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Recommended Practices Evaluating Short Term Proppant Pack Conductivity
American Petroleum Institute Washington, D.C. 20005
API RP-61 establishes standardized test procedures to determine reference conductivity of various proppants. The tests were not intended for use in obtaining absolute values of proppant conductivity, as they fail to address fines migration, multiphase flow, non-Darcy flow, proppant embedment or other critical parameters that substantially reduce effective fracture conductivity.
In these short-term tests, proppant is placed between steel platens. Stress is increased at a maximum of 500 psi/minute until the desired stress is reached. Stress is maintained for 15 to 60 minutes depending on proppant type and stress level. A single-phase liquid (water) is pumped through the proppant pack at extremely low velocities ranging from 1 to 10 milliliters per minute. The superficial velocity in the pack is often a fraction of one inch per minute.
From the observed pressure drop across the proppant pack, the proppant permeability and conductivity can be calculated via Darcy’s Law.
Comment: Various improvements to the API RP-61 procedure have been adopted by the industry, as described in SPE papers 15067, 16453, and 16900. Notable improvements include:
- Proppant is placed between platens of medium-hard sandstone to allow some embedment.
- Proppant stress is maintained for 50 hours to include some degree of long-term packing and degradation of the proppant.
- Single phase water is pre-saturated with silica and deoxygenated to better represent formation fluids and minimize fluid corrosion of the proppant and sandstone platens.
Note, the long term reference conductivity tests generally do not address gel damage, fines migration, multiphase or non-Darcy flow. Substantial incremental adjustments to the reported values are required to accurately predict effective downhole performance of the proppant pack. |
