**4. Discussion**

The goal of the effort was to quantitatively consider the change in natural fracture shear (shear being analogous with microseismicity generation and the potential stimulation of the natural fractures providing increased production from the hydraulic fracture) for multi-well comple‐ tions. It is commonly believed that by configuring the geometry and injection behavior from parallel wellbores (e.g., simultaneous fracturing, zipper-fracs, and modified zipper-fracs), shear of the natural fractures can be enhanced (thereby increasing production).

fracture friction also depends upon the underlying natural fracture pattern (and stress ratio). The area of shear generated for the '180°' DFN at a friction angle of 15° was nearly equal to the area of natural fracture shear for the '145°' DFN at a friction angle of 25° (5740m2 versus

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Less so for the '145°' DFN and more so for the '180°' DFN, the higher fracture friction tended to push the point of maximum total length of natural fracture shear towards longer Xf2 halflengths; however, these longer half-lengths also represented conditions when there was a net loss of natural fracture shear for two dual hydraulic fractures over two equivalent independent

Figures 25 and 32 suggest that hydraulic fracture separation did not significantly affect the maximum total length of natural fracture shear (more so for the '180°' DFN and less so for the '145°' DFN). However, the influence of separation spacing was more apparent when the Xf2

Though Figures 25 and 32 may suggest a somewhat limited influence of hydraulic fracture spacing, this is clearly not the whole picture. As shown in Figures 9 through 12 in particular, and somewhat in Figures 13 through 20, the critical issues for hydraulic fracture separation are to: 1) shear as much total formation as possible; and 2) not cause a net loss of natural fracture shear by placing hydraulic fractures too close. Figure 10 shows that at a 45m hydraulic fracture separation distance (for dual, 125m-long hydraulic fractures and a natural fracture friction angle of 15°) the shear area from the two hydraulic fractures still overlapped (when the hydraulic fractures act independently). In contrast, Figure 12 shows that a 45m separation

The simulation results (especially Figures 25 and 32) show that the amount of natural fracture shear is significantly influenced by the half-length of the Xf2 hydraulic fracture in a dual fracture configuration. The overall trend of the results is that keeping the half-length of Xf2 small enough to prevent the tip of Xf2 from getting closer than 25m to the tip of Xf1 (that is, no overlap of the hydraulic fractures) creates the maximum total length of natural fracture shear. Further, as shown in Figures 9 through 12, keeping the half-length of Xf2 small enough may also cause a net increase in natural fracture shear (over that from two independent

**•** As natural fracture orientation (relative to the orientation of a hydraulic fracture) signifi‐ cantly influences the amount and location of natural fracture shear, multi-well completion

**4.3. Observations on the influence of hydraulic fracture separation distance**

distance may be too much when natural fracture friction angle is 25°.

**4.4. Observations on the influence of hydraulic fracture Xf2 half-length**

hydraulic fractures), which is the goal of a dual hydraulic fracture configuration.

5250m2 ).

hydraulic fractures.

**5. Conclusions**

half-length was 100m or longer.

During the evaluations presented in this paper, the following parameter effects were consid‐ ered:


#### **4.1. Observations on the influence of fracture network**

As shown in Figures 6 and 8, the natural fracture shear pattern coming off the tip of a propa‐ gating hydraulic fracture depends upon the orientation and nature of the natural fracture system. For the '180°' DFN, natural fracture shear extended a bit beyond the hydraulic fracture tip, but mainly lay in a symmetrical pattern perpendicular to the direction of hydraulic fracture propagation. In contrast, for the '145°' DFN, the natural fracture shear pattern was asymmetric and lead the tip of the propagating hydraulic fracture. Clearly as observed in previous publications (Nagel et al. 2011a), interpreting microseismic event locations cannot be done without consideration of the general orientation of the natural fracture pattern.

The natural fracture pattern also plays a role in the amount of natural fracture shear (and, by analogy, the number of microseismic events). For the same natural fracture friction (and same in-situ stress), the total area of natural fracture shear for the '180°' DFN was only 42% of that for the '145°' DFN (2220m2 versus 5250m2 ). However, as shown in the graphs in Figures 25 and 32, the overall trends in the cumulative length of natural fracture shear from dual hydraulic fractures was similar (with the exception of the 35m spacing for the '145°' DFN in which natural fracture shear was very low).

#### **4.2. Observations on the influence of natural fracture friction**

As evident from the figures of natural fracture shear and the quantitative results in Figures 25 and 32, natural fracture friction plays a significant role in determining the amount of natural fracture shear (and, by analogy, the number of microseismic events). The influence of natural fracture friction also depends upon the underlying natural fracture pattern (and stress ratio). The area of shear generated for the '180°' DFN at a friction angle of 15° was nearly equal to the area of natural fracture shear for the '145°' DFN at a friction angle of 25° (5740m2 versus 5250m2 ).

Less so for the '145°' DFN and more so for the '180°' DFN, the higher fracture friction tended to push the point of maximum total length of natural fracture shear towards longer Xf2 halflengths; however, these longer half-lengths also represented conditions when there was a net loss of natural fracture shear for two dual hydraulic fractures over two equivalent independent hydraulic fractures.

#### **4.3. Observations on the influence of hydraulic fracture separation distance**

Figures 25 and 32 suggest that hydraulic fracture separation did not significantly affect the maximum total length of natural fracture shear (more so for the '180°' DFN and less so for the '145°' DFN). However, the influence of separation spacing was more apparent when the Xf2 half-length was 100m or longer.

Though Figures 25 and 32 may suggest a somewhat limited influence of hydraulic fracture spacing, this is clearly not the whole picture. As shown in Figures 9 through 12 in particular, and somewhat in Figures 13 through 20, the critical issues for hydraulic fracture separation are to: 1) shear as much total formation as possible; and 2) not cause a net loss of natural fracture shear by placing hydraulic fractures too close. Figure 10 shows that at a 45m hydraulic fracture separation distance (for dual, 125m-long hydraulic fractures and a natural fracture friction angle of 15°) the shear area from the two hydraulic fractures still overlapped (when the hydraulic fractures act independently). In contrast, Figure 12 shows that a 45m separation distance may be too much when natural fracture friction angle is 25°.

#### **4.4. Observations on the influence of hydraulic fracture Xf2 half-length**

The simulation results (especially Figures 25 and 32) show that the amount of natural fracture shear is significantly influenced by the half-length of the Xf2 hydraulic fracture in a dual fracture configuration. The overall trend of the results is that keeping the half-length of Xf2 small enough to prevent the tip of Xf2 from getting closer than 25m to the tip of Xf1 (that is, no overlap of the hydraulic fractures) creates the maximum total length of natural fracture shear. Further, as shown in Figures 9 through 12, keeping the half-length of Xf2 small enough may also cause a net increase in natural fracture shear (over that from two independent hydraulic fractures), which is the goal of a dual hydraulic fracture configuration.

### **5. Conclusions**

**4. Discussion**

542 Effective and Sustainable Hydraulic Fracturing

ered:

DFN);

and

for the '145°' DFN (2220m2

fracture shear was very low).

The goal of the effort was to quantitatively consider the change in natural fracture shear (shear being analogous with microseismicity generation and the potential stimulation of the natural fractures providing increased production from the hydraulic fracture) for multi-well comple‐ tions. It is commonly believed that by configuring the geometry and injection behavior from parallel wellbores (e.g., simultaneous fracturing, zipper-fracs, and modified zipper-fracs),

During the evaluations presented in this paper, the following parameter effects were consid‐

**2.** Natural fracture friction angle (15° and 25° for the '180°' DFN and 25° and 35° for the '145°'

**4.** Hydraulic fracture half-length from the second wellbore (Xf2 helf-lengths of 50m to 125m);

**5.** In-situ stress (from a horizontal stress ratio – SHmax/Shmin - of 1.18 to a ratio of 1.03).

As shown in Figures 6 and 8, the natural fracture shear pattern coming off the tip of a propa‐ gating hydraulic fracture depends upon the orientation and nature of the natural fracture system. For the '180°' DFN, natural fracture shear extended a bit beyond the hydraulic fracture tip, but mainly lay in a symmetrical pattern perpendicular to the direction of hydraulic fracture propagation. In contrast, for the '145°' DFN, the natural fracture shear pattern was asymmetric and lead the tip of the propagating hydraulic fracture. Clearly as observed in previous publications (Nagel et al. 2011a), interpreting microseismic event locations cannot be done

The natural fracture pattern also plays a role in the amount of natural fracture shear (and, by analogy, the number of microseismic events). For the same natural fracture friction (and same in-situ stress), the total area of natural fracture shear for the '180°' DFN was only 42% of that

32, the overall trends in the cumulative length of natural fracture shear from dual hydraulic fractures was similar (with the exception of the 35m spacing for the '145°' DFN in which natural

As evident from the figures of natural fracture shear and the quantitative results in Figures 25 and 32, natural fracture friction plays a significant role in determining the amount of natural fracture shear (and, by analogy, the number of microseismic events). The influence of natural

). However, as shown in the graphs in Figures 25 and

without consideration of the general orientation of the natural fracture pattern.

versus 5250m2

**4.2. Observations on the influence of natural fracture friction**

**3.** Hydraulic fracture separation (offset between injection points) from 20m to 45m;

shear of the natural fractures can be enhanced (thereby increasing production).

**1.** Fracture network orientation (i.e., the '180°' DFN and the '145°' DFN);

**4.1. Observations on the influence of fracture network**

**•** As natural fracture orientation (relative to the orientation of a hydraulic fracture) signifi‐ cantly influences the amount and location of natural fracture shear, multi-well completion optimization (wherein the goal is to maximize natural fracture shear, i.e., maximize 'complexity') requires the evaluation and consideration of natural fracture orientation.

[4] King, G. E. (2010). Thirty Years of Gas Shale Fracturing: What Have We Learned?", Paper SPE 133456 presented at the SPE Annual Technical Conference and Exhibition,

Quantitative Evaluation of Completion Techniques on Influencing Shale Fracture 'Complexity'

http://dx.doi.org/10.5772/56304

545

[5] Nagel, N, & Sanchez-nagel, M. (2011). Stress Shadowing and Microseismic Events: A Numerical Evaluation", Paper SPE 147363 presented at the SPE Annual Technical

[6] Nagel, N, Damjanac, B, Garcia, X, & Sanchez-nagel, M. (2011b). Discrete Element Hy‐ draulic Fracture Modeling- Evaluating Changes in Natural Fracture Aperture and Transmissivity", Paper SPE 148957 presented at the Canadian Unconventional Re‐

[7] Nagel, N, Gil, I, Sanchez-nagel, M, & Damjanac, B. (2011a). Simulating Hydraulic Fracturing in Real Fractured Rock- Overcoming the Limits of Pseudo3D Models", Pa‐ per SPE 140480 presented at the SPE Hydraulic Fracturing Technology Conference

[8] Nagel, N, Sanchez-nagel, M, & Lee, B. T. (2012a). Gas Shale Hydraulic Fracturing: A Numerical Evaluation of the Effect of Geomechanical Parameters", Paper SPE 152192 presented at the SPE Hydraulic Fracturing Technology Conference and Exhibition,

[9] Nagel, N. B, Garcia, X, Lee, B, & Sanchez-nagel, M. (2012d). Hydraulic Fracturing Optimization for Unconventional Reservoirs- The Critical Role of the Mechanical Properties of the Natural Fracture Network", Paper SPE 161934 presented at the SPE Canadian Unconventional Resources Conference, Calgary, Alberta, Canada, 30 Octo‐

[10] Nagel, N. B, Sanchez-nagel, M, Zhang, F, Garcia, X, & Lee, B. (2013). Coupled Nu‐ merical Evaluations of the Geomechanical Interactions Between a Hydrualic Fracture Stimulation and a Natural Fracture System in Shale Formations", Rock Mechanics

[11] Nagel, N. B, Sanchez-nagel, M, Garcia, X, & Lee, B. (2012b). A Numerical Evaluation of the Geomechanical Interactions Between a Hydraulic Fracture Stimulation and a Natural Fracture System", ARMA presented at the 46th Rock Mechanics / Geome‐

[12] Nagel, N. B, Sanchez-nagel, M, Garcia, X, & Lee, B. SRV": A Numerical Investigation of "Wet" vs. "Dry" Microseismicity During Hydraulic Fracturing", Paper SPE 159791 presented the SPE Annual Technical Conference and Exhibition held in San Antonio,

[13] Sneddon, I. N. (1946). The Distribution of Stress in the Neighbourhood of a Crack in

Conference and Exhibition, Denver, Colorado, USA, 30 October-2 November.

sources Conference, Calgary, Alberta, Canada, November., 15-17.

and Exhibition, The Woodlands, Texas, USA, January., 24-26.

The Woodlands, Texas, USA, February., 6-8.

and Rock Engineering, pending publication.

chanics Symposium, Chicago, Illinois, 24-27 June., 12-287.

an Elastic Solid", Proc. R. Soc. London, Ser. A. , 195, 229-260.

ber- 1 November.

Texas, USA, October., 8-10.

Florence, Italy, September., 19-22.

