Critical stress intensity factor (fracture toughness) K1c represents a rock’s resistance to the fracture propagation. In some cases during stimulation jobs (low slurry rate, low fluid viscosity, formation strength homogeneity) determines essentially the net fluid pressure as well as fracture’s rise dynamics and geometry. Laboratory fracture toughness evaluation is usually performed by testing Cracked Chevron Notched Brazilian Disc (CCNBD) core plugs drilled out parallel to the rock bedding. Due to the time- and labour-consuming preparation and testing process, well core inaccessibility or invalidity, during the HF modeling, fracture toughness values are often used "by default" (usually about 1000 kPa·m1/2) or various correlations between fracture toughness and common well log data are utilized.
The authors made an attempt of fracture toughness evaluation by scratching full-size core of aimed formation while designing hydrofrac jobs for clastic reservoirs of Gazprom Dobycha Tomsk JSC in Tomsk region (West Siberia). The results of core scratching were compared to the classic CCNBD strength tests. As a fast and non-destructive method, core scratching gives satisfying correlation level (R2 = 0,83) comparing to the CCNBD test results. It is essentially higher than fracture toughness – rock density (R2 = 0,67) and fracture toughness – acoustic P-wave velocity (R2 = 0,58) correlations. Further, the rock plug preparation, testing methods and lab equipment are described.
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