Environmental Engineering Reference
In-Depth Information
Successful completion or frac'ing of the lateral wells is very dependent on
the characteristics of the shale penetrated. ''Brittle'' shales (silica or car-
bonate-rich) have much better completion capabilities and hence flow-rates
than ductile, organic-matter/clay-rich shales. Therefore, placement of the
lateral in a ductile zone preferably located above an organic-rich layer po-
tentially offers the best production scenario. Hydraulic fracturing of the
lateral well-bore is sequentially undertaken, generally at regular intervals
separated by movable seals (packers), with up to 20 or more stages per well,
avoiding any geo-hazards as noted above. However, microseismic moni-
toring of fracture initiation and propagation during individual completion
stages often indicates variable results (see Figure 15), with some intervals
showing little fracture activity, suggesting that lateral variations in shale
parameters can be significant. 55 Non-completion of such intervals can po-
tentially lead to considerable savings in costs with little effect on overall well
productivity.
Currently, completion is largely undertaken using water as the frac me-
dium due the large volumes required for each stage. Minor amounts of
chemical additives (typically 0.05% by volume) are added to enhance pene-
tration of the shale structure (surfactants and friction reducers, as well as
biocides, scale prevention agents, etc.). As noted above, proppants form the
other major component of the frac fluid, comprising quartz grains or arti-
ficial ceramic material. Flow-back of frac fluids typically occurs during initial
production from a well; however, the volumes of fluid produced are typically
Figure 15 Micro-seismic data from hydraulic fracture stages on horizontal
wellbores.
 
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