Geomechanical wellbore stability analysis for the wells drilled in the shallow sediments of the Mediterranean Sea

Abstract

Gas hydrates are considered as near-future energy resources in the world. Due to high gas hydrate potential of the Mediterranean Sea, gas hydrate exploration wells are expected to be drilled. In this study, for the first time, the caliper logs of the wells drilled during ODP (Ocean Drilling Program) Leg 160 and ODP Leg 161 in the Eastern and Western Mediterranean Sea were evaluated together with the python codes predicting wellbore stresses, unconfined compressive strength (Co) and lower-hemisphere projection. The required Co in the Eastern Mediterranean Sea ranges from 60 psia to 2250 psia from seafloor to target depth. However, the estimated Co of the sediments in the Eastern Mediterranean in this study by using the python codes developed in this study and the caliper logs of ODP Leg 160 ranges from 15 psia to 1250 psia with depth. Similarly, the required Co to keep the wells in gauge and the estimated Co of the sediments in ODP Leg 161 range between 100-12000 psi and 30-11000 psia, respectively. Mostly, Co values of the sediments were less than the required Co values for stable wellbore so extensive washouts were detected both in the wells of ODP Leg 160 and Leg 161. This is because the sediments within gas hydrate stability zone in the Mediterranean Sea are weak and almost unconsolidated. Moreover, in this study, it was shown that it is possible to reduce the degree of washouts by increasing the density of drilling fluid until fracture pressure. According to the estimations in this study, the wellbore enlargements for the wells in ODP Leg 160 and ODP Leg 161 were mostly reduced below 2 inches, below which better wellbore stability conditions, better well log data and better casing cement quality might be obtained.