High-water-saturation (Sw) bioturbated zones are generally bypassed due to the perception that those are non-oil-bearing, nonproductive zones. The major challenge in bioturbated pay zones is low resistivity, which leads to high Sw calculation. The hydrocarbon could then be bypassed in the existing completion. The high Sw also contributes to an inaccurate static model and stock tank oil originally in place (STOIIP) calculation. This study is to prove that bioturbated zones can add oil flow (as part of behind-casing opportunities from the current idle wells in many fields in Malaysia) and contribute meaningfully to the STOIIP.
The objective of this paper is to use multifunction pulsed neutron service to identify and quantify the remaining oil saturation in bioturbated reservoirs. The examples shown are from a mature field located in offshore Sarawak.
Pulsed neutron logging (PNL) is a well-established technology in casedhole reservoir monitoring and evaluation. However, the challenging environment with dual-tubing completions makes quantitative interpretation very difficult. The new generation slim pulsed neutron logging tool used in this study improves both sigma (∑) and carbon/oxygen (C/O) ratio. It also provides fast neutron cross section (FNXS), a new measurement very sensitive to formation gas and dry weight total organic carbon (DWTOC). More reliable answers help the subsurface team to select the perforation intervals confidently.
The hydrocarbon zones in bioturbated reservoirs were clearly identified. The remaining oil saturation was computed from much improved C/O ratios and the independent DWTOC method. The positive production results confirmed that the measurements and interpretation are accurate and more robust.