Water-flood management is an exploitation strategy adopted in Mumbai High field to maintain the reservoir health. Pressure sinks are seen developing in Block X of the south field as a result of inadequate water injection. Efforts to increase water injection in this area based on analysis of injection wells using various water-flood surveillance tools resulted in arresting decline, increase in water retention and reservoir pressure.
As a part of the study, the performance of injectors was analysed with their effect on nearby producers. Various classical techniques for water-flood surveillance viz. injection performance curve vs. time incorporating events, Hall's plot, ABC plot, backwash analysis, PLT report analysis, pressure vs. time plot, production performance curve vs. time incorporating events, water cut and GOR vs. time plot, WOR vs. cumulative oil production plot and bubble mapping were utilized to monitor the efficiency and take corrective actions to improve the health of the reservoir.
The study area is located in the peripheral part to the west of WI-X1 injection platform of the MH south field. The area was developed by platforms I1, I2 and I3 (1993-94) by conventional wells, but had limited productivity/ injectivity. Further development was done by horizontal/ multilateral wells from I4 (2002-05), where the horizontal wells produced with high initial rates, that declined sharply, more prominently in wells towards west away from injection platforms. The high decline could be attributed to negligible pressure support from western natural aquifer influx and tight reservoir characteristics leading to poor pressure transmissibility from the eastern injection platforms. The problem was aggravated by less water injection than envisaged rate resulting in decline in production, drop in reservoir pressure, low water retention and low incremental voidage replacement ratio (0.45). To increase the injection support 3 more water injectors were added that resulted in an increase of injection from 17,500 to 31,800 BWPD. This led to increase in incremental VRR to 1.80 and water retention to 88%, which has shown positive response in jacking up pressures in this area and arresting of oil production decline per string from 14% to 4% with flattened WOR.
The present study emphasises the importance of maintaining incremental VRR greater than 1.0 and reducing circulation of injection water by conversions in appropriate places with respect to producers along with optimised rates in existing injectors to improve the recovery in tight reservoir conditions.