Abstract
Multiple fractures created around a well bore can enhance the oil and gas productivity of the fractured well. Recent analyses have shown that if the rate of pressure loading used to induce fracture is sufficiently high, the resulting plastic flow and elastic rebound can set up a “cage” of highly compressive residual stress around the borehole which effectively prohibits crack growth into the formation. The present analysis considers the lower end of the loading rate spectrum and shows that for pressure loading rates sufficiently low so that quasi-static conditions prevail, an initial crack opening under pressure creates highly compressive circumferential stresses around the well bore and the clamping effect of these stresses prohibits additional crack initiation. The existence of these theoretical upper and lower bounds on loading rates to create multiple fractures in well boreholes has been experimentally demonstrated for at least one geologic formation.
Résumé
Des ruptures multiples produites autour d'un puits de forage peuvent accrôitre la productivité de prélèvement du pétrole ou du gaz. Des analyses récentes ont montré que si la vitesse de mise en charge de pression utilisée pour provoquer la rupture est suffisamment éleée, l'écoulement plastique et la rétention élastique qui en résultent peuvent provoquer la création d'une “cage” de contrainte résiduelle de compression élevée autour du trou de forage ce qui empêche une croissance effective de la fissure dans la formation. La présente analyse considère la portion inférieure du spectre de charge et montre que pour des vitesses de mise en charge de pression suffisamment basses telles qu'on se trouve en condition essentiellement quasi statique, une ouverture d'une fissure initiale sous pression provoque des contraintes circonférencielles de compression autour du trou foré et l'effet de fermeture de ces contraintes empêche l'amorçage de fissures additionnelles. L'existence de ces limites théoriques supérieure et inférieure dans les vitesses de mise en charge susceptibles de créer des ruptures multiples dans les trous de forage a été expérimentalement démontrée dans au moins une formation géologique.
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This article sponsored by the U.S. DOE under Contract DE-AC04-76-DP00789.
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Warren, W.E. The quasi-static stress field around a fractured well bore. Int J Fract 18, 113–124 (1982). https://doi.org/10.1007/BF00019636
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DOI: https://doi.org/10.1007/BF00019636