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FRACTURE INTENSITY -- SCHAFER'S METHOD
Fracture Intensity and Initial Flow Rate (Schafer’s
A quantitative expression of fracture intensity (SFI) was derived from the dipmeter log and empirically related to established production histories. The SFI of 16 wells was then plotted against second month average daily production for each and an equation was selected to fit the observable relationship. Finally, economics of well pay out were applied in order to assign a commercial cut off SFI value.
equations developed by Shafer are:
Payout is expected when SFI > 2.0 for oil wells with no gas sales, and when SFI > 1.6 for oil wells with gas sales. The constants in equation 1 should be calibrated for each area and will vary with average gas/oil ratio.
Large borehole elongations in fractured reservoirs indicate the intersection of major fractures, which pass completely through the borehole. Flow capacity (millidarcy feet) will increase as A and B footage values increase. These two parameters quantify the fracture indicators that contribute most significantly to production.
Parameter C indicates small scale fractures limited in extent. Single pad fracture footage on the FIL generally has little or no corresponding hole washout. Fractures of this nature will contribute hydrocarbons initially, especially after artificial stimulation, but due to the small potential reservoir production will rapidly drop off.
The degree of borehole ellipticity, D, was chosen to be an indicator of fracture width or intensity of fracture spacing. Fracture width or spacing intensity will determine permeability and therefore affect well capacity. Assuming this, then borehole ellipticity will be inversely proportional to well productivity.
dipmeter log parameters described above were determined for the
Austin Chalk in Texas and Louisiana. They are used as quantitative
indicators of well capacity and correlate reasonably with initial
well flow rate. However, other variables which complicate the
relationship must be considered, such as gas/oil ratio, fracture
treatments, well mechanical problems, partial reservoir depletion,
and reservoir changes external to the borehole. Gas/oil ratio,
which can vary greatly even between offsetting wells, will affect
flow rate. Well stimulations, such as acid wash or hydraulic fracturing
, will normally increase initial production beyond that predicted
by the above correlations.