Accuracy and Uncertainty Evaluation of Measuring the ISO Long-Term Conductivity of Proppants

Abstract

One of the critical factors in planning a hydraulic fracturing treatment is proppant selection. Conductivity is an important property for ensuring efficient reservoir stimulation. Conductivity data obtained during laboratory measurements help determine which proppant to use. ISO provides standard testing procedures and methodology for measuring the long-term conductivity of proppants but does not document expected measuring uncertainty. This paper addresses accuracy and uncertainty estimation of ISO 13503-5 conductivity measurements.For the purposes of this study, three proppant samples were used. Two identical samples of high-strength ceramic proppant (HSP) and one sample of natural silica sand were distributed among six laboratories that participated in this study. The resulting data from measuring conductivity and permeability of the samples were used to calculate repeatability and reproducibility in terms of standard deviations in accordance with ISO 5725-2. Uncertainty of measuring the ISO long-term conductivity of proppants was calculated in accordance with ISO 21748.The evaluation shows that conductivity and permeability values of ceramic proppant and natural sand measured by the ISO 13503-5 method have quite high interlaboratory uncertainty. Uncertainty of conductivity is from 19 to 100% depending on closure stress for values of conductivity from 3000 to 200 md-ft respectively. Uncertainty of long-term conductivity measurements increases dramatically while conductivity decreases. At the same time, internal evaluation of uncertainty in measuring conductivity within one laboratory resulted in approximately 15% as reference value of uncertainty for a single laboratory and never exceeded 30% in given conductivity range.ISO 13503-5 prescribes testing procedures; however, it does not provide information about measurement uncertainty. This information is essential for comparing different proppants to enable reasonable selection criteria for a job, to identify proppant property inconsistency and quality decline over time due to changes in production process or raw material, or to resolve discrepancies in different laboratory measurements. Uncertainty values were estimated for a wide range of conductivity data and different proppant types.