Managing Waterflood in Deepwater Soft Rock: The Bonga Experience to Date

Abstract

Abstract The Bonga field came on stream in November 2005 with 16 predrilled wells (9 producers and 7 water injectors). The field is characterised by unconsolidated rock but with permeabilities in the multi darcy range. The reservoirs are undersaturated and contain light crudes in the 28-35 deg API range. The field contains STOIIP of over 4 billion barrels across 6 reservoirs and it is developed by waterflood. The reservoir characteristics have culminated in high injection and production rates per well with injection rates above 60 kbpd and production rates above 50 kbpd achieved for some wells. Plateau field production and injection was achieved soon after start up with production and injection at or near nameplate capacities of 225 kbopd and 300kbwpd respectively. This continued until H1 2008 when injectivity decline set in. As water breakthrough had already occurred, the reduced deliverability of the water injectors further compromised the voidage replacement capacity of the system. Net result was that the field fell off plateau but production was soon restored to near plateau level. Since then, production has been off plateau intermittently with the integrated team taking action to ameliorate the situation and to bring the field back to near plateau level of some 200 kbopd. The key issues/challenges that have resulted in production decline include: injector impairment, inadequate topsides injection capacity, injection well failures, baffles mitigating injector-producer connectivity, producer impairment, ScSSV failures, injector longevity, injection water quality, produced water handling, premature water breakthrough. Cross discipline corrective and innovative measures have been taken to address the above challenges and maximise value from this field. The measures cut across subsurface, wells, topsides, subsea and production operations disciplines and include: Backflush/refrac, acid stimulation, increased injection capacity, improved injection water quality, fracture mode injection only, drilling of horizontal wells as preferred option for injectors, breaking down of connectivity baffles by sustained high rate water injection etc. This paper captures the key learnings from the Bonga experience and the corrective measures applied. The field overview and performance history of the field are illustrated in figs 1&2.