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Chemical Control and Fluid Flow in Shale Microfractures
Project Number
FWP 100211

SLAC National Accelerator Laboratory

Schematic illustration of fracture system across length scales. Top panel: Following stimulation, fluids are more acidic and reactive in the periphery of the fracture network. Barium and other constituents leached from mud by acid in the wellbore environment and transported into fractures can drive mineral scale precipitation. Middle and bottom panels: Mineral scale precipitation occludes porosity in fractures and in shale matrix, attenuating permeability and production. Bottom: matrix abutting fractures, through which gas and oil must flow to be produced, is most strongly impacted by acid attack, which creates secondary porosity, and by scale precipitation. This important region is referred to herein as the “altered zone”.

Project Description

During stimulation, shale porosity can be initially enhanced when injected concentrated acid imbibes into fracture surfaces and dissolves minerals, promoting the opening secondary porosity and widening of micro-fractures. This near-surface “altered zone” is the main conduit through which oil and gas flow out of matrix. The thickness and permeability of this altered zone are therefore extremely important to improving recovery from matrix. During shut-in, mineral dissolution increases both the pH and salinity of the solution. As the pH of brine increases, scale precipitation becomes favorable, and permeability can drop below that of unreacted shale. Our previous research shows that the extent and rates of scale precipitation in shale matrix are dependent on shale composition, in addition to the types and concentrations of drilling mud and acid used during stimulation. Thus, the key to scale mitigation lies in optimizing fluid compositions and injection practices. Moreover, we have observed that fluid and gas exchange into/out of shale matrix is strongly enhanced by micro-fractures and the development of secondary porosity. These observations point a way to improving recovery from matrix by manipulating chemical reactions to promote mineral dissolution while simultaneously controlling scale precipitation.

Shale compositions and stimulation practices vary substantially between basins, while showing intra-basin similarities. Stimulation practices, and particularly fluid chemistry, not only vary in the total number of chemicals used but also in the type of chemical used for a specific purpose (friction reducer, corrosion inhibitor, anti-scaling, etc.). These variations necessitate a focus on basin-specific parameters to more deeply understand chemical controls over scale precipitation and to understand intra-region variability, crucial to improving efficiency/recovery factor.

This project is focusing on two strategic geochemistry-based research thrusts where new knowledge can immediately begin to improve unconventional gas and oil recovery factors. First, we are evaluating mineral scale precipitation processes specific to major shale formations and fracture stimulation practices and developing geochemistry-based approaches to mitigate it. This knowledge has an additional benefit of improving our ability to reuse flowback and produced water without causing formation damage. The focus of this work will be to compare and contrast conditions specific to Marcellus (dry gas) and Midland (oil) basins. We are also conducting research to understand how geochemistry can be used to manipulate the thickness and permeability of the altered zone by focusing on controlling microscale chemical and mechanical features such as secondary porosity created during stimulation, the connectivity of this porosity across the altered zone, and irreversible mineral scale precipitation within the altered zone. Our ultimate goal is to develop approaches to manipulate the thickness and permeability of the altered zone during stimulation to increase access to matrix and thus production recovery factors.

To monitor scale precipitation and microstructure evolution within shales, we are using a combination of laboratory, synchrotron X-ray imaging, computed tomography, electron microscopy, and seismic techniques. Research is being performed in consultation with industrial experts to help facilitate technology transfer from the laboratory to the field.


Project Benefits

The fundamental geochemical-microstructural knowledge being developed by this project is helping to improve the efficiency and environmental impact of unconventional stimulation practices. Our ultimate goal is to develop approaches to manipulate the thickness and permeability of the altered zone during stimulation to increase access to matrix and thus production recovery factors. These improvements are helping to advance the DOE-FE mission of improving hydrocarbon long-term recovery, reducing environmental impact, and helping the U.S. to establish energy dominance.

Additional Information