Atlas of Natural and Induced Fractures in Core
Understanding fractures and fracture networks has become one of the most important elements in designing completions and stimulations in shales, as well as in the case of carbonates and clastics. Welcome to an interview with two renowned experts in fractures, John Lorenz and Scott Cooper, who have just published a groundbreaking Atlas of Natural and Induced Fractures in Core with AAPG. They will also run their popular short course at URTeC this year.
What are your names and experience in the oil industry?
John Lorenz: I worked on reservoir-characterization research projects for 26 years for Sandia National Laboratories. In 2007 I began to turn that experience into an applied consulting operation specializing in understanding naturally-fractured reservoirs, working for the hydrocarbon industry. Scott Cooper and I partnered as FractureStudies LLC in 2008.
Scott Cooper: I have spent the last 21 years working on projects related to outcrop and subsurface fracture studies with applications to reservoir characterization, production and CO2 sequestration. I was a Senior Member of the Technical Staff at Sandia National Laboratories, a Department of Energy Research Laboratory until 2008, since then I have been working in partnership with Dr. John Lorenz at FractureStudies LLC on various projects around the world.
How did you get interested in fractures?
John: we kept finding them in the cores we hauled out of the ground in the early 1980s but there was little understanding of their origin or significance. It was a poorly-understood part of the geological sciences that needed answers if the unconventional reservoirs were to be efficiently produced.
Scott: I first became interested in natural fractures years ago, as a cave tour guide and working at a trout hatchery. The formation of cave passageways and the flow of subsurface water to springs are both strongly connected to natural fractures. Later in graduate school my research was an outcrop and modeling project at Teapot Dome in central Wyoming under academic advisors Dr. Laurel Goodwin and Dr. John Lorenz. The database from that project continues to be used as a teaching database in various industry modeling programs.
Why is it important to understand fractures now more than ever?
With greater production in unconventional reservoirs it is the combination of technology, horizontal drilling, hydraulic fracturing and their interaction with the natural fracture system that are making these plays successful. All fracture types are not equal in their contributions to a reservoir plumbing system, so recognizing and differentiating fracture types is important in understanding and exploiting these systems.
Describe the difference between induced and natural fractures.
Our Atlas includes the all-important discussion on how to differentiate natural from induced fractures in core. One does not want to build a fracture or fluid flow model based on natural fractures incorrectly-identified as natural fractures - it will simply be wrong and cost everyone time and money. However, induced fractures can be very useful for determining the orientations of the in situ horizontal principal stresses and by proxy the orientation of the natural fractures. Differentiating natural from induced fractures is not always an easy or cut and dried exercise, a wide range of experience with many cores and many fracture types is essential.
What is your Atlas and what does it contain?
The Atlas contains over 550 color photographs and figures, with complete detailed descriptions that illustrate the multitude of variations in natural and induced fractures in core. The atlas is divided into three main sections: 1) Natural Fractures, 2) Induced Fractures, and 3) Artifacts. The natural fractures section highlights extension and shear fractures and various other natural fracture types plus the mineralization (real and false) or dissolution that may alter the surface of these fractures and thus their permeabilities. The induced fractures section details the multiple fracture types that can form during the drilling and handling processes. This section includes how to differentiate natural from induced fractures in core and what induced fractures can be useful for, such as determining the orientations of the in situ horizontal principal stresses. They also offer orientation references for the orientations of natural fractures relative to the in situ stresses and to each other in un-oriented cores. The last section describes the fascinating artifacts you may come across during a lifetime of logging core.
What makes your Atlas useful? How should it be used?
Assessing a core for fractures is so much more than counting fractures and measuring their dips and strikes. Our hope in developing the Atlas was for it to be an invaluable reference that helps geologists recognize and differentiate the many types of natural fractures, induced fractures and artifacts found in cores. But more importantly the natural and induced fracture data contained in cores provides a wealth of information about the reservoir permeability system once they are recognized and properly interpreted.
What is the best way to investigate fractures? Cores? Passive seismic?
The best way to investigate natural fractures involves using the full variety of tools available, including image logs, production logs, drilling logs, CT scans, seismic, information on lost circulations zones and many other tools. Each provide a piece of the puzzle we are all trying to reconstruct and understand - the reservoir fluid flow/permeability system. However, core is definitively one of the best tools: hands on the actual fractures in the rock provides quality control and a calibration of the geophysical fracture signatures. Thoroughly assessing fractures in a core is a labor-intensive exercise involving use of the core slabs and butts together, as well as the removal of the core from the core boxes so that all core surfaces can be examined.
Do you have any plans for the future?
Current plans are to continue working with our industry partners and clients on fractured-reservoir studies, leading field trips and teaching classes in our backyard that includes the San Juan and Permian basins or anywhere in the Rocky Mountains, Gulf Coast, Mid-Continent, South America, Kurdistan and beyond. It is all an adventure and the rocks and core have stories to be told.