Experimental Study of In Situ Fracture Generation and Fluid Migration in Shale Last Reviewed November 2017

FWP FE 406/408/409

Goal
Hydraulic fracturing of unconventional oil and gas resources is inefficient with recoveries at less than 15 percent. One of the limitations to improving efficiency is the lack of knowledge of the fundamental properties of the fracture network generated by hydraulic fracturing operations. In the field, the ability to observe fracture and flow consists mostly of microseismic data on the number and location of fracture events. As valuable as these data are in delimiting the location and extent of fracture clouds, they provide neither the resolution nor the detail required to provide an understanding of processes occurring in unconventional reservoirs. In this task, the objective is to conduct experiments at reservoir conditions that provide fundamental insights into fracture growth, penetration, permeability, and surface area as essential components of developing methods of improving hydraulic fracturing performance in the field.

Key objectives:

  • Evaluate the relative significance of tensile fractures directly generated during hydraulic fracturing and shear fractures activated by hydraulic fracturing in permeability and hydrocarbon production.
  • Measure surface areas of tensile and shear fracture systems using microfluidics and triaxial coreflood systems and relate this to fracture-matrix interactions and the production of hydrocarbon.
  • Replicate fracture systems created in the triaxial experiments within shale microfluidics models and investigate the efficacy of hydrocarbon removal by brine.
  • Integrate results of x-ray tomography characterization, acoustic emission analysis of fracture formation, surface area data on fracture penetration, and sweep efficiency to create understanding of how fractures propagate, penetrate, and ultimately liberate hydrocarbon.

Performer
Los Alamos National Laboratory (LANL), Los Alamos, NM 87545

Background
Experimental studies of fracture-permeability relations generated under in situ reservoir conditions are limited. Most studies of hydraulic fracturing have used triaxial devices to examine rock mechanics (stress-strain behavior) and acoustic emissions associated with rock damage. Key features of hydraulic fracturing remain unexplored in these experimental studies. What are fracture properties (aperture and geometry) at reservoir conditions? How permeable are these fractures, and how does permeability evolve with fluid flow and time? What is the surface area of fractures in relation to the amount of matrix and hydrocarbon that can easily diffuse into the fractures? How does the injected working fluid migrate into and sweep hydrocarbon out of the fracture system?

LANL will make use of unique experimental capabilities in a triaxial coreflood system with at-conditions x-ray tomography and high-pressure microfluidics. These allow the research team to overcome limitations of earlier work by making direct observations at in situ conditions so that fractures are generated and characterized without changing the environment. This removes the ambiguity created by ex situ analyses where fracture properties may change following decompression (e.g., apertures may open or fractures may even be created). The research team uses these in situ observations to characterize fracture density, connectivity, surface areas, and permeability.

Impact
LANL combines simultaneous triaxial-induced fracturing and x-ray tomography with a high-pressure microfluidics system for direct optical observations of fluid flow behavior to provide a unique opportunity to investigate the dynamics of fracture initiation and growth, fluid movement, and hydrocarbon production. This effort will yield new insights into how applied stress, fluid pressure, and injection dynamics impact fracture penetration, apertures, and permeability. The project will correlate acoustic emission activity with tomographic observations of fracture growth to enhance field understanding of microseismic surveys. Finally, this study will yield basic understanding of how to effectively displace and mobilize hydrocarbon from complex fracture networks.

Accomplishments
Chesapeake Energy has supplied Utica shale core from the unconventional plays in Ohio and Pennsylvania. The researchers have data on the mineralogy, porosity, and organic carbon content of these samples. In addition, they have acquired Marcellus shale in outcrop.

Researchers developed a novel technique for inducing hydraulic fractures in a triaxial device. Previous studies have required specialized equipment or sample geometries, but their approach works with standard rock core and allows permeability measurements of the fracture system. The researchers have conducted simultaneous fracturing of shale in a triaxial device while conducting x-ray tomography imaging, an experimental first that will be key to understanding in situ fracture properties. They have developed and implemented an acoustic emission system to monitor fracture development.

Researchers have made numerous measurements of fracture permeability of Utica shale conducted in traditional compression experiments and using direct shear methods. These have provided detailed information on the effect of confining stress (or depth) and time on fracture permeability and evolution. Fracture permeability tests were performed at high effective confining pressure and showed that in spite of substantial sample shortening, permeabilities remained less than 1 mD until the pressure was released.

The researchers have conducted preliminary sweep efficiency experiments with a high-pressure microfluidics system to characterize hydrocarbon removal during water injection and developed improved fracture etching methods for representing complex fracture networks in shale.

The researchers have conducted preliminary sweep efficiency experiments with a high-pressure microfluidics system to characterize hydrocarbon removal during water injection and developed improved fracture etching methods for representing complex fracture networks in shale.An experimental study that quantifies permeability of fractured, carbonate-rich Marcellus shale (Bedford Quarry, Pennsylvania) was completed. A newly modified version of LANL’s triaxial direct-shear device was used, which enabled improved resolution of permeability as well as a quantification of uncertainty of the permeability measurements. The results of the experiments demonstrate the importance of considering the stresses at which fractures are created when predicting the permeability of fractured, low permeability rocks. In LANL experiments on a carbonate-rich Marcellus shale, LANL investigated triaxial direct-shear specimen permeability as functions of: 1) increasing stresses at which fractures are created through initially intact material; 2) increasing confining stress on an existing fracture; 3) increasing shear displacement across an existing fracture; and 4) combined time-dependent effects such as mechanical creep, chemical precipitation, and particle mobilization.

  • The effect of increasing the stress at which fractures were created was most significant.
  • Increasing the effective confining stress on an existing fracture was also found to cause permeability reduction.
  • Increasing the shear displacement across an existing fracture had varying effects, dependent upon the effective confining stress and magnitude of displacement.
  • Permeability after fracturing was generally observed to decrease with time.

In addition, experiments were completed on one Marcellus core, which was acquired from the Marcellus Shale Energy and Environment Laboratory (MSEEL). The sample lithology is much more clay-rich than what was used in previous experiments. Core preparation has begun for this study, which involves subsampling the 4"-diameter slabbed material to create 1"-diameter core that the research team used in the triaxial device. The team will conduct experiments similar to previous work but will apply new approaches (described in Section 2.2) of characterizing the tributary fracture zone.

Current Status (November 2017)
Work continues on tributary zone fractures’ contributions to hydrocarbon production in the Marcellus Shale. The team is working with the quantification of fracture-network permeabilities as well as examining the impact of reservoir stress conditions on fracture permeability and the integration of tributary fracture zone properties with DFN simulations.

An additional set of experiments that will focus on Marcellus core acquired from the Marcellus Shale Energy and Environment Laboratory (MSEEL) is planned. This is a different lithology (more clay-rich) than used in previous experiments.

Project Start: October 1, 2014
Project End:  July 31, 2018

Project Cost Information:
Phase 1 – DOE Contribution: $233,000
Phase 2 – DOE Contribution: $233,000

Planned Total Funding:
DOE Contribution:  $467,000

Contact Information:
NETL – Bruce Brown (Bruce.Brown@netl.doe.gov or 412-386-5534)
Los Alamos National Laboratory – Bill Carey (bcarey@lanl.gov )

Additional Information:

Experimental Study of In Situ Fracture Generation and Fluid Migration in Shale (Aug 2017)
Bill Carey, Los Alamos National Laboratory, 2017 Carbon Storage and Oil and Natural Gas Technologies Review Meeting, Pittsburgh, PA