APPLICATION OF WELL LOGS AND 4D SEISMIC FOR ASSESSING AND MONITORING RESEVOIRS IN ‘ROYAL’ FIELD, NIGER DELTA

Abstract

Advanced surveillance tools are needed to identify areas of complex geometry and nonhomogeneous reservoirs which cannot be properly resolved through conventional 3D methods. The Niger Delta has several depleted and abandoned oil fields. ‘Royal ‘Field has experienced hydrocarbon production for over a decade and is nearing abandonment. 4D Seismic and Well log data were utilized in assessing and monitoring reservoirs in ‘Royal ‘Field, Niger Delta in order to determine internal reservoir architecture and identify depleted and un-swept zones for enhancement of future oil production in the field. The reservoirs were delineated based on low Gamma Ray and high Resistivity logs signatures and they were continuous across the field. The results of the petrophysical analysis indicated good porosity (13 - 28%), very good permeability (285 to 670 millidarcies), low water saturation (5-28%) and high hydrocarbon saturation (72-85%). The modelled petrophysical parameters indicated that the reservoir rocks were hydrocarbon bearing. The reservoirs have enough pore spaces for hydrocarbon accumulation as well as effective interconnected migration pathway for fluid transmission within the pore spaces of the rocks without causing structural changes in the formation. The calibration and correlation of 4D seismic volumes revealed good correlation and high repeatability between the base and the monitor seismic dataset. Structurally, the field was characterized by structural high dominated by synthetic and antithetic fault which assisted in proper accumulation of hydrocarbon within the study area. Rock physics analysis performed in 3-D cross plot spaces established four distinct clustering zones. Gassmann’s fluid substitution modelling for 20-60%, 20-80% and 25 to 100% hydrocarbon and brine saturations over reservoir A and B showed good sensitivity of Density, Acoustic impedance, Poisson ratio and Lambda rho to varying saturations. 4D inversion were used to transform seismic amplitude to acoustic (elastic) impedance by integrating Check-shot corrected well data as well as 3 interpreted horizons to produce detailed views of the subsurface. Based on rock physics results, Density, Acoustic impedance and Lambda rho were extracted from time-lapse seismic volumes around and away from the producing well locations. Low values of Density, Acoustic impedance and Lambda rho were associated with hydrocarbon saturations, while high values corresponded to brine (water) saturation and shale. The monitor horizon slices exhibited noticeable increases in Density, Acoustic impedance and Lambda rho values compared to the base data indicative of pressure depletion, hydrocarbon extraction and probably reservoir compaction. No production or pressure data was available so, the extent of depletion of reservoirs was inferred by attributes increased in the monitor horizon slices for Royal A, C and D as a result of production and pressure depletion. It indicated It indicated that Royal A, C and D have probably watered out and not producing again while Royal B associated constantly with low Density, Acoustic impedance and Lambda rho was still producing. Two new prospect areas (PA and PB) were predicted at the northeastern and southeastern parts of the producing wells location and these were recommended for new drilling opportunities in the field.