FROM DRILLING HAZARD TO ECONOMICAL RESERVES: THE EVOLUTION OF FORMATION EVALUATION IN A SHALLOW GAS RESERVOIR

摘要

Shallow gas in the Mahakam PSC is considered arnmajor operational risk during drilling operations.rnThe status as geo-hazard for shallow gas reservoirrnhas had an impact on well logging data acquisitionrnfor decades. Nuclear logs, such as density andrnneutron tool, were banned during loggingrnacquisition to minimize the risk during the shallowrndrilling phase. As a consequence, most of the wellsrnonly have Gamma Ray and Resistivity (GR-RES)rnlogs in the shallow section. No ‘porosity’ logs arernavailable, except for a dozen exploration wells,rnwhere a complete logging suite was recorded.rnFollowing recent evaluation of these shallow gasrnresources, it has been decided to propose and applyrndevelopments plans to produce these reservoirs.rnShallow gas must be mapped and its volumernestimated from the limited data available. Anrnempirical transform was developed to computerneffective porosity from the GR log. This method isrnbased on a derived effective porosity from arnfunction of compaction and shaliness. Trends ofrnclean porosity versus subsea depth and porosityrnreduction versus shaliness were established from thernwells having full logging suites.rnThe shaliness is computed from the GR by pickingrnshale and sand values and using a linearrnrelationship. The effective porosity is computedrnfrom subsea vertical depth and shaliness usingrnempirical transforms. Water saturation is thenrncomputed from effective porosity, shaliness, andrnresistivity.rnThis method was applied in several hundred wellsrnwith success. In the meantime, more data werernacquired, such as full diameter cores, to have arnbetter geological understanding of the reservoir and to calibrate the petrophysicalrnparameters used for interpretation. A PulsedrnNeutron Logging campaign was carried out torndifferentiate gas from coal and to detect saturationrnchanges after production. This paper presents thernevolution of petrophysical interpretation fromrnshallow gas occurrences, once considered a drillingrnhazard, to economical reserves.