In an underbalanced drilling process, using diesel as an oleic additive to drilling mud increases the chance of explosion if the drilling fluids mix with natural gas or injected air and reach the flammable limits. Although some investigations about flammability limits of drilling fluids have been conducted on the basis of diesel components, further laboratory experimental study needs to be done. Initially, the properties of the two diesel samples, including heating value, molecular weight, densities and simulated distillation characteristics are measured and compared. Then, an experimental study is conducted to investigate the flammability limits of two diesel fuel samples (diesel A and B) at varied conditions of elevated temperature and pressure using a specially designed apparatus.

Based on the experimental results, generally, diesel B exhibits wider flammability range than diesel A. However, diesel A is easier to ignite than diesel B. In addition, the effect of pressure on the flammability limit of diesel B is more profound compared to diesel A.

This research can help diesel selection for specific conditions in the field. Moreover, it is suggested that, to maintain a low probability of ignition or explosion, the minimum diesel concentration of drilling fluid is 4.29% for both diesels under certain conditions.

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