Publications

Shallow gas in the Mahakam PSC is considered a major operational risk during drilling operations. The status as geo-hazard for shallow gas reservoir has had an impact on well logging data acquisition for decades. Nuclear logs, such as density and neutron tool, were banned during logging acquisition to minimize the risk during the shallow drilling phase. As a consequence, most of the wells only have Gamma Ray and Resistivity (GR-RES) logs in the shallow section. No ‘porosity’ logs are available, except for a dozen exploration wells, where a complete logging suite was recorded. Following recent evaluation of these shallow gas resources, it has been decided to propose and apply developments plans to produce these reservoirs. Shallow gas must be mapped and its volume estimated from the limited data available. An empirical transform was developed to compute effective porosity from the GR log. This method is based on a derived effective porosity from a function of compaction and shaliness. Trends of clean porosity versus subsea depth and porosity reduction versus shaliness were established from the wells having full logging suites. The shaliness is computed from the GR by picking shale and sand values and using a linear relationship. The effective porosity is computed from subsea vertical depth and shaliness using empirical transforms. Water saturation is then computed from effective porosity, shaliness, and resistivity. This method was applied in several hundred wells with success. In the meantime, more data were acquired, such as full diameter cores, to have a better geological understanding of the reservoir and to calibrate the petrophysical parameters used for interpretation. A Pulsed Neutron Logging campaign was carried out to differentiate gas from coal and to detect saturation changes after production. This paper presents the evolution of petrophysical interpretation from shallow gas occurrences, once considered a drilling hazard, to economical reserves.