**3. Quantitative numerical evaluation of modified zipper-fracs**

#### **3.1. Natural fracture shear from a single hydraulic fracture**

A first series of base case simulations were conducted in order to evaluate the natural fracture shear from a single fracture. The simulations looked at the growth of the Xf1 hydraulic fracture as well as the Xf2 hydraulic fracture for two different fracture friction angles. These base case simulations are important because, in order to correctly evaluate the benefit or detriment of the dual frac modified zipper-frac completion, the effect of the two fractures Xf1 and Xf2 completely independent of each other needs to be considered.

Figure 5 shows the natural fracture shear region (in blue) for both the 15° (plot A) and the 25° friction angle simulations for the '180°' DFN model when Xf1 had a fracture half-length of 100m. As an example, the total cumulative length of natural fractures at shear condition in plot A (the sum of the length of the natural fractures in blue in Figure 5) was 300.1m versus 80.8m in plot B. Figure 6 shows the combined shear for the five Xf1 length simulations and the combined sheared area (shaded area) for the propagation of Xf1 from the wellbore to a 125m half-length. As expected, the area of shear for the lower friction simulations is considerably greater (5740m2 ) than for the higher friction simulations (2220m2 ).

The shaded area in Figure 6 and others, adjusted for the length of Xf2 in the dual fracture simulations, represents the sheared area for Xf2 when Xf2 was created independently of Xf1. This shaded area then allows for comparison of independent Xf1 and Xf2 hydraulic fracture effects to modified zipper-frac effects.

**2.4. Simulation matrix**

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greater (5740m2

effects to modified zipper-frac effects.

In total, nearly 100 simulations were performed in order to explore the behavior of the modified zipper-frac completion scheme. 20 simulations were performed to look at the shear results from a single hydraulic fracture with the '180°' (DFN#1 from Tables 1 and 2) varying the length of both fractures Xf1 and Xf2 separately (Xf1 represents the left-side hydraulic fracture - the solid black line in Figure 4 - and Xf2 represents the right-side hydraulic fracture – the dashed lines in Figure 4) from 25m to 125m in 25m increments with a friction angle of 15 degrees (10 simulations) and repeating these with a 25 degree friction angle (10 simulations). Then 60 simulations were performed to look at the efficacy of the modified zipper-frac completion by performing simulations with dual hydraulic fractures with both DFN models (the '180°' and '145°') varying the length of Xf2 (0m, 50, 75m, 100m, and 125m) for a constant Xf1 of 125m length for three separation spacings (20m, 35, and 45m, where separation is the horizontal offset of the Xf1 and Xf2 fractures as shown in Figure 4) and for two friction angles (15 degrees and 25 degrees in the '180°' model and 25 degrees and 35 degrees in the '145°' model). Finally, an additional 15 simulations were performed with the '180°' model varying the initial in-situ

stress (see Table 2) and Xf2 length, and keeping the friction angle at 25 degrees.

**3. Quantitative numerical evaluation of modified zipper-fracs**

A first series of base case simulations were conducted in order to evaluate the natural fracture shear from a single fracture. The simulations looked at the growth of the Xf1 hydraulic fracture as well as the Xf2 hydraulic fracture for two different fracture friction angles. These base case simulations are important because, in order to correctly evaluate the benefit or detriment of the dual frac modified zipper-frac completion, the effect of the two fractures Xf1 and Xf2

Figure 5 shows the natural fracture shear region (in blue) for both the 15° (plot A) and the 25° friction angle simulations for the '180°' DFN model when Xf1 had a fracture half-length of 100m. As an example, the total cumulative length of natural fractures at shear condition in plot A (the sum of the length of the natural fractures in blue in Figure 5) was 300.1m versus 80.8m in plot B. Figure 6 shows the combined shear for the five Xf1 length simulations and the combined sheared area (shaded area) for the propagation of Xf1 from the wellbore to a 125m half-length. As expected, the area of shear for the lower friction simulations is considerably

The shaded area in Figure 6 and others, adjusted for the length of Xf2 in the dual fracture simulations, represents the sheared area for Xf2 when Xf2 was created independently of Xf1. This shaded area then allows for comparison of independent Xf1 and Xf2 hydraulic fracture

).

) than for the higher friction simulations (2220m2

**3.1. Natural fracture shear from a single hydraulic fracture**

completely independent of each other needs to be considered.

**Figure 5.** Natural fracture shear (blue lines) for a 100m-long Xf1 hydraulic fracture. A) Shear for the 15° fracture fric‐ tion case; and B) Shear for the 25° fracture friction case.

**Figure 6.** Cumulative natural fracture shear (shaded area) from simulations at 25 to 125m hydraulic fracture half length. A) Shear for the 15° friction case with an area of 5740m2; and B) Shear for the 25° friction case with an area of 2220m2.

Figure 7 shows the growth of sheared natural fracture length as a function of hydraulic fracture half-length for the 15° and 25° natural fracture friction cases for all 20 single fracture simula‐ tions. Not surprisingly, given the slight variability in the statistics for natural fracture gener‐ ation, there are slight, insignificant differences between the results for the Xf1 and Xf2 simulations.

In summary, Figures 5 through 8 suggest the following:

for the 25° case and 2490 m2

2012a).

5250m2

friction cases.

friction angle.

significant (5740m2

the '180°' DFN).

**•** Natural fracture shear from the total stress change caused by the inflated hydraulic fracture travels with the tip of a growing hydraulic fracture (as reported in Nagel et al. 2011a and

**•** The length of natural fractures being sheared increases significantly with length (Figure 7). **•** The length of natural fractures being sheared is strongly a function of natural fracture

**•** The area (and by default the volume) of formation sheared by a single fracture can also be

**•** The orientation of the natural fractures significantly affects natural fracture shear for a given fracture friction (at 25° friction, more than twice the shear occurred for the '145°' DFN as for

Figures 9 through 12 show the superimposed natural fracture shear areas from independent hydraulic fractures for multi-well completions with hydraulic fracture separations ranging from zero (equivalent to either the simultaneous or zipper-fracs) to 45m for both fracture

**Figure 9.** Superimposed natural fracture shear areas for the 15° friction case when Xf1 and Xf2 are both 125m in halflength. A) Zero separation between the two hydraulic fractures; and B) A 20m separation between fractures.

Within the figures, the regions of overlap would likely represent areas of 'wasted' hydraulic fracture shear (and, perhaps, a negative effect on production as excess shear will cause the natural fractures to reclose and even fill with gouge). Ideally, the best effect, assuming no

for the 35° case of the '145°' DFN).

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for the 15° case and 2220m2

**3.2. Natural fracture shear superimposing two hydraulic fractures**

**Figure 7.** Cumulative natural fracture shear length versus hydraulic fracture half-length for single hydraulic fracture simulations.

A similar evaluation to Figure 6 was performed for the '145°' DFN case as shown in Figure 8. Note that in plot A, natural fracture friction was 25° and in plot B natural fracture friction was 35°.

**Figure 8.** Cumulative natural fracture shear (shaded area) from simulations with the '145°' DFN. A) Shear for the 25° friction case with an area of 5250m2; and B) Shear for the 35° friction case with an area of 2490m2.

In summary, Figures 5 through 8 suggest the following:


#### **3.2. Natural fracture shear superimposing two hydraulic fractures**

**Figure 7.** Cumulative natural fracture shear length versus hydraulic fracture half-length for single hydraulic fracture

A similar evaluation to Figure 6 was performed for the '145°' DFN case as shown in Figure 8. Note that in plot A, natural fracture friction was 25° and in plot B natural fracture friction

**Figure 8.** Cumulative natural fracture shear (shaded area) from simulations with the '145°' DFN. A) Shear for the 25°

friction case with an area of 5250m2; and B) Shear for the 35° friction case with an area of 2490m2.

simulations.

524 Effective and Sustainable Hydraulic Fracturing

was 35°.

Figures 9 through 12 show the superimposed natural fracture shear areas from independent hydraulic fractures for multi-well completions with hydraulic fracture separations ranging from zero (equivalent to either the simultaneous or zipper-fracs) to 45m for both fracture friction cases.

**Figure 9.** Superimposed natural fracture shear areas for the 15° friction case when Xf1 and Xf2 are both 125m in halflength. A) Zero separation between the two hydraulic fractures; and B) A 20m separation between fractures.

Within the figures, the regions of overlap would likely represent areas of 'wasted' hydraulic fracture shear (and, perhaps, a negative effect on production as excess shear will cause the natural fractures to reclose and even fill with gouge). Ideally, the best effect, assuming no geomechanical interaction between the two hydraulic fractures, may be when the natural fracture shear regions just touch each other (not unlike the situation in Figure 12A).

Figures 9 and 10 suggest that overlapping the lengths of the hydraulic fracture (as proposed in the modified zipper-frac completion) creates large overlapping natural fracture shear areas for the 15° fracture friction case. Further, increasing the hydraulic fracture separation out to 45m still results in considerable overlap of the shear regions. In contrast, with the reduction in shear area due to the increase in natural fracture friction in the 25° friction case in Figures 11 and 12, the shear region overlap goes away at a 35m hydraulic fracture spacing, and for the 45m separation case an unsheared region (Figure 12, plot B) occurs between the hydraulic fractures.

**Figure 12.** Superimposed natural fracture shear areas for the 25° friction case when Xf1 and Xf2 are both 125m in half-length. A) A 35m separation between the two hydraulic fractures; and B) A 45m separation between fractures.

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Figures 13 through 20 show the generation of natural fracture shear from combinations of the two hydraulic fractures Xf1 and Xf2 as a function of Xf2 length and natural fracture friction for the '145°' DFN with a hydraulic fracture separation of 20m. Plot A shows the sheared natural fractures in blue and open fractures in red; plot B shows the same data with an overlay of sheared natural fracture area (similar to Figure 8) as if hydraulic fractures Xf1 and Xf2

**Figure 13.** Natural fracture shear in blue from propagating hydraulic fractures Xf1 (from the left at 125m) and Xf2 (from the right at 50m) for a natural fracture friction of 25° and 20m hydraulic fracture separation. Red represents open fractures. A) Shear and open fractures only; and B) Shear and open fractures with overlay of shear area as if Xf1

**3.3. Natural fracture shear from dual, competing hydraulic fractures**

*3.3.1. Shear results for the '145°' DFN and 20m hydraulic fracture separation*

propagated independent of each other.

and Xf2 propagated independently.

**Figure 10.** Superimposed natural fracture shear areas for the 15° friction case when Xf1 and Xf2 are both 125m in half-length. A) A 35m separation between the two hydraulic fractures; and B) A 45m separation between fractures.

**Figure 11.** Superimposed natural fracture shear areas for the 25° friction case when Xf1 and Xf2 are both 125m in half-length. A) Zero separation between the two hydraulic fractures; and B) A 20m separation between fractures.

**Figure 12.** Superimposed natural fracture shear areas for the 25° friction case when Xf1 and Xf2 are both 125m in half-length. A) A 35m separation between the two hydraulic fractures; and B) A 45m separation between fractures.

#### **3.3. Natural fracture shear from dual, competing hydraulic fractures**

geomechanical interaction between the two hydraulic fractures, may be when the natural

Figures 9 and 10 suggest that overlapping the lengths of the hydraulic fracture (as proposed in the modified zipper-frac completion) creates large overlapping natural fracture shear areas for the 15° fracture friction case. Further, increasing the hydraulic fracture separation out to 45m still results in considerable overlap of the shear regions. In contrast, with the reduction in shear area due to the increase in natural fracture friction in the 25° friction case in Figures 11 and 12, the shear region overlap goes away at a 35m hydraulic fracture spacing, and for the 45m separation case an unsheared region (Figure 12, plot B) occurs between the hydraulic fractures.

**Figure 10.** Superimposed natural fracture shear areas for the 15° friction case when Xf1 and Xf2 are both 125m in half-length. A) A 35m separation between the two hydraulic fractures; and B) A 45m separation between fractures.

**Figure 11.** Superimposed natural fracture shear areas for the 25° friction case when Xf1 and Xf2 are both 125m in half-length. A) Zero separation between the two hydraulic fractures; and B) A 20m separation between fractures.

fracture shear regions just touch each other (not unlike the situation in Figure 12A).

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#### *3.3.1. Shear results for the '145°' DFN and 20m hydraulic fracture separation*

Figures 13 through 20 show the generation of natural fracture shear from combinations of the two hydraulic fractures Xf1 and Xf2 as a function of Xf2 length and natural fracture friction for the '145°' DFN with a hydraulic fracture separation of 20m. Plot A shows the sheared natural fractures in blue and open fractures in red; plot B shows the same data with an overlay of sheared natural fracture area (similar to Figure 8) as if hydraulic fractures Xf1 and Xf2 propagated independent of each other.

**Figure 13.** Natural fracture shear in blue from propagating hydraulic fractures Xf1 (from the left at 125m) and Xf2 (from the right at 50m) for a natural fracture friction of 25° and 20m hydraulic fracture separation. Red represents open fractures. A) Shear and open fractures only; and B) Shear and open fractures with overlay of shear area as if Xf1 and Xf2 propagated independently.

**Figure 14.** Natural fracture shear in blue from propagating hydraulic fractures Xf1 (from the left at 125m) and Xf2 (from the right at 50m) for a natural fracture friction of 35°.

**Figure 16.** Natural fracture shear in blue from propagating hydraulic fractures Xf1 (from the left at 125m) and Xf2

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**Figure 17.** Natural fracture shear in blue from propagating hydraulic fractures Xf1 (from the left at 125m) and Xf2

(from the right at 75m) for a natural fracture friction of 35°.

(from the right at 100m) for a natural fracture friction of 25°.

**Figure 15.** Natural fracture shear in blue from propagating hydraulic fractures Xf1 (from the left at 125m) and Xf2 (from the right at 75m) for a natural fracture friction of 25°.

**Figure 16.** Natural fracture shear in blue from propagating hydraulic fractures Xf1 (from the left at 125m) and Xf2 (from the right at 75m) for a natural fracture friction of 35°.

**Figure 14.** Natural fracture shear in blue from propagating hydraulic fractures Xf1 (from the left at 125m) and Xf2

**Figure 15.** Natural fracture shear in blue from propagating hydraulic fractures Xf1 (from the left at 125m) and Xf2

(from the right at 50m) for a natural fracture friction of 35°.

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(from the right at 75m) for a natural fracture friction of 25°.

**Figure 17.** Natural fracture shear in blue from propagating hydraulic fractures Xf1 (from the left at 125m) and Xf2 (from the right at 100m) for a natural fracture friction of 25°.

**Figure 20.** Natural fracture shear in blue from propagating hydraulic fractures Xf1 (from the left at 125m) and Xf2

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The simulations were conducted such that hydraulic fracture Xf1 always had a fracture halflength of 125m and 'snapshots' were taken for hydraulic fracture Xf2 half-lengths of 50m, 75, 100m, and 125m. The simulated wellbores that Xf1 and Xf2 propagated from were set at 225m

**•** For the 20m separation cases shown, the greatest 'extra' natural fracture shear (shear beyond what would have occurred from two independent hydraulic fractures) occurred when Xf2 was 50m in length. This was true for both natural fracture friction cases (Figures 13 and

**•** As Xf2 grew beyond 50m in length, the 'extra' formation shear decreased and, most importantly, when Xf2 was 100m or 125m in length, there was a net loss of sheared natural

**•** When Xf2 was 100m in length (so that the fracture tips from Xf1 and Xf2 just overlapped), the effect was a complete cancellation of natural fracture shear and a significant opening of natural fractures between Xf1 and Xf2 (likely allowing significant pressure communication)

**•** Once Xf2 exceeded 100m in length, natural fracture shear re-occurred, though it was significantly reduced (Figures 19 and 20). Note that in Figure 19 (natural fracture friction of 25°), the hydraulic fractures blunted the sheared fractures coming from the tip of the other hydraulic fracture acting as a form of release surface preventing transmission of shear on

apart so that once Xf2 reached 100m or longer, it overlapped hydraulic fracture Xf1.

(from the right at 125m) for a natural fracture friction of 35°.

14).

as shown in Figures 17 and 18.

the other side of the hydraulic fracture.

*3.3.2. Observations for the '145°' DFN and 20m separation simulations*

The significant observations from the simulation results include:

fractures as compared to two independent hydraulic fractures.

**Figure 18.** Natural fracture shear in blue from propagating hydraulic fractures Xf1 (from the left at 125m) and Xf2 (from the right at 100m) for a natural fracture friction of 35°.

**Figure 19.** Natural fracture shear in blue from propagating hydraulic fractures Xf1 (from the left at 125m) and Xf2 (from the right at 125m) for a natural fracture friction of 25°.

**Figure 20.** Natural fracture shear in blue from propagating hydraulic fractures Xf1 (from the left at 125m) and Xf2 (from the right at 125m) for a natural fracture friction of 35°.

#### *3.3.2. Observations for the '145°' DFN and 20m separation simulations*

**Figure 18.** Natural fracture shear in blue from propagating hydraulic fractures Xf1 (from the left at 125m) and Xf2

**Figure 19.** Natural fracture shear in blue from propagating hydraulic fractures Xf1 (from the left at 125m) and Xf2

(from the right at 100m) for a natural fracture friction of 35°.

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(from the right at 125m) for a natural fracture friction of 25°.

The simulations were conducted such that hydraulic fracture Xf1 always had a fracture halflength of 125m and 'snapshots' were taken for hydraulic fracture Xf2 half-lengths of 50m, 75, 100m, and 125m. The simulated wellbores that Xf1 and Xf2 propagated from were set at 225m apart so that once Xf2 reached 100m or longer, it overlapped hydraulic fracture Xf1.

The significant observations from the simulation results include:


#### *3.3.3. Shear results for the '145°' DFN and other hydraulic fracture separations*

Figures 21 to 24 show a comparison of natural fracture shear for hydraulic fracture separations of 20m, 35m, and 45m for both natural fracture friction cases for Xf2 lengths of 75m and 125m.

**Figure 21.** Natural fractures at shear as shown in blue for an Xf2 half-length of 75m and natural fracture friction of 25°. Red represents open fractures. A) A 20m hydraulic fracture separation; B) A 35m separation; and C) A 45m sepa‐ ration.

**Figure 23.** Natural fractures at shear as shown in blue for an Xf2 half-length of 125m and natural fracture friction of

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**Figure 24.** Natural fractures at shear as shown in blue for an Xf2 half-length of 125m and natural fracture friction of

Figure 25 presents a graph of the cumulative length of natural fracture shear for the 30

35°. A) A 20m hydraulic fracture separation; B) A 35m separation; and C) A 45m separation.

simulations with the '145°' DFN.

25°. A) A 20m hydraulic fracture separation; B) A 35m separation; and C) A 45m separation

**Figure 22.** Natural fractures at shear as shown in blue for an Xf2 half-length of 75m and natural fracture friction of 35°. A) A 20m hydraulic fracture separation; B) A 35m separation; and C) A 45m separation.

*3.3.3. Shear results for the '145°' DFN and other hydraulic fracture separations*

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Figures 21 to 24 show a comparison of natural fracture shear for hydraulic fracture separations of 20m, 35m, and 45m for both natural fracture friction cases for Xf2 lengths of 75m and 125m.

**Figure 21.** Natural fractures at shear as shown in blue for an Xf2 half-length of 75m and natural fracture friction of 25°. Red represents open fractures. A) A 20m hydraulic fracture separation; B) A 35m separation; and C) A 45m sepa‐

**Figure 22.** Natural fractures at shear as shown in blue for an Xf2 half-length of 75m and natural fracture friction of

35°. A) A 20m hydraulic fracture separation; B) A 35m separation; and C) A 45m separation.

ration.

**Figure 23.** Natural fractures at shear as shown in blue for an Xf2 half-length of 125m and natural fracture friction of 25°. A) A 20m hydraulic fracture separation; B) A 35m separation; and C) A 45m separation

**Figure 24.** Natural fractures at shear as shown in blue for an Xf2 half-length of 125m and natural fracture friction of 35°. A) A 20m hydraulic fracture separation; B) A 35m separation; and C) A 45m separation.

Figure 25 presents a graph of the cumulative length of natural fracture shear for the 30 simulations with the '145°' DFN.

**•** In all the cases, when the half-length of Xf2 grew to 125m, the cumulative length of natural fracture shear increased, but only modestly and significantly less than before the two hydraulic fractures overlapped. This suggests that overlapping hydraulic fractures do not enhance natural fracture shear but cause a net loss of shear relative to two independent

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**•** While for the 20m and 45m separation cases the effect of higher natural fracture friction was to significantly reduce the cumulative length of natural fracture shear (by 50% to 75%), for the 35m separation case the higher natural fracture friction resulted in greater cumulative natural fracture shear than the lower natural fracture friction case. While the full cause of this is not defined, a likely contributor is the ability of the rock mass in the low friction case

Figures 26 and 27 show the natural fracture shear (in blue) for Xf2 half-length cases of 50m, 75, 100m, and 125m for the '180°' DFN with a 20m separation distance and a natural fracture

**Figure 26.** Natural fracture shear in blue from propagating hydraulic fractures Xf1 (from the left at 125m) and Xf2 (from the right) for a natural fracture friction of 15° and 20m hydraulic fracture separation. Red represents open frac‐ tures and shaded regions represent the expected shear area for two independent hydraulic fractures. A) Xf2 length

Similar in fashion to Figures 13 to 20 for the '145°' DFN, Figures 26 and 27 show that there is an increase in natural fracture shear over two independent hydraulic fractures when Xf2 is less than about 75m. Further, when Xf2 exceeds a half-length of more than 75m (or, better, when the tip of Xf2 is within 25m of overlapping the tip of Xf1), then the result is a net loss of

natural fracture shear over two independent hydraulic fractures.

to accommodate greater deformation without reaching the shear condition.

*3.3.5. Shear results for the '180°' DFN and 20m hydraulic fracture separation*

hydraulic fractures.

friction of 15°.

equal to 50m; and B) Xf2 length equal to 75m.

**Figure 25.** Graph of cumulative natural fracture shear length versus hydraulic fracture Xf2 half-length for separation cases 20m, 30m, and 45m for natural fracture friction of 25° and 35° for the '145°' DFN.

#### *3.3.4. Observations for the '145°' DFN dual hydraulic fracture simulations*

Within Figures 21 to 24, the simulation results for each of the three hydraulic fracture separa‐ tion distances (20m, 35m, and 45m) are shown. Again, blue lines represent natural fractures at a shear condition at the moment the two hydraulic fractures are at their given half-length (125m for Xf1 and 75m or 125m for Xf2). Red lines represent open fractures (meaning there is no longer contact between the two sides of the fracture).

The significant observations from the simulation results include:


#### *3.3.5. Shear results for the '180°' DFN and 20m hydraulic fracture separation*

**Figure 25.** Graph of cumulative natural fracture shear length versus hydraulic fracture Xf2 half-length for separation

Within Figures 21 to 24, the simulation results for each of the three hydraulic fracture separa‐ tion distances (20m, 35m, and 45m) are shown. Again, blue lines represent natural fractures at a shear condition at the moment the two hydraulic fractures are at their given half-length (125m for Xf1 and 75m or 125m for Xf2). Red lines represent open fractures (meaning there is no

**•** Perhaps not surprisingly, the greatest total length of shear occurs for the 20m separation distance (at an Xf2 half-length of 50m); however, most interesting is that the total length of shear for the 45m separation distance is greater than that for the 35m separation distance. This suggests that natural fracture shear created between two hydraulic fracture tips is both

**•** The simulation results suggest that the Xf2 half-length at which the maximum induced length of natural fracture shear occurs is related to the hydraulic fracture separation. For the 20m separation case, maximum shear occurred at Xf2 equal to 50m while for the 45m

**•** In all the cases, when the half-length of Xf2 was equal to 100m (so that the tips of Xf1 and

a function of the separation distance and the orientation of the natural fractures.

separation case, maximum shear occurred when the half-length of Xf2 was 75m.

cases 20m, 30m, and 45m for natural fracture friction of 25° and 35° for the '145°' DFN.

*3.3.4. Observations for the '145°' DFN dual hydraulic fracture simulations*

longer contact between the two sides of the fracture).

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The significant observations from the simulation results include:

Xf2 just overlapped), natural fracture shear was at a minimum.

Figures 26 and 27 show the natural fracture shear (in blue) for Xf2 half-length cases of 50m, 75, 100m, and 125m for the '180°' DFN with a 20m separation distance and a natural fracture friction of 15°.

**Figure 26.** Natural fracture shear in blue from propagating hydraulic fractures Xf1 (from the left at 125m) and Xf2 (from the right) for a natural fracture friction of 15° and 20m hydraulic fracture separation. Red represents open frac‐ tures and shaded regions represent the expected shear area for two independent hydraulic fractures. A) Xf2 length equal to 50m; and B) Xf2 length equal to 75m.

Similar in fashion to Figures 13 to 20 for the '145°' DFN, Figures 26 and 27 show that there is an increase in natural fracture shear over two independent hydraulic fractures when Xf2 is less than about 75m. Further, when Xf2 exceeds a half-length of more than 75m (or, better, when the tip of Xf2 is within 25m of overlapping the tip of Xf1), then the result is a net loss of natural fracture shear over two independent hydraulic fractures.

**Figure 27.** Natural fracture shear in blue from propagating hydraulic fractures Xf1 (from the left at 125m) and Xf2 (from the right) for a natural fracture friction of 15° and 20m hydraulic fracture separation. Red represents open frac‐ tures and shaded regions represent the expected shear area for two independent hydraulic fractures. A) Xf2 length equal to 100m; and B) Xf2 length equal to 125m.

Figures 28 to 31 show a comparison of natural fracture shear for hydraulic fracture separations of 20m, 35m, and 45m for both natural fracture friction cases (15° and 25°) for Xf2 lengths of 75m and 125m. Figure 32 shows a graph of the cumulative length of natural fracture shear versus Xf2 half-length for the 15° and 25° simulations (30 in total) for the '180°' DFN.

**Figure 29.** Natural fractures at shear (blue) for an Xf2 half-length of 75m and natural fracture friction of 25° for the '180°' DFN. Red represents open fractures. A) A 20m hydraulic fracture separation; B) A 35m separation; and C) A 45m

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**Figure 30.** Natural fractures at shear (blue) for an Xf2 half-length of 125m and natural fracture friction of 15° for the '180°' DFN. Red represents open fractures. A) A 20m hydraulic fracture separation; B) A 35m separation; and C) A 45m

separation.

separation.

**Figure 28.** Natural fractures at shear (blue) for an Xf2 half-length of 75m and natural fracture friction of 15° for the '180°' DFN. Red represents open fractures. A) A 20m hydraulic fracture separation; B) A 35m separation; and C) A 45m separation.

**Figure 29.** Natural fractures at shear (blue) for an Xf2 half-length of 75m and natural fracture friction of 25° for the '180°' DFN. Red represents open fractures. A) A 20m hydraulic fracture separation; B) A 35m separation; and C) A 45m separation.

Figures 28 to 31 show a comparison of natural fracture shear for hydraulic fracture separations of 20m, 35m, and 45m for both natural fracture friction cases (15° and 25°) for Xf2 lengths of 75m and 125m. Figure 32 shows a graph of the cumulative length of natural fracture shear

**Figure 27.** Natural fracture shear in blue from propagating hydraulic fractures Xf1 (from the left at 125m) and Xf2 (from the right) for a natural fracture friction of 15° and 20m hydraulic fracture separation. Red represents open frac‐ tures and shaded regions represent the expected shear area for two independent hydraulic fractures. A) Xf2 length

**Figure 28.** Natural fractures at shear (blue) for an Xf2 half-length of 75m and natural fracture friction of 15° for the '180°' DFN. Red represents open fractures. A) A 20m hydraulic fracture separation; B) A 35m separation; and C) A 45m

separation.

versus Xf2 half-length for the 15° and 25° simulations (30 in total) for the '180°' DFN.

equal to 100m; and B) Xf2 length equal to 125m.

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**Figure 30.** Natural fractures at shear (blue) for an Xf2 half-length of 125m and natural fracture friction of 15° for the '180°' DFN. Red represents open fractures. A) A 20m hydraulic fracture separation; B) A 35m separation; and C) A 45m separation.

*3.3.6. Observations for the '180°' DFN dual hydraulic fracture simulations*

(meaning there is no longer contact between the two sides of the fracture).

decreased by more than 90% between an Xf2 half-length of 50m and 75m.

The significant observations from the simulation results include:

had more sheared length of natural fractures.

fractures influence each other.

tially causing screenout events.

initial and revised stresses.

*3.3.7. Shear results for the '180°' DFN and altered in-situ stress*

Within Figures 28 to 31, the simulation results for each of the three hydraulic fracture separa‐ tion distances (20m, 35m, and 45m) are shown for the '180°' DFN. Again, blue lines represent natural fractures at a shear condition at the moment the two hydraulic fractures are at their given half-length (125m for Xf1 and 75m or 125m for Xf2). Red lines represent open fractures

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**•** As with the simulations for the '145°' DFN, the higher friction cases generally resulted in less total length of sheared natural fractures than the lower friction cases; however, when Xf2 was 100m (so the tips of Xf1 and Xf2 just overlapped), the higher friction cases generally

**•** For all three separation cases, the greatest total length of natural fracture shear occurred when the Xf2 half-length was 50m. As the separation distance increased between the hydraulic fractures, the total length of natural fracture shear became increasing sensitive to Xf2 half-length. For the 45m separation case, the total length of natural fracture shear

**•** For the '180°' DFN, the cumulative length of natural fracture shear was not as sensitive at an Xf2 half-length of 100m as was the '145°' DFN. This, again, shows that the orientation of the natural fractures is important in creating natural fracture shear when two hydraulic

**•** As with the '145°' DFN, the amount of open fractures in the '180°' DFN cases appeared to influence the amount of natural fracture shear. Further, open natural fractures will be more conductive and, likely, allow pressure communication between hydraulic fractures poten‐

Recall from Table 2 that a number of simulations were conducted with the '180°' DFN model wherein the in-situ stress field was altered. As shown in Table 2, the vertical stress Sv, maximum horizontal stress SHmax, and pore pressure were kept constant and the minimum horizontal stress was increased by 5.6 MPa, which resulted in near-isotropic horizontal stress conditions. Figures 33 and 34 show the sheared natural fractures for the 20m separation case and natural fracture friction of 15° and 25° and with the revise in-situ stress. Figure 35 shows a graph of the length of natural fracture shear for the 25° simulations from and Table 6 and

*3.3.8. Observations for the '180°' DFN dual fracture simulations with revised in-situ stress*

The significant observations from the simulation of the change in in-situ stress include:

**Figure 31.** Natural fractures at shear (blue) for an Xf2 half-length of 125m and natural fracture friction of 25° for the '180°' DFN. Red represents open fractures. A) A 20m hydraulic fracture separation; B) A 35m separation; and C) A 45m separation.

**Figure 32.** Graph of cumulative natural fracture shear length versus hydraulic fracture Xf2 half-length for separation cases 20m, 30m, and 45m for natural fracture friction of 15° and 25° for the '180°' DFN.

#### *3.3.6. Observations for the '180°' DFN dual hydraulic fracture simulations*

Within Figures 28 to 31, the simulation results for each of the three hydraulic fracture separa‐ tion distances (20m, 35m, and 45m) are shown for the '180°' DFN. Again, blue lines represent natural fractures at a shear condition at the moment the two hydraulic fractures are at their given half-length (125m for Xf1 and 75m or 125m for Xf2). Red lines represent open fractures (meaning there is no longer contact between the two sides of the fracture).

The significant observations from the simulation results include:


#### *3.3.7. Shear results for the '180°' DFN and altered in-situ stress*

**Figure 31.** Natural fractures at shear (blue) for an Xf2 half-length of 125m and natural fracture friction of 25° for the '180°' DFN. Red represents open fractures. A) A 20m hydraulic fracture separation; B) A 35m separation; and C) A 45m

**Figure 32.** Graph of cumulative natural fracture shear length versus hydraulic fracture Xf2 half-length for separation

cases 20m, 30m, and 45m for natural fracture friction of 15° and 25° for the '180°' DFN.

separation.

538 Effective and Sustainable Hydraulic Fracturing

Recall from Table 2 that a number of simulations were conducted with the '180°' DFN model wherein the in-situ stress field was altered. As shown in Table 2, the vertical stress Sv, maximum horizontal stress SHmax, and pore pressure were kept constant and the minimum horizontal stress was increased by 5.6 MPa, which resulted in near-isotropic horizontal stress conditions. Figures 33 and 34 show the sheared natural fractures for the 20m separation case and natural fracture friction of 15° and 25° and with the revise in-situ stress. Figure 35 shows a graph of the length of natural fracture shear for the 25° simulations from and Table 6 and initial and revised stresses.

#### *3.3.8. Observations for the '180°' DFN dual fracture simulations with revised in-situ stress*

The significant observations from the simulation of the change in in-situ stress include:

**Figure 33.** Natural fractures at shear (blue) for an Xf2 half-length of 75m for the '180°' DFN at a 20m separation. Red represents open fractures. A) 15° natural fracture friction; B) 25° fracture friction; and C) 25° fracture friction and re‐ vised in-situ stress.

**Figure 35.** Graph of natural fracture shear length versus hydraulic fracture Xf2 half-length for separation cases 20m, 30m, and 45m for natural fracture friction of 25° for the '180°' DFN with the initial and revised in-situ

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**•** As shown in Figure 35, the maximum cumulative length of natural fracture shear occurred

**•** Clearly, moving towards more of an isotropic in-situ horizontal stress reduced the total length of natural fracture shear. Furthermore, the overall behavior also changed such that the maximum cumulative length of natural fracture shear occurred at 100m (the point of

**•** Particularly at larger separation distances (the 45m separation case), the isotropic in-situ stress appeared to make the two hydraulic fractures cancel the shear from each other until

**•** Perhaps even more so than the initial stress cases, the cumulative length of natural fracture shear when the tips of Xf1 and Xf2 overlapped (e.g., the 125m Xf2 case) dropped to nearzero for the revised stress simulations. This suggests that, even with a revised in-situ stress field, overlapping the tips of hydraulic fractures from parallel wellbores creates a net loss

at the 45m separation distance for either in-situ stress case.

tip-to-tip overlap) for the near-isotropic stress case.

the tips of Xf1 and Xf2 were close or overlapped.

of natural fracture shear.

stress (see Table 2).

**Figure 34.** Natural fractures at shear (blue) for an Xf2 half-length of 125m for the '180°' DFN at a 20m separation. Red represents open fractures. A) 15° natural fracture friction; B) 25° fracture friction; and C) 25° fracture friction and re‐ vised in-situ stress.

**Figure 35.** Graph of natural fracture shear length versus hydraulic fracture Xf2 half-length for separation cases 20m, 30m, and 45m for natural fracture friction of 25° for the '180°' DFN with the initial and revised in-situ stress (see Table 2).

**Figure 33.** Natural fractures at shear (blue) for an Xf2 half-length of 75m for the '180°' DFN at a 20m separation. Red represents open fractures. A) 15° natural fracture friction; B) 25° fracture friction; and C) 25° fracture friction and re‐

**Figure 34.** Natural fractures at shear (blue) for an Xf2 half-length of 125m for the '180°' DFN at a 20m separation. Red represents open fractures. A) 15° natural fracture friction; B) 25° fracture friction; and C) 25° fracture friction and re‐

vised in-situ stress.

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vised in-situ stress.

