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Sustaining Production from Mature, Mainly Single Layer Reservoir Fields Through New Concepts from Bypassed “Fringe-Oil” and Delineation of New Hydrocarbon Plays for Alternate, Unconventional Reservoirs - A Case Study from Rudrasagar Field, Upper Assam, India

Sanjiv R. Gupta1 and Ashok Gupta2
1GEOPIC, ONGC, Dehradun, India
2NAS, ONGC, Jorhat, India

The paper discusses two new concepts, (a) “ fringe oil” -bypassed oil at lower structural level along the pool edges- and (b) alternate unconventional reservoirs, that have been applied to a mature, mainly single reservoir field in Upper Assam, India, to look for new avenues for oil.

The performance analysis of wells from from the main block of this field indicates a watering out tendency (water -cut upto 82%) after producing only 48 % of reserves. However,most wells along the field periphery and often structurally lower indicate better performance, often with lesser water cut. This phenomenon of abnormal flooding in structurally higher wells has been documented by Cased hole formation resistivity ( CHFR ) logs in two recent wells and also by the initial open-hole logs and production testing in one of the recently drilled infill well that indicated flooding of the pay zone.

Analysis of the pressure- production trends indicate that the performance of wells may not be entirely related to its absolute pay level but at a later stage of production the bottom- water encroachment overtakes the effect of edge-water-particularly in areas of high well density and drainage- that results in premature flooding of the pay zone at relatively higher levels, and in the process leaves substantial undrained pockets at lower structural levels. This phenomenon is particularly well documented along the pool edges bounded by sealing faults.

The other concept of alternate and unconventional reservoirs is derived from a new hydrocarbon prospect in an apparently unfavorable facies (mainly clay section) which has accreted new reserves and has been linked to a major lineament by integrating it with regional seismics. Its log interpretation and production testing gives a new dimension to inferring new hydrocarbon plays and reassessment of tight reservoirs.

Presentation GEO India Expo XXI, Noida, New Delhi, India 2008©AAPG Search and Discovery