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 Understanding Proppant Closure Stress
S.K. Schubarth, SPE, Halliburton Energy Services, S.L. Cobb, SPE, Carbo Ceramics Inc.; R.G. Jeffrey, SPE, CSIRO Petroleum
This paper explains that the frac gradients typically used in the industry are actually "closure pressures" or "parting pressures" that correspond to a fracture width of essentially zero. During a fracture stimulation treatment, the reservoir rock is not allowed to completely rebound – the proppant prevents the fracture from closing. Many engineers fail to consider this additional stress during fracture design. The authors present a mathematical method to calculate the excess pressure, which is shown to range from 200 to 1500 psi for a variety of reservoir conditions. For 20/40 Ottawa sand, this excess pressure often causes the fracture conductivity to be 50% lower than predicted using traditional techniques in thin reservoirs with high Young's Moduli.
Instead of mathematically calculating the excess pressure, one method suggested is to conservatively assume that the fracture is unable to relax and add the full net pressure generated during the frac job to the traditional frac closure stress to estimate the effective proppant stress.
This paper also briefly mentions that proppant in non-uniformly propped fractures will be subjected to considerably higher stresses. Note that all lab data are measured with perfectly uniform proppant distribution. |
