Water-flood Performance Monitoring of Fluvial Reservoir Through Saturation logging - A Case Study of Large Onshore Mangala Field
Authors: Prasad Dhruva , Verma Sumil , Kumar Pankaj , Singh Aditya Kumar , Tandon Rohit , V Ravichandran , Shankar Pranay , Singh Pradeep 
Published: May 2016

Abstract
This paper discusses the application of saturation logging to characterize the water flood performance, sweep, importance of vertical conformance in moderately varying to high net to gross fluvial system, at one of the largest onshore field Mangala in Barmer basin, India containing ~1.3 billion initial oil in-place. Mangala field was discovered in 2004 and was brought on production with hot water flooding in August 2009. Structurally Mangala is a tilted fault block consisting main oil bearing reservoirs of Fatehgarh group of Cretaceous/ Paleocene age as the main sandstone reservoir unit (~250 meters) dipping at around 9 degrees to the east. The Fatehgarh group is subdivided into 5 major reservoir layers litho-stratigraphically termed FM1 (top) to FM5 (base). The lower Fathegarh Formations (FM3 to FM5) are dominated by well-connected sheet flood and braided channel sands having net to gross ~ 80%, whilst the Upper Fathegarh Formation (FM1 and FM2) is dominated by more sinuous, laterally migrating fluvial channel sands transitioning into lacustrine depositional system at the top and having net to gross &lt;50%. The reservoir in general is of high quality with multi-darcy permeability, porosity &gt; 25%; with relatively viscous (15cp) and waxy crude. The FM1 and FM2 are developed with downdip edge line drive and inverted 9- spot pattern. The massive FM3 and FM4 sands have been developed with a downdip edge line drive and up-dip horizontal producers.
Saturation logging with Production logging is very important tool in monitoring the field injection performance. Time lapsed saturation logging data suggested that the FM-3 is sweeping very nicely from the bottom whereas in FM-4, the intra-shale layers are extended and thus not allowing the bottom sweep in some area. The FM-1 has come up with the conformance issues which suggest that the injection is not getting uniformly distributed across layers, resulting in the non-uniform sweep.
Saturation log has helped in monitoring varying sweep in different reservoir units, sand to sand correlation in highly heterogeneous FM1 reservoir unit with the integration of Production Logging and other data in Mangala field. The improved understanding of conformance, production and injection has helped in locating the un-swept areas targeted for selective injection and drilling infill wells.

Introduction
The Mangala field was discovered in January 2004 by the discovery well N-B-1 in the license block RJ-ON-90/1 (Figure-1). The main reservoir unit in Mangala is the Fatehgarh Group, which has been sub-divided into the Lower Fatehgarh Formation dominated by amalgamated well-connected sheet flood and braided channel sands, and the Upper Fatehgarh Formation dominated by sinuous, meandering, fluvial channel sands. Five reservoir units (FM1-FM5) have been named from the top down. Fatehgarh sand properties are excellent, with porosities of 21–28% and permeability of 200–20,000 milliDarcies (mD). Average permeability is ~5 Darcy. The Mangala structure is a simple tilted fault block dipping at ~9º to the east - southeast (Figure 2).