Why you should care about reservoir heterogeneity – Part 1: Hydraulic fracture geometry

Computer simulation of hydraulic fractures is typically part of an optimization study. Some of the problems that engineers and geophysicists need to address are

  • determining the best landing zone
  • well spacing optimization

Propagation of hydraulic fractures is a complicated physical process. The shale formations are often highly heterogeneous as a result of their multi-layered structure. This means that the physical properties can be different from one layer to another. In the span of a few feet, Young’s modulus can change 40% or more. This is the reality of shale formations and as I will demonstrate in this post, you should not ignore it.

This figure shows a few curves from a well log from a location in the Permian basin in Texas. According to these curves, values can change drastically every few feet.

When it comes to landing zone optimization, one needs to estimate the extent and conductivity of the hydraulic fractures propagating from perforation located at a number of alternative depths. How can you know which one is better? Firstly, you need to know your reservoir well. Studying well logs tells you a lot about your reservoir. Modern logs give you a wealth of data with a good level of detail, with step sizes at about 0.5 ft. However, then the question becomes, “Can you use the data?” The answer to this question is “probably not!“. Unfortunately, most hydraulic fracture simulation software packages available on the market are incapable of including heterogeneity in material properties of the rock with such level of detail. If you somehow force the software to do that, then you might encounter numerical instabilities or at the very least many hours of computing for just one fracture of a single stage. This is not great news when you want to optimize the landing zone where mechanical properties change every few feet. Happily however, FullRank Software is here with the solution!

VirtuaFrac is a 3D hydraulic fracture simulator developed by FullRank Software. One of the many features of this software is its capability to create planar 3D hydraulic fracture models with very high resolution in a very short amount of time. In layman’s terms, you can pick grid spacing of a few feet and the hydraulic fracture simulation is going to be ready in just a few short minutes on a workstation desktop. This software is designed to be numerically stable and at the same time preserve as much data from logs as it is available. We posted a report on benchmarking this software earlier. See here.

The hydraulic fracture model you see above was generated using VirtuaFrac and utilizes quantities from well logs directly. You can see that any change in reservoir properties influences the hydraulic fracture geometry directly. For instance, when you have relatively high pressure the fracture becomes more constrained. This feature of VirtuaFrac can be effectively utilized for landing zone optimization problems. Because of the high fidelity of this software and its models to the original reservoir data input, any change in local reservoir condition is directly reflected.

See how a change of 150 ft in the location of landing zone can affect the fracture extent in the figure above. The overall length of fracture is up about 14% and aperture near the wellbore is about 40% higher (leading to better conductivity). This is due to reservoir heterogeneity. We are able to see and measure this difference because VirtuaFrac is able to include all the details, jumps and drops in reservoir properties.

The key takeaway from this study is that you should preserve as much reservoir information as you can. If the simulation software you use smoothens input data, then you should not trust it. When it comes to landing zone optimization every foot matters and the model should adhere to your well log data (and not the other way around). VirtuaFrac is a breakthrough in numerical simulation of hydraulic fractures that can preserve unprecedented levels of performance. It takes VirtuaFrac just a few minutes of computing on a desktop workstation to create a robust model that addresses practical optimization problems.