Hydraulic fracturing experiments to investigate circulation losses

In recent years the oil and gas industry has been drilling more challenging wells due to long deviated wells, drilling through already depleted reservoirs, sub salt wells and increasing water depth. A major challenge these wells create is to prevent fluid loss into the formation and wellbore breakouts by having accurately determined the mud weight operational window. In addition to accurately determine the fracture gradient, additives in the drilling fluid have been used to enhance the fracture gradient in an industry process named wellbore strengthening. In order to study the phenomenon of fracture gradient alteration, a hydraulic fracturing apparatus was developed to replicate downhole conditions. Different lithologies were tested by performing hydraulic fracturing experiments in order to compare and contrast their original breakdowns and re-opening pressures. Results showed that original breakdown pressures for non-permeable cores tend to vary depending on which fracturing fluid is used. The more viscous fluids, the higher breakdown pressure was obtained. A re-opening pressure cycle was performed after reaching breakdown pressure. The values obtained for re-opening pressures do not present a large variation with respect to the fracturing fluid. Thus, it can be said that the re-opening pressure does not have a significant change with respect to the mechanical properties of the core as well as fluid properties.