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       Fracture Intensity and Initial Productivity   

Fracture Intensity and Initial Flow Rate (Schafer’s Method)
Since the productive potential of fractured wells cannot be determined easily with conventional logs and Archie's saturation equation, an alternative technique of open hole analysis was proposed by Schafer to identify and eliminate poor wells which would not pay out.

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.

Shafer’s fracture intensity (SFI) analysis based on dipmeter anomalies

The equations developed by Shafer are:
      1. SFI = KF1 * (2.5 * (A + B) + C) / (70 * D)
      2. Qi = KF2 * (SFI ^ 0.5) * Bo

Where:
  A = total opposite pad fracture length on FIL in perforated intervals (ft or m)
  B = total length of borehole width elongation greater than 25% of hole diameter (ft or m)
  C = total single pad fracture length on FIL in perforated intervals (ft or m)
  D = maximum borehole ellipticity (short / long diameters)
  SFI = fracture intensity index (unitless)
  Qi = initial flow rate (bbl or m3)
  Bo = oil formation volume factor (vol per vol)
  KF1 = 1.00 for English units
  KF1 = 0.3048 for Metric units
  KF2 = 1.00 for English units
  KF2 = 0.159 for Metric units

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.

The 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.
 

Copyright © E. R. (Ross) Crain, P.Eng.  email
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