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Cements and In-Situ Widths of Natural Fractures, Almond Formation, Green River Basin, Wyoming
In-situ fracture width and extent of cementation are evaluation characteristics useful in the exploration and development of naturally fractured reservoirs. A joint field test of the Amoco Champlin No. 254B-2H, a horizontal well in the Almond Formation at the northern extent of the Echo Springs Field, Green River Basin, Wyoming, provided an opportunity to examine natural fracture cementation histories and widths at in-situ pressures using slant and horizontal cores. In this field, natural fractures in the Mesaverde Group provide increased production rates and ultimate recoveries of natural gas. Conventional core plug samples were taken from this well so as to orient the trace of natural fractures parallel to the plug axis. These samples were epoxy impregnated at an effective horizontal pressure of 2500 psi, representative of reservoir conditions. Polished thin sections were used to image the in-situ fracture widths using back-scattered scanning electron microscopy.
The fracture cementation includes quartz overgrowths locally followed by sparse kaolinite cement. Patchy barite cement followed kaolinite. Sparry calcite was the latest mineralization. Both barite and calcite prop open the fractures; quartz and kaolinite do not.
In-situ fracture width (both open and mineralized) measurements were collected perpendicular to the length of the fracture traces. Average widths range from 59 to 118 μm (micrometers). Apparent one-inch plug permeabilities can be calculated from open fracture widths, and range from 297 md to 3108 md. In-situ fracture permeabilities of this magnitude should provide high rates of production if the fractures are numerous, have extensive areal dimension, and are not plugged with either natural cements or drilling and completion fluids. Borehole imaging of the sampled well indicates numerous fractures at a spacing of one to two meters, yet production rates are less than the production anticipated from the calculated permeability. Since both this study and borehole imaging indicate the presence of open fractures at in-situ conditions, then either the areal extent of the fractures is limited or the fracture apertures have been reduced during drilling and completion or by the presence of natural cements.
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