This paper presents an investigation of the effect of pressuredependent natural-fracture permeability on production from shale-gas wells. The motivation of the study is to provide data for the discussion of whether it is crucial to pump proppant into natural fractures in shale plays. Experiments have been conducted on Bakken-shale core samples to select appropriate correlations to represent fracture conductivity as a function of pressure (the actual characterization of fracture conductivity under stress for a specific formation is not an objective of the study). Correlations have been used in a flow model to demonstrate the potential impact of natural-fracture closure as pressure drops during production. Although the correlations indicate up to an 80% reduction in fracture permeability over practical ranges of pressure, the results of the flow model do not warrant the claims that fracture closing plays a significant role in the productivity losses of shalegas wells. A history match of the performances of two wells in the Barnett and Haynesville formations also indicates that the effect of pressure-dependent natural-fracture permeability on shale-gas-well production is a function of the permeability of the matrix system. If the matrix system is too tight, then the retained permeability of the natural fractures may still be sufficient for the available volume of the fluid when the system pressure drops.

This paper presents the results of an experimental study of pressure-dependent natural-fracture permeability in tight, unconventional reservoirs. Bakken cores are used in the experiments. For the purpose of this paper, pressure-related permeability losses in hydraulic fractures and matrix system are not considered. Experimental data are used to screen the stress-dependent matrix-permeability correlations available in the literature for application to shale fractures. Selected correlations are matched with the data to delineate the reasonable ranges of the correlation coefficients for shale fractures. The applications of the correlations over practical ranges of pressure drop in shale reservoirs indicate over 80% reduction in fracture permeability, with most of the permeability loss occurring during the initial pressure drop. To appraise the effect of pressure-dependent natural-fracture permeability on shale-gas production, experimentally developed correlations are incorporated in an analytical model of a fractured horizontal well surrounded by a stimulated reservoir volume. The model is used to history match the performances of two wells in the Barnett and Haynesville formations. It is shown that the effect of pressure-dependent natural-fracture permeability on shale-gas-well production is a function of the permeability of the matrix system. If the matrix system is too tight, then the retained permeability of the natural fractures may still be sufficient for the available volume of the fluid when the system pressure drops.

Liquid-rich shale reservoirs contribute immensely to the United States oil and gas production. Because Bakken, Lower Eagle Ford, and Niobrara formations have different mineralogy, pore structure, organic content, and fluid compositions, it is critical to differentiate the unique characteristics of each formation for field development and oil and gas production. The latter information is also useful in well stimulation design and hydraulic fracturing. This paper presents an experimental study of mineralogy, pore-size distribution, pore geometry, and spatial correlation between minerals and pores to identify the effect of micro-scale properties on flow behavior. Porosity and permeability of several core samples from the Middle Bakken, Lower Eagle Ford, and Niobrara formations were studied and the results, using mercury injection capillary pressure (MICP), X-Ray diffraction (XRD), and scanning electron microscopy (SEM), were shown. Finally, a workflow that estimates cementation factor combining the results obtained from MICP measurements and GRI crushed core analysis will be presented.

No abstract

No abstract