Abstract
Hydrocarbon phase determination in depleted fields has been a major challenge. This main uncertainty affects completion decisions and, consequently, oil recovery. As the field matures, the reservoir fluids start to exist in a two-phase configuration due to gas released from reservoir oil or gas trapped due to aquifer movements during production. A traditional petrophysical evaluation, neutron density separation, can mislead under such conditions. An oil reservoir can be interpreted as gas by showing a big separation in neutron density logs. However, a fundamental question arises: Are these apparent "gas zones" genuinely filled with free gas, or could a substantial portion contain oil?
This paper introduces an integrated work process between petrophysical logs and Gas Oil Ratio (GOR) prediction based on advanced mud gas, as well as historical fluid samples and field production history. The effectiveness of this method is demonstrated through a case study from the Oseberg Sor field, a late-life production field on the Norwegian Continental Shelf. The fluid identification technology based on mud gas data predicts a large oil saturation in reservoirs. In contrast, interpretations derived from conventional petrophysical logging tend to overemphasize minimal gas saturation and incorrectly predict the reservoir fluid type. This critical interpretation from mud gas has enabled the completion of the well in the oil zone. With the successful implementation of this innovative approach, questionable "gas zones" is reinterpreted as oil zones, making them viable production prospects.
The integrated methodology reveals a robust solution for a longstanding industry challenge and emphasizes incorporating a comprehensive understanding of petrophysical logs, advanced mud gas (AMG) data, and PVT data from nearby wells. This approach remains unaffected by minor quantities of released or entrapped gas within reservoirs, offering the potential to complete a higher number of oil zones for production. Given the increasing activity in matured fields, the business value of this innovative method is substantial.
