Sealed coring technology has been widely used in the Bohai Bay Basin in the past decades. The temperature and pressure changes experienced by the core from in-place borehole to lab conditions generate inconsistencies between the measured and the in-place saturations. In the Bohai Bay Basin, the as-received air-filled saturation measured in a laboratory has reached 10 ~ 30%, leading to significant uncertainty when using measured saturation to calibrate log-derived saturation.
Saturation correction must be performed when attempting to assess the risks, economics, and potential of a targeted resource play. It is commonly assumed that the void space at surface was previously occupied by hydrocarbons. However, this statement clearly overestimates the content of hydrocarbons, especially in waterflood intervals where there is a simultaneous loss of oil and water. The influence of montmorillonite dehydration on Dean-Stark saturation has not been extensively investigated. Although physical experiments can simulate the effects of degasification on fluid saturations, they are not practical for vendors and operators due to their lengthy duration.
Too much attention has been focused on updating coring technology and saturation measurement methods, while the key question has been neglected: How has the core saturation changed due to the temperature and pressure changes? The interlayer water of montmorillonite bleeds from the core at high temperatures. The volume of pores and fluids changes as the core is brought to the surface. Some of the fluids in the pore space get volatile because of degasification. All these phenomena can lead to errors in laboratory saturation.
This manuscript presents a workflow that includes correction models for pore volume expansion, clay dehydration, and degasification to bridge the gap between measured and in-situ saturations. The corrected saturation shows a good agreement with log-derived and air-mercury injection saturations, indicating the effectiveness of the workflow. Compared with the measured water saturation from standard practice, the corrected water saturation increased by an average of 7.4 saturation units (s.u.) and was found to be statistically significant (P = 0.0268 < 0.05). The workflow is based on experiments that were conducted on hydrophilic clay-quartz samples, so the application of the workflow is limited by the wettability and lithology of the formation.
