool. Probe/gauges bonded in place; (6) Overcore the Borre Probe and recover to surface in core barrel (After Ljunggren & Klasson) overcoring - Borre probe
  • 19. Displacements from stress concentrations around a borehole are given by K1-K4 are correction factors 𝜀 𝜃 = 1 𝐸 [ 𝜎𝑥 + 𝜎 𝑦 𝐾1 − 2 1 − 𝜇2 𝜎𝑥 − 𝜎 𝑦 cos 2𝜃 + 2𝜏 𝑥𝑦 sin 2𝜃 𝐾2 − 𝜇𝜎𝑧 𝐾4] 𝜀 𝑧 = 1 𝐸 [𝜎𝑧 − 𝜇 𝜎𝑥 + 𝜎 𝑦 ] overcoring - Borre probe
  • 20. USBM Deformation Probe When the probe is inserted in a borehole, six ‘buttons’ press against the borehole wall and their diametral position is measured by strain gauges bonded to steel cantilevers supporting the buttons. When the borehole is overcored by a larger diameter borehole, the stress state in the resulting hollow cylinder is reduced to zero, the diameter of the hole changes, the buttons move, and hence different strains are induced in the strain gauges.
  • 21. USBM Deformation Probe
  • 22. Conical Strain Cell The hemi-spherical or conical strain cell is attached to the hemi-spherical or conical bottom of the borehole. It also do not require a pilot hole. After the cell has been positioned properly at the end of the borehole and readings of the strain gauges have been performed, the instrument is overcored. During overcoring, the changes in strain/deformation are recorded
  • 23. Using a hemispherical or conical strain cell for measuring rock stresses, a borehole is first drilled. Its bottom surface is then reshaped into a hemispherical or conical shape using special drill bits. Thereafter, the strain cell is bonded to the rock surface at the bottom of the borehole. Conical Strain Cell
  • 24. The Doorstopper cell is attached at the polished flat bottom of a borehole. Hence, it does not require a pilot hole. After the cell has been positioned properly at the end of the borehole and readings of the strain gauges have been performed, the instrument is over cored. During Overcoring, the changes in strain/deformation are recorded. Doorstopper
  • 25. Leeman indicates that a doorstopper technique was used as early as 1932 to determine stresses in a rock tunnel below the Hoover Dam in the United States, and also in Russia in 1935. The cell is pushed forward by compressed air and glued at the base of a drill hole. Reading of the strain gauges is taken before and after overcoring of the cell. Hence, they do not require a pilot hole. Doorstopper
  • 26. DDGS A modified doorstopper cell called the Deep Doorstopper Gauge System (DDGS) has been developed lately. The DDGS was designed to allow Overcoring measurements at depths as great as 1000m in sub vertical boreholes. Installation of the DDGS: (1) After attaining and cleaning of the bottom, the instruments are lowered down the hole with the wire line cables. (2) When the DDGS is at the bottom the orientation of the measurement is noted in the orientation device and the strain sensor is glued. (3) The IAM and Doorstopper gauge are removed from the installation equipment. (4) The installation assembly is retrieved with the wire line system. (5) The monitoring and over drilling start, the strain change in the bottom is measured by the time. (6) When over drilling is completed, the core is taken up and a bi-axial pressure test done to estimate the Young’s modulus.
  • 27. DDGS
  • 28. Successful measurements have been performed in Canada – borehole depths as great as 518m (943m depth from surface), where both hydraulic fracturing and triaxial strain cells were not applicable at depths deeper than 360m because of the high stress situation. An advantage for the Doorstopper, as well as the conical or spherical methods, is that they do not require long overcoring lengths, i.e. only some 5 cm, as compared to the pilot hole methods (at least 30 cm). Compared to triaxial cells, a Doorstopper measurement requires less time, and 2–3 tests can be conducted per day. Furthermore, the end of the borehole must be flat which require polishing of the hole bottom. Another limitation is their poor success in water-filled boreholes. The disadvantage with the doorstopper is, however, that measurement at one point only enables the stresses in the plane perpendicular to the borehole to be determined. ADVANTAGES & Disadvantages
  • 29. Core Discing Fig. 5 Discing between 1,920 and 1,931.2 m depth. The core is oriented with depth increasing from top to bottom and from left to right
  • 30. The pre-loaded nature of rock masses has consequences in rock stress observation. The process of boring of holes to obtain cores results in stress concentrations directly at the coring bit/rock interface. As the core is formed, the annular groove causes the in situ stresses to be redistributed, creating high-induced stresses across the core. This can result in damage (irrecoverable strains and microcracks) to the core. If the in situ stresses are high, and the rock brittle, this can result in ‘core discing’— the core is produced in the form of thin ‘poker chips’. The thickness of the chips decreases as the stress intensity increases; in extreme cases, the discs can become so thin that they have the appearance of milles feuilles, or flaky pastry. Observation of discing in cores is often taken as evidence of high stress zones in the rock. Core Discing
  • 31. The following minimum information is needed for the interpretation: • the tensile strength of the rock, • Poisson’s ratio of the rock, • the UCS of the rock, • the mean disc spacing, • the shape of the fracture (morphology) • the extent of the fracture in the core. The confidence of the interpretation can be increased considerably if the same information can be achieved from both normal coring and overcoring at the same depth level. In practice, core discing can only be used as an indicator for estimation of rock stresses. When core discing occurs, one can of course also conclude that rock stress concentrations are higher than the rock strength. Such information, obtained already during the drilling stage, is of course valuable and a guide for further decision. Core Discing
  • 32. In brittle rocks it has been observed that discing and breakouts usually occur over the corresponding lengths of core and borehole. The thinner the discs the higher the stress level. However, the formation of discs depends significantly on the properties of the rock and the magnitude of the stress in the borehole axial direction. In addition, the type and technique of drilling, including the drill thrust, can significantly affect the occurrence of discing. It is therefore unlikely that observation and measurements of discing will be successful in quantifying the magnitudes of in situ stresses If the discs are symmetrical about the core axis, as shown in figure above, then it is probable that the hole has been drilled approximately along the orientation of one of the principal stresses. Core discs symmetrical with respect to the core axis Core Discing
  • 33. Nevertheless, the shape and symmetry of the discs can give a good indication of in situ stress orientations (Dyke, 1989). A measure of the inclination of a principal stress to the borehole axis can be gauged from the relative asymmetry of the disc. For unequal stresses normal to the core axis, the core circumference will peak and trough as shown in figure next to text. The direction defined by a line drawn between the peaks of the disc surfaces facing in the original drilling direction indicates the orientation of the minor secondary principal stresses. Core discs resulting with unequal stresses normal to the core axis Core Discing
  • 34. Non-symmetrical core discing, indicating that the core axis is not a principal stress direction. Lack of symmetry of the discing, as shown in figure above, indicates that there is a shear stress acting across the borehole axis and that the axis is not in a principal stress direction. Core Discing
  • 35. Refrence In situ stress Marek Cała – Katedra Geomechaniki, Budownictwa i Geotechniki In Situ Stresses & Stress Measurement Dr. Erik Eberhardt Insitu Stress Measurements U.Siva Sankar Sr. Under Manager Project Planning Singareni Collieries Company Ltd Stress measurements in deep boreholes using the Borre (SSPB) probe J. Sj .oberg*, H. Klasson SwedPower AB, Lule ( a, Sweden Accepted10 July 2003 Geotechnisches Ingenieurbüro Prof. Fecker & Partner GmbHStress-relief Methods In situ rock stress determinations in deep boreholes at the Underground Research Laboratory P.M. Thompson, N.A. Chandler