Modeling Interaction between Natural Fractures and Hydraulic Fractures in Block Cave Mining

Transcription

1 ARMA Modeling Interaction between Natural Fractures and Hydraulic Fractures in Block Cave Mining He, Q. Suorineni, F. and Oh, J. School of Mining Engineering, University of New South Wales, Sydney, New South Wales, Australia Copyright 2015 ARMA, American Rock Mechanics Association This paper was prepared for presentation at the 49 th US Rock Mechanics / Geomechanics Symposium held in San Francisco, CA, USA, 28 June- 1 July This paper was selected for presentation at the symposium by an ARMA Technical Program Committee based on a technical and critical review of the paper by a minimum of two technical reviewers. The material, as presented, does not necessarily reflect any position of ARMA, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper for commercial purposes without the written consent of ARMA is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 200 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgement of where and by whom the paper was presented. ABSTRACT: Hydraulic fracturing has been utilized as a pre-conditioning method in block cave mining to improve cavability and fragmentation sizes in recent years. A successful hydraulic fracturing operation depends largely on the interaction between natural fractures and hydraulic fractures. This process has been extensively studied in the oil & gas industry where hydraulic fractures are commonly vertical planar types. In this case, the effects of natural fracture dip angle and vertical in-situ stress are neglected. However, in block cave mining, hydraulic fractures are generally horizontal radial types. The applicability of the existing conclusions for block cave mining needs to be examined. In this paper, the interaction between natural fractures and horizontal radial hydraulic fractures is investigated by theoretical analysis and numerical modeling. It indicates that both natural fracture dip angle and approach angle influence the interaction result, as well as all the three in-situ stresses. It shows that hydraulic fractures are more likely to cross natural fractures if rock tensile strength is sufficiently low, or the product of flow rate and fluid viscosity is sufficiently high. Rock masses in block cave mining have mixed-qualities. High flow rate and fluid viscosity are recommended in regions where hydraulic fractures are not able to cross natural fractures. 1. INTRODUCTION Hydraulic fracturing was firstly experimentally used in 1947, and its first commercially successful application was realized in 1949 [1]. From then on, this technique has been widely utilized in the oil & gas industry for reservoir stimulation. In recent decades, hydraulic fracturing has been introduced into the mining industry. Initially, it was applied in coal mining for methane extraction since the 1970s in the United States and to control hard roof rockburst [2-5]. Then, it was used in cave mining for either caving inducement or preconditioning [6-9]. When hydraulic fracturing is utilized in cave mining, it is more economic compared with the traditional drilling and blasting, and has the ability to improve both cavability and fragmentation sizes [10]. As is known to all, hydraulic fractures (HFs) propagate perpendicularly to the minimum principle stress. The objective of hydraulic fracturing in block cave (BC) mining is either to create horizontal radial hydraulic factures (HRHFs) that can propagate across the existing vertical or inclined natural fractures (NFs), or to create inclined hydraulic fractures in a horizontal or subhorizontal NFs dominated region as shown in Figure 1. By either of these two approaches, cavability and fragmentation sizes can be improved. Fig. 1. Hydraulic fracturing in block cave mining When HFs intersect NFs, generally the following four results may occur [11]: (i) HFs are temporarily arrested by NFs; (ii) HFs propagate across NFs; (iii) HFs are diverted from their original directions, and propagate into NFs;

2 (iv) HFs simultaneously dilate NFs and propagate into their original directions. Therefore, the interaction between NFs and HFs plays a key role in a successful hydraulic fracturing operation in BC mining. Numerous experimental and theoretical studies have been carried out to investigate the interaction between NFs and HFs. It should be noted that all these studies were based on the interaction between NFs and a vertical planar hydraulic fractures (VPHF). This interaction type is common in the oil & gas industry, and is illustrated in Figure 2. Lamont and Jassen [12] carried out an early experimental study, and found that an existing NF has little influence on the propagating HF (due to the small rock model they used and relatively high treatment parameters). Daneshy [13] reported that the interaction result is mainly influenced by three factors: NF strength, the approach angle and the horizontal differential stress (i.e. the difference between horizontal in-situ stresses). He also concluded that HFs are more likely to cross NFs with small apertures. Blanton [14, 15] stated that HFs can cross NFs only under high approach angle and high horizontal differential stress. The results of mineback and laboratory experiments from Warpinski and Teufel [16] indicated that the interaction process is also influenced by treatment parameters. Renshaw and Pollard [17] found that the HF can cross the NF if the normal stress applied on the NF surface could provide enough shear resistance to avoid NF slippage. Recently, laboratory tests showed that crossing is more likely to occur under high flow rate and high fluid viscosity [18-21]. From the above, the interaction between NFs and HFs is a complex process, besides NF properties, rock mass properties and treatment parameters, it is significantly influenced by: (i) The approach angle; (ii) The horizontal differential stress. Fig. 2. Interaction between a NF and a VPHF These conclusions have been highly recognized and commonly used in the oil & gas industry. In the VPHF case, the effects of NF dip angle and vertical in-situ stress are neglected. However, due to different HF type and engineering environment, the applicability of these conclusions for BC mining needs to be examined. As shown in Figure 3, in the HRHF case, the resultant force applied perpendicularly to the NF surface depends on all the three in-situ stresses, the NF dip angle and the NF strike angle. This resultant force makes an important influence on final interaction results. Fig. 3. Interaction between a NF and a HRHF In this paper, the influences of NF dip angle, NF strike angle and all the three in-situ stresses on the interaction between NFs and HRHFs are investigated. Then, the effects of rock tensile strength, flow rate and fluid viscosity are examined. Finally, recommendations are proposed for improving hydraulic fracturing applications in BC mining. 2. ANALYTICAL CRITERIA FOR FRACTURE INTERACTION 2.1. Analytical Criteria for Vertical Planar Hydraulic Fracture Case Several analytical criteria have been proposed in the past to predict the interaction between NFs and VPHFs. These criteria have the following common features: (i) The hydraulic fracture height is assumed to be constant; (ii) The effect of natural fracture dip angle is neglected, and only the effect of approach angle is considered; (iii) The effect of vertical in-situ stress is neglected. Blanton [14] put forward an analytical criterion to predict interaction results. He assumed that HFs will be blunted and arrested at least temporarily when they intersect NFs. NF dilation occurs if the pore pressure at the intersection point (in a plane strain problem) exceeds the normal stress applied on the NF surface; and crossing occurs if this pressure exceeds the pressure required for HF re-initiation. He considered the HF reinitiation direction is perpendicular to the NF surface, so the required pressure for re-initiation is equal to the sum of the stresses applied parallel to the NF surface and rock tensile strength. This analytical criterion can be mechanically written as: (1)

3 for crossing, and (2) for dilation, where p is the pore pressure at the intersection point, is stress applied parallel to the NF surface, is stress applied perpendicularly to the NF surface and T is rock tensile strength. Based on the elastic solution of stresses in the interaction zone, crossing occurs if the following inequality is satisfied: (3) where is the approach angle, and the parameter b can be calculated by the approach proposed by Blanton. Sarmadivaleh [22] stated that the Blanton s criterion overestimates the needed normal stress applied on the NF surface where crossing occurs. This is also reported by Lianos [23] through comparison with experimental results. Furthermore, this criterion implies that if no slippage occurs, HFs can always cross NFs. This is inconsistent with real situations. Warpinski and Teufel [16] noted that the HF will be arrested by the NF if the normal stress applied on the NF surface is not sufficient enough to avoid NF slippage, and that HF will propagate into NF if the pore pressure exceeds the normal stress applied on the NF surface. Based on the Warpinski and Teufel s criterion, shear slippage occurs if the following inequality is satisfied: (4) where c is NF cohesion, p n is the net pressure (the difference between the pore pressure and the minimum principle stress), and f is the coefficient of friction. HFs will propagate into NFs if the following inequality is satisfied: From Eqs. (4) and (5), it shows both approach angles and horizontal differential stresses make important influences on shear slippage and dilation. HFs are more likely to cross NFs if approach angles and horizontal differential stresses are sufficiently high. Renshaw and Pollard [17] provided another crossing criterion. In this criterion, HFs will either cross or be arrested by NFs. They assumed that crossing will occur if NF shear slippage does not happen. The linear fracture theory was applied to build up the following crossing criterion:... (5) (6) If Eq. (6) is satisfied, crossing will occur. This criterion is only valid for orthogonal NF cases, and was modified by Gu and Weng for non-orthogonal cases [24, 25]. According to the above analytical criteria, the resultant stress applied perpendicularly to the NF surface makes an important influence on NF dilation. In these criteria, the effects of NF dip angle and vertical principle stress are neglected. This is reasonable in the VPHF case. As shown in Figure 4, the HF plane intersects the NF plane in a vertical line. Due to the assumption of constant HF height, the dilation direction is perpendicular to this intersection line, and no fluid flows upwards or downwards. Actually, the resultant force is perpendicular to the NF surface in a plane strain problem, and it is parallel to the horizontal plane in the three-dimensional space. Thus, only the two horizontal in-situ stresses and the approach angle have effects on the resultant stress in the VPHF case. On the contrary, when the NF is intersected by a HRHF, the intersection line is horizontal, and the dilation direction is upward or downward as shown in Figure 5. The compressive stress to resist NF dilation or slippage is really applied perpendicularly to the NF surface. In this situation, the effects of both NF fracture dip angle and vertical in-situ stress need to be considered. side view plan view Fig. 4. Resultant stress direction in the VPHF case side view (a) side view (b) Fig. 5. Resultant stress direction in the HRHF case 2.2. Modified Interaction Criterion for Horizontal Radial Hydraulic Fracture Cases Based on the foregoing analytical criteria, a simple interaction criterion which accounts for the effects of NF dip angle and vertical in-situ stress in the HRHF case is

4 proposed. No NF slippage is considered in the current study. It is assumed that the HF will be arrested in the intersection line at least temporarily, and then the pore pressure in the intersection line will increase. If the pore pressure exceeds the stress applied perpendicularly to the NF surface, the NF will be dilated and mechanically written in Eq. (7): (7) If the pore pressure exceeds the sum of rock tensile strength and the vertical in-situ stress, the HF will propagate into its original direction and mechanically written in Eq. (8): (8) The in-situ stress tensor in coordinate xyz, in which each axis coincides with the corresponding in-situ stress direction, can be written as below: (9) To calculate the stress applied perpendicularly to the NF surface, we set up a new coordinate x'y'z' in which the x' axis is perpendicular to the intersection line, the y' axis is parallel to the intersection line and the z ' axis coincides with the vertical in-situ stress direction. The components of in-situ stress tensor in coordinate xyz can be transformed to those in coordinate x'y'z' by Eqs. (10) and (11). (10) 1 (11) where is the component of in-situ stress tensor in the i'th row and j'th column, is the component of in-situ stress tensor in the ith row and jth column, and is the component of tensor in the ith row and i'th column. The stress tensor in coordinate x'y'z' can be calculated as below: σ cos θσ sin θσ sinθcosθ σ σ sinθcosθ σ σ sin θσ cos θσ (12) σ We define e n as the NF plane direction vector in the coordinate x'y'z': sinβ 0 cosβ (13) where β is the NF dip angle, then the resultant stress,, applied perpendicularly to the NF surface can be calculated as below: (14) From Eq. (7) and (8), it shows that HFs tend to cross NFs if the following inequality is satisfied: 0 (15) Substituting Eq. (14) into the above inequality, we can derive the simple crossing criterion as below: (16) It shows that both the approach angle and the NF dip angle need to be considered. Besides horizontal in-situ stresses, the vertical in-situ stress also makes an influence on the interaction result. Meanwhile, HFs are more likely to cross NFs in rock masses with low tensile strength. 3. MODELING VALIDATION Up to now, no experiments have been carried out to investigate the interaction between NFs and HRHFs. In this paper, the commercial software 3DEC 5.0 has been utilized to study the effects of NF dip angle, approach angle, in-situ stresses, rock tensile strength and treatment parameters on the interaction result. 3DEC has been applied to predict the interaction between NF networks and a VPHF [26-31]. In these researches, a fictitious fracture has been pre-defined to represent the HF propagation plane. The properties of this fictitious fracture were calculated by [32]: (17) (18) where G is the shear modulus, JKS is the fictitious joint shear stiffness, JKN is the fictitious joint normal stiffness and E is the Young s modulus Model Establishment The following assumptions have been adopted to establish the models: (i) Fluid is injected from a point source, and the HRHF will always propagate in the pre-defined horizontal plane which is normal to the vertical in-situ stress; (ii) To study whether if the propagating HF will cross or dilate the NF under the given testing parameters, it is assumed that no shear slippage occurs on the NF surface; (iii) As shown in Figure 6, the distance between the injection point and the intersection line is 4 m. It implies that the NF is assume to be 4 m away from the injection point; (iv) For simplicity, the angles between the intersection line and the positive x axis are assumed to be 90 degrees in all the tests.

5 The selected input parameters in most tests are listed in Table 1. The rock properties are designed to represent the mining block condition at Northparkes, Australia [5]. The properties of the pre-defined fictitious fracture are calculated by Eq. (17) and (18). The properties of NFs are set to be mid-range values [33]. The fracture aperture after in-situ stress initialization and the allowable minimum fracture aperture are 10-3 m and 10-4 m, respectively. If the aperture of either HF or NF exceeds its initial value, it implies the HF propagates or the NF is dilated. In BC mining, the injection flow rate is typically from 5~10 L/s, so the injection flow rate in most tests is 10-2 m 3 /s. The fluid viscosity is 0.58 Pa s, which coincides with that used in the experiments carried out by Bunger [34]. The injected fluid is assumed to be incompressible. The maximum mechanical cycles per fluid cycle are 1000, and the limits for the average and the maximum unbalanced force is 10-3 Pa and 10-2 Pa, respectively. The model size is 40 m 40 m 20 m, and the element edge size is 1 m. Fig. 6. Schematic of the established model Table 1. Input parameters Rock property Hydraulic fracture Natural fracture Injected fluid Young s modulus Poisson ratio Tensile strength 60 GPa MPa Density 2500 kg/m 3 Normal Shear Tensile stiffness stiffness strength 6000 GPa/m 4200 GPa/m 10 MPa Normal Shear Tensile stiffness stiffness strength 60 GPa/m 30 GPa/m 0 MPa Flow rate Viscosity Density 10-2 m 3 /s 0.58 Pa s 1000 kg/m Effect of Natural Fracture Dip Angle According to Eq. (16), HFs are more likely to cross NFs with the increase of NF dip angle β. The following insitu stress conditions and approach angle are considered: 30, 20, 90. Five modeling tests were carried out. In these tests, the NF dip angles change from 30 degrees to 90 degrees at 15 degree intervals, and the other input parameters are the same as those in Table 1. The modeling results are given in Figure 7 and Table 2. Table 2. Modeling results of NF dip angle variation Modeling test Dip angle/degree Interaction result 1 90 Crossing 2 75 Crossing 3 60 Crossing 4 45 Dilation 5 30 Dilation 45 Fig. 7. Effect of natural fracture dip angle 60 From the modeling results, it indicates that: (i) Different from the VPHF case, not only the approach angle, but also the NF dip angle influences the interaction result. In all the above tests, the approach angles were always set to be 90 degrees, in which condition the HF is more likely to cross the NF in the VPHF case. However, the interaction results may change when NF dip angles change. (ii) The HF tends to cross the NF with big dip angle, while it is more likely to dilate the NF if the NF dip angle is relatively small Effect of Approach Angle Six modeling tests were carried out to investigate the effect of approach angle on the interaction results. In these tests, the in-situ stress conditions and the NF dip angle are: 50, 40, 50, 45. As mentioned in assumption (4), the approach angles are always set to be 90 degrees in all the tests. To represent the change of approach angles in each test, the in-situ stress conditions are transformed by Eq. (12). The approach angles change from 15 degrees to 90 degrees at 15 degree intervals, and the other input parameters are the same as those in Table 1. The modeling results are given in Figure 8 and Table 3. The modeling results indicate that the change of approach angle will influence the interaction result. When the approach angle increases, the maximum horizontal principle stress can provide more resistance on the NF surface to avoid NF dilation. The HF is more likely to cross the NF with big approach angle.

6 Table 3. Modeling results of approach angle variation Modeling test Approach angle/degree Interaction result 1 90 Crossing 2 75 Crossing 3 60 Crossing 4 45 Dilation 5 30 Dilation 6 15 Dilation and the HF crosses the NF. When either of D 1 and D 2 decreases from 25 MPa to 0 MPa in test 2 and test 3, NF dilation occurs as shown in Figure 9. It shows that the interaction result is determined by both the horizontal differential stress and the difference between the minimum horizontal in-situ stress and the vertical in-situ stress. (ii) In test 4, D 1 decreases from 25 MPa to 12 MPa, the HF can still cross the NF. In test 5, D 2 decreases from 25 MPa to 12 MPa. However, in this situation NF dilation occurs as shown in Figure 9. It shows that the interaction result is more sensitive to D 2, the difference between the vertical in-situ stress and the minimum horizontal in-situ stress. Table 4 Modeling results of in-situ stresses variation 45 θ 60 Fig. 8. Effect of approach angle 3.4. Effect of In-situ Stresses D 1 /MPa D 2 /MPa Interaction Result Test Crossing Test Dilation Test Dilation Test Crossing Test Dilation In this study the difference between and is expressed as D 1, and the difference between and as D 2. This can be mathematically written as: (19) (20) then σ and σ can be expressed as: (21) (22) Substituting D 1 and D 2 into Eq. (16), the following inequality can be derived: (23) If Eq. (23) is satisfied, HFs tend to cross NFs. It shows that both D 1 and D 2 may have influence on the interaction result. Meanwhile, this result may be more sensitive to D 2. D 1 =0 MPa D 2 =25 MPa D 1 =12 MPa D 2 =25 MPa Fig. 9. Effect of in-situ stresses D 1 =25 MPa D 2 =0 MPa D 1 =25 MPa D 2 =12 MPa Five modeling tests were set up to investigate the effects of D 1 and D 2 on the interaction results. In all these tests, the NF dip angles and the approach angles are set to be 45 degrees, and the other input parameters are the same as those in Table 1. In test 1, D 1 and D 2 are the same as 25 MPa; in test 2 and 3, D 1 and D 2 decreases from 25 MPa to 0 MPa, respectively; in test 4 and 5, D 1 and D 2 decreases from 25 MPa to 12 MPa, respectively. The modeling results are given in Figure 9 and Table 4. From the modeling results, it is found that: (i) Both D 1 and D 2 have influences on the interaction results. In test 1, D 1 and D 2 are the same as 25 MPa, 3.5. Effect of Rock Tensile Strength Besides NF orientation and in-situ stress conditions, rock tensile strength also has an influence on the interaction results. According to Eqs. (3) and (6), the HF tends to cross the NF if rock tensile strength is sufficiently low in a VPHF case. Rock masses in BC mining usually have higher tensile strength compared with those in the oil & gas industry, and sometimes have mixed-quality. Five modeling tests were carried out to investigate the effect of rock tensile strength on the interaction between NFs and a HRHF. In all these tests, the following in-situ stress conditions, NF dip angle and approach angle are

7 considered: 40, 30, 45. The rock tensile strength in these tests changes from 1 MPa to 10 MPa as in Table 5, and the other input parameters are the same as those in Table 1. The modeling results are given in Figure 10 and Table 5. Table 5 Modeling results of rock tensile strength variation Tensile strength/mpa Interaction result Test 1 1 Crossing Test Crossing Test 3 5 Crossing Test Dilation Test 5 10 Dilation Table 6 Modeling results of treatment parameters variation Flow rate/m 3 /s Fluid viscosity/pa s Product m 3 Pa/s 2 Interaction result Test Dilation Test Dilation Test Crossing Test Crossing Test Crossing 0.01 / / 2 T=5 MPa T=7.5 MPa Fig. 10. Effect of rock tensile strength From the modeling results, it is found that: (i) The same as the VPHF case, HRHFs tend to cross NFs if rock tensile strength is sufficiently low. (ii) Due to the mixed-quality of rock masses in BC mining, The success of a hydraulic fracturing operation in a mining block with low tensile strength does not necessarily ensure the same success in a mining block with high tensile strength Effect of Flow Rate and Fluid Viscosity Experimental and numerical studies demonstrated that the interaction between NFs and a VPHF is influence by flow rate and fluid viscosity [18-21, 35, 36]. It indicated that it is the product of flow rate and fluid viscosity which influences the final result. In hydraulic fracture operations, the controllable parameters are flow rate and fluid viscosity (and sometimes the borehole location). By increasing either of these two parameters, HFs are more likely to cross NFs. Five modeling tests were carried out to investigate the effect of flow rate and fluid viscosity on the interaction of NFs and a HRHF. In all these tests, the in-situ stress conditions and the NF orientation are: 35, 30, 45. The other input parameters are the same as those in Table 1. The treatment parameters are listed in Table 6, and the modeling results are given in Figure 11 and Table / 1 Fig. 11. Effect of treatment parameters It is clear from the modeling results that: (i) 0.02 / 5 When HFs dilate NFs, the pore pressure in the intersection line will increase. HRHFs tend to reinitiate if the product of flow rate and fluid viscosity is sufficiently high. This is consistent with the VPHF case. (ii) It is the product of flow rate and fluid viscosity that makes an influence on the interaction result. In test 1, the treatment parameters are the same as those in Table 1. In this case, the NF is dilated and the HF propagation is arrested. In test 2 and test 3, either flow rate or fluid viscosity increases, and the product value in test 3 is higher than that in test 2. In test 2, the NF is dilated and no crossing occurs. In test 3, the HF can cross the NF due to the relatively high product value as shown in Figure 11. (iii) In test 4 and test 5, the product of flow rate and fluid viscosity is equal, though either of these two parameters has marked difference. In both tests, the HFs can cross the NFs because of much higher product as shown in Figure 11.

8 4. CONCLUSIONS AND RECOMMENDATIONS Hydraulic fractures in the oil & gas industry are commonly vertical planar types. However, in block cave mining they are generally horizontal radial types. The interaction between natural fractures and a vertical planar hydraulic fracture has been widely studied, but the applicability of the derived conclusions in block cave mining should be examined. In the vertical planar hydraulic fracture case, the effects of vertical in-situ stress and natural fracture dip angle are not accounted for. In this paper, the effects of natural fracture dip angle, approach angle, in-situ stresses, rock tensile strength and treatment parameters on the interaction between natural fractures and a horizontal radial hydraulic fracture are investigated by theoretical analysis and numerical modeling. The following conclusions and recommendations have been achieved: (i) Not only the approach angle, but also the natural fracture dip angle has an influence on the interaction result. Hydraulic fractures may be arrested by natural fractures with big approach angles but small dip angles. Crossing tends to occur if both the approach angle and the natural fracture dip angle are sufficiently high. (ii) All the three in-situ stresses should be considered to predict the interaction result in block cave mining. Not only the difference between horizontal in-situ stresses, but also the difference between the minimum horizontal in-situ stress and the vertical insitu stress has an influence on the interaction result. Hydraulic fractures are more likely to cross natural fractures if both of these differential stresses are sufficiently high. Meanwhile, the interaction result is more sensitive to the latter one. (iii) Consistent with the results in the vertical planar hydraulic fracture case, rock tensile strength has an influence on the interaction result. Crossing tends to occur if rock tensile strength is sufficiently low. The success of a hydraulic fracturing operation in a mining block with low tensile strength does not necessarily ensure the same success in a mining block with high tensile strength. (iv) High flow rate and fluid viscosity are in favor of the re-initiation of hydraulic fracture from the intersection line when natural fracture dilation occurs. The product of these two parameters determines whether if re-initiation occurs. High flow rate and fluid viscosity are recommended in regions where hydraulic fractures are not able to cross natural fractures. REFERENCES 1. Clark, J A hydraulic process for increasing the productivity of wells. Journal of Petroleum Technology. 1(01): p Chernov, O. and N. Kyu Oriented rupture of solids by highly viscous fluid. Journal of Mining Science. 32(5): p Fan, J., et al Directional hydraulic fracturing to control hard-roof rockburst in coal mines. International Journal of Mining Science and Technology. 22(2): p He, H., et al Deep-hole directional fracturing of thick hard roof for rockburst prevention. Tunnelling and Underground Space Technology. 32: p Van As, A. and R. Jeffrey Caving induced by hydraulic fracturing at Northparkes mines. in 4th North American Rock Mechanics Symposium. American Rock Mechanics Association. 6. Catalan, A., et al How Can an Intensive Preconditioning Concept Be Implemented At Mass Mining Method? Application to Cadia East Panel Caving Project. in 46th US Rock Mechanics/Geomechanics Symposium. American Rock Mechanics Association. 7. Chen, Z. and R. Jeffrey Tilt monitoring of hydraulic fracture preconditioning treatments. in 43rd US Rock Mechanics Symposium & 4th US-Canada Rock Mechanics Symposium. American Rock Mechanics Association. 8. Kaiser, P.K., et al Hydraulic Fracturing Mine Back Trials Design Rationale and Project Status. 9. Van As, A. and R. Jeffrey Hydraulic fracturing as a cave inducement technique at Northparkes Mines. Proceedings, MassMin. p Chacon, E., et al Hydraulic fracturing used to precondition ore and reduce fragment size for block caving. A. Karzulovic and MA Al faro (Eds), MassMin August. p Dahi Taleghani, A. and J.E. Olson How Natural Fractures Could Affect Hydraulic-Fracture Geometry. SPE Journal. (Preprint). 12. Lamont, N. and F. Jessen The effects of existing fractures in rocks on the extension of hydraulic fractures. Journal of Petroleum Technology. 15(02): p Daneshy, A.A Hydraulic fracture propagation in the presence of planes of weakness. in SPE European Spring Meeting. Society of Petroleum Engineers. 14. Blanton, T Propagation of hydraulically and dynamically induced fractures in naturally fractured reservoirs. in SPE Unconventional Gas Technology Symposium. Society of Petroleum Engineers. 15. Blanton, T.L An experimental study of interaction between hydraulically induced and preexisting fractures. in SPE Unconventional Gas Recovery Symposium. Society of Petroleum Engineers. 16. Warpinski, N. and L. Teufel Influence of geologic discontinuities on hydraulic fracture

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