Andang Bachtiar 1, Yudi S. Purnama 1, Bayu Nugroho 2,
Maradona Mansyur 3, Andi Krisyunianto 4
1Exploration Think Thank Indonesia, Jakarta, Indonesia
2Harvest Natural Resources, Jakarta, Indonesia
3Salamander Energy, Jakarta, Indonesia
4GDA Consulting, Jakarta, Indonesia
Basically, a well postmortem is carried out in order to find out as to why there is no producible hydrocarbon in a well despite the existence of favorable geological factors and convincing hydrocarbon shows either during drilling or based on well evaluation of E-logs. Subsequently, result of the postmortem can then be used to determine strategy for the next exploration/appraisal plan. A well postmortem often come up with findings that geologically the well is not really dry but was either considered as dry or inconclusive due to non-geological related factors.
This paper describes methods, case studies and implication of a well postmortem. Postmortem procedures applied in this study consist of Mudlog review, Pressure Analysis of drilling and testing data, Geochemical analysis of laboratory results as well as test result verification using chloride matching analysis and test curve review. All procedures are complemented by log analyses of case studies.
Mudlog review of oil and gas shows derives conclusiveness of the reservoir content. It is noted that the first occurrence of C3+ indicates either the penetrated section starts to enter early mature window for oil generation or, hydrocarbon has migrated to shallower section. Moreover, the occurrence of C4 and C5 in may also be considered as oil fraction indicator. Oil show characters also give direct clues to hydrocarbon type such as blooming means possible oil with gas accumulation. Pressure analysis combined with hydrocarbon shows, help to understand the relationships of overbalance with formation damage, of overpressure/subnormal pressure and hydrocarbon column as well as to determine possible hydrocarbon column. Test result verification is crucial especially in concluding the conclusiveness of a tested zone. There are practical clues in test verification, such as water salinity analysis which often misinterpreted as formation water, pressure chart that shows characters of formation damage or tight formation. Geochemical analyses results of well cuttings and/or fluids from well testing -if available- are also key elements in a well postmortem, in order to understand how the petroleum system works in the area. The clues are determined by comparing hydrocarbon shows (i.e. C3 and C4 readings from mudlog) against Vitrinite Reflectance (in order to verify Top Oil Window/early oil generation) and by resolving hydrocarbon origin of tested zones.
Results of postmortem evaluations of three areas may potentially change the chance-of-success ratio from 14% to 86% in Central Sumatra Basin Highs, from 50% to 73% in Jambi Sub-Basin, and from 13% to 50% in South Sumatra Mountain Front. Immediate implication of postmortem result is, the originally considered inconclusive wells may in fact turn out to be candidates for re-entering, re-test or even re-drill with minimum cost and effort to possibly give new reserve or even increase in oil production.