Reservoir Characterisation and Monitoring using Integrated 4-D Seismic, Production and Geologic Data from Oil Fields Central and Coastal Swamps, Niger Delta

Description

Reservoir Characterisation and Monitoring using Integrated 4-D Seismic, Production and Geologic Data from Oil Fields Central and Coastal Swamps, Niger Delta

Abstract
Monitoring of reservoir properties prior to well placements and production is a requirement for
optimisation and efficient oil and gas production. This is usually done using well log analyses and 3-D
seismic, which are often prone to errors. However, 4-D (Time-lapse) seismic, incorporating numerous 3-
D seismic surveys overtime, could be utilised because it generates better resolution. There is however
dearth of information on the applicability of this approach in the Niger Delta. This study was therefore
designed to apply 4-D seismic, production and geologic data in monitoring of reservoirs in EK and SK
fields of Niger Delta. Data covering two 4-D vintages acquired over fifteen-year interval were obtained
from oil companies operating in EK (1200 km2) and SK (400 km2) fields. The data were analysed to
determine the seismic structures, horizons, Well-to-Seismic Tie (WST), and wavelets. Well logs, facies
and production history data from forty selected wells were also collected from the Oil companies.
Formation evaluation, petrophysical analysis and inversion alongside geological data were undertaken
using Petrel, Shell-nDi, Techlog and Jason Software. Gassmann’s fluid substitution and the empirical
rock physics relationships were analysed using Petrel and RokDoc. Rock physics and cross-plot analysis
of these well-logs were used to determine lithology, fluid properties and movements in reservoirs. The
vintages were interpreted, and changes in the reservoirs were defined by the differences in Acoustic
Impedance (AI) inversions of the base and the monitor seismic. The AI rock properties were estimated
from all the seismic amplitudes using controlled sparse-spike inversion. The structural analysis showed
dominance of NW-SE trending rollover collapsed-crest anticlines in EK with hydrocarbons trapped
northwards. Rollover anticlines, growth faults, trending E-W predominate in SK. There were good ties in
wells EK 27, 39 and SK 13, 26 and 30. These had wavelets with consistent amplitude and phase.
Consequently, a good match between the inverted impedance and the well data were achieved.
Reservoirs properties, rock physics and seismic attributes correlated. The comparison between the base
and the current monitor and their AI maps showed clear difference that helped in locating untapped
hydrocarbons and determining production effects. The monitor and the inverted sections through EK 27,
39 (within 3101 m – 3695 m), indicated depletion in the reservoirs. The extent of the present non-uniform
gas-oil contact and oil-water contact movements were 3554 to 3575 m and 3633 to 3612 m, for EK and
SK, respectively. More prospects were identified discovered, and mapped; consequently, new wells were
proposed for drilling in EK and SK. The 4-D seismic approach led to better reservoir monitoring, well
development and the location of deeper and bypassed hydrocarbon reservoirs.