Horizontal hydraulic fracture propagation is believed to be widespread in shale plays where the frac gradient approaches the overburden – such as the Vaca Muerta, Utica, and Montney. However, horizontal propagation is nearly always ignored in hydraulic fracture modeling. In ResFrac, we are obsessed with ‘getting the physics right’, and naturally, we extended our simulator to handle horizontal fracturing. The first version of this new capability was released earlier this year. We are eager to start collecting feedback from users, which will help us to fine tune the algorithm and workflow.

In my previous blog post, I reviewed field evidence that indicates the presence of horizontal hydraulic fractures. This includes tilt meter data [1], microseismic data [2], the unambiguous observation of a propped horizontal fracture in a vertical core [3], and casing deformation data [4]. In addition, a recent paper [5] provided further confirmation of the presence of horizontal fractures by means of fiber optic measurements. In particular, data from Sichuan Basin (China) indicated the existence of a sub-horizontal hydraulic fracture that experienced both tensile (opening) and shear (slipping) modes of deformations that was persistent for several stages [5].

The primary purpose of this blog post is to cover specifics of horizontal fracture propagation, as implemented in ResFrac. Under what conditions are horizontal hydraulic fractures formed? What is an appropriate ‘initiation criterion’ for the model?

As is demonstrated in a laboratory experiment [6], tensile strength anisotropy of rock can play a crucial role on hydraulic fracture initiation. Because of the layered nature of shales, it is natural to consider that tensile strength exhibits anisotropic behavior, in which the vertical tensile strength (Tstrv) or the strength required to initiate horizontal fractures is noticeably smaller than its counterpart needed for initiating vertical fractures (Tstr). Horizontal hydraulic fractures initiate when fluid pressure exceeds Sv+Tstrv, where Sv is the total vertical stress. At the same time, vertical hydraulic fractures initiate when fluid pressure exceeds Sh+Tstr, where Sh is the total minimum horizontal stress. Note that the total stress depends on the initial state of stress, as well as it accounts for the stress changes associated with operations, such as fluid injections and withdrawals. In ResFrac, such initiation conditions are checked at perforations, at frictional interfaces, as well as at a user specified depth interval, such as every 50 ft, to engage multiple initiation locations. Vertical hydraulic fractures might form initially during stimulation, but then stress shadow could increase the minimum horizontal stress up to a point when horizontal fractures start to form; or alternatively, vertical hydraulic fractures might propagate upwards to high stress layers, where they turn horizontally.

Once horizontal hydraulic fractures are initiated, they will continue to propagate laterally, unless they encounter a vertical hydraulic fracture or a natural fracture. However, even if a horizontal fracture partially terminates against such a feature, it still may be able to propagate around it. Horizontal hydraulic fractures connect to vertical fractures hydraulically and are able to sustain mechanical opening and transport proppant. In ResFrac, horizontal fractures propagate primarily in mode I (i.e., opening mode). While shear deformation can also be important, especially for casing deformation situations, it is probably the opening mode that is able to spread fracture deformation over long distances. The alternative would be pure sliding at the weak interface, which can potentially increase hydraulic conductivity along the horizontal plane. The accelerated pressure diffusion will further promote sliding. However, the absolute magnitude of hydraulic conductivity will still be much less than that of a mechanically open fracture. As a result, without opening, the spatial extent of shear deformation will be relatively localized. At the same time, an open horizontal fracture can propagate far and is unable to sustain shear stress, which will further promote sliding. A lower fracture toughness value may be used for horizontal fractures because in a highly laminated formations they will propagate along a layer interface. It is easier for the horizontal fracture to propagate along the bedding interface than for a vertical fracture to propagate through layers. To account for this effect, a new parameter called ‘debonding toughness’ or ‘KIc debonding’ is used in ResFrac. Several examples are shown below to illustrate this new capability.

The first example shown in the picture below shows a simulation of three injection stages along a single well, in which the state of stress is close to isotropic. The fracture colors are proportional to aperture, and the screenshot is taken during the third stimulation stage. Vertical hydraulic fractures form initially, but as stress shadow builds up, horizontal features develop during the second and third stages. In addition, there is a horizontal fracture that is initiated along the weak bedding plane or frictional interface, where the vertical tensile strength is particularly low.

The second example shows a situation that is qualitatively similar to that observed in [2], in which the well trajectory and formation structure are such that the well meanders between two different formations. To represent this behavior in ResFrac, a sub-horizontal well with a finite dip angle is placed such that some stages are located within one rock formation, while other in another. The total of three stages with eight clusters each are simulated. As can be seen from the results, vertical hydraulic fractures are initiated from stages located in the upper formation, while horizontal hydraulic fractures are initiated in the lower formation that has a different state of stress. The fracture colors are proportional to fluid pressure in this example, and the screenshot is taken during long-term pressure depletion. Even though strong pressure depletion occurs in the horizontal fractures, they have low productivity because the vertical permeability of the formation is significantly lower than the horizontal permeability.

 

 

The third example demonstrates one of the most complex scenarios that includes the presence of dipping faults alongside vertical and horizontal hydraulic fractures, as shown in the image below. Most of the time, hydraulic fractures are unable to propagate across preexisting natural fractures. However, sometimes they will still able to grow around the preexisting fractures because of their finite size.

The last example demonstrates why the fracture opening of the sub-horizontal fracture is necessary for spreading the shear deformation away from the stimulating stage. A sub-horizontal preexisting fracture is placed above the horizontal part of the well such that it crosses the vertical part of the well. Two scenarios are considered, each with a slightly different state of stress. In both cases, three stages with eight clusters are considered and it is assumed that the vertical hydraulic fractures are arrested at the preexisting fracture. The picture below shows the first case, in which the state of stress is insufficient to open up the bedding plane. The left picture shows the fracture aperture, while the right picture shows the sliding displacement. As can be seen from the result, the shear sliding is very localized around the location of the vertical hydraulic fractures.

The second scenario is depicted below, in which the state of stress is such that the bedding plane is able to mechanically open. As for the previous case, the left picture shows the aperture and the right picture shows the sliding displacement. As can be seen, there is an observable aperture of the preexisting fracture, which spreads noticeably towards the well. Note that the dip of the preexisting fracture is such that the latter becomes shallower closer to the well, which further reduces the magnitude of stress and promotes fluid migration towards the well or upwards opposite to the dip direction. In this example, the shear slip is ‘carried’ by the fracture opening and it reaches the vertical section of the well. This could cause loss of the well as a result of casing deformation.

While the first few examples focused on the new capabilities of ResFrac with respect to horizontal fractures and dipping faults, the last example clearly highlights seriousness of the practical aspects related to horizontal and/or sub-horizontal fractures. In particular, a relatively small change of stress state (0.02 psi/ft for the minimum stress in this case) can lead to drastically different results for the potential of casing deformation. This illustrates a very peculiar feature of the solution of having a bifurcation point, i.e. a relatively small change of the conditions can lead to significant qualitative change the overall result.

It is also instructive to make estimates regarding formation of horizontal fractures to assess whether we can expect them in a particular formation or not. Assume that the vertical stress gradient is 1.0 psi/ft, while the minimum horizontal stress gradient is 0.8 psi/ft. Let’s say the well is drilled at 8000 ft. Therefore, we have Sv = 8000 psi and Sh = 6400 psi initially. Let’s further assume that the effective tensile strength for initiating vertical fractures is Tstr = 1000 psi, while due to laminations the effective vertical tensile strength for initiating horizontal fractures is Tstrv = 0 psi. For the very first stage, the fractures will be vertical because Sv+Tstrv = 8000 psi, while Sh+Tstr = 7400 psi. Therefore, once the bottomhole pressure exceeds 7400 psi, vertical hydraulic fractures will be formed. Nowadays, however, reservoir stimulation is typically done by completing several wells within the same pad, which leads to large fluid injections into the formation. This can substantially elevate the minimum horizontal stress, from several 100s to 1000s of psi. For instance, if the total accumulated stress shadow (SS) is 500 psi, then the total minimum horizontal stress now is 6900 psi. Then, vertical fracture initiation will occur at fluid pressure exceeding Sh+SS+Tstr = 7900 psi. This is still under the 8000 psi limit for initiation of horizontal fractures and therefore vertical fractures will be initiated. However, if the total accumulated stress shadow exceeds 600 psi, then, in the considered example, horizontal fractures will form. This hypothetical example highlights that horizontal fractures are affected by the initial stress state (i.e. Sh vs. Sv), by the rock type (i.e. Tstrv vs. Tstr), as well as by operations (stress can increase substantially due to fluid injection). While the rock and the initial state of stress cannot be changed, we do have the ability to optimize our pump schedule and the magnitude of stress buildup. This example demonstrates that even for a relatively modest value of minimum horizontal stress gradient of 0.8 psi/ft, it is still possible to have horizontal fractures, especially for large pads with closely spaced wells and high fluid loading.

The horizontal fracture capability in ResFrac will enable users to make informed decisions about understanding and mitigating the effects associated with horizontal fractures. As is shown in the above conceptual example, the operational part plays an important role in forming and/or preventing horizontal fractures. The degree of influence also varies from one geologic location to another because it strongly depends on rock and the state of stress. The fully coupled fracturing and reservoir modeling approach that is adopted in ResFrac will therefore allow operators to fully quantify the effect of various completion designs on the resultant fracture geometry, the presence of horizontal fractures and their influence on casing deformation, as well as the associated production.

References

[1] C. A. Wright, E. J. Davis, L. Weijers, W. A. Minner, C. M. Hennigan, G. M. Golich. Horizontal Hydraulic Fractures: Oddball Occurrences or Practical Engineering Concern? Paper presented at the SPE Western Regional Meeting, Long Beach, CA. SPE-38324-MS, 1997.

[2] A. A. Alalli and M. D. Zoback. Microseismic evidence for horizontal hydraulic fractures in the Marcellus Shale, southeastern West Virginia. The Leading Edge, 2018.

[3] R. Suarez-Rivera, W.D. Von Gonten, J. Graham ,S. Ali , J. Degenhardt, and A. Jegadeesan. Analyzing the dependence of well production on lateral landing depths. Invited presentation, University of Texas, Austin TX, 2016 (courtesy of W.D. Von Gonten Engineering).

[4] J. A. Uribe-Patino, A. Casero, D. Dall’Acqua, E. Davis, G. E. King, H. Singh, M. Rylance, R. Chalaturnyk and G. Zambrano-Narvaez. A Comprehensive Review of Casing Deformation During Multi-Stage Hydraulic Fracturing in Unconventional Plays: Characterization, Diagnosis, Controlling Factors, Mitigation and Recovery Strategies. Paper presented at the SPE Hydraulic Fracturing Technology Conference and Exhibition, The Woodlands, Texas, USA, SPE-217822-MS, 10.2118/217822-MS, 2024.

[5] W. Wang, J. Mjehovich, L. Chen, G. Jin, J. Li, and X. Fu. Bedding Plane Slippage and Fault Reactivation Captured by Cross-Well LFDAS Monitoring During Hydraulic Fracturing. Paper presented at the SPE Hydraulic Fracturing Technology Conference and Exhibition, The Woodlands, Texas, USA. SPE-223538-MS, 2025.

[6] A. Abdelaziz, and G. Grasselli. Crack Opening and Slippage Signatures During Stimulation of Bedded Montney Rock Under Laboratory True-Triaxial Hydraulic Fracturing Experiments. Rock Mech Rock Eng, 57, 9827–9845, 2024.