Time Lapse (4D) Seismic for Reservoir Management: Case Studies from Offshore Niger Delta, Nigeria
Authors: Amoyedo Sunday Oluleye , Ekut Emmanuel , Salami Rasaki , Goncalves-Ferreira Liliana , Desegaulx Pascal 
Published: October 2014

We present in this paper case studies focused on the interpretation and integration of seismic monitoring from several fields located in conventional offshore and deep-water Niger Delta. The fields are characterized by different, geological settings and development maturity stages. We show different applications varying from qualitative to quantitative use of 4D information.

In the first case study, which is located in shallow water, the field has specific reservoir development challenges, simple geology, and is in phased development. On this field, 4D seismic, which was acquired several years ago, was not of optimum quality due to poor seismic repeatability. Nevertheless, we show that due to improvements from the reprocessing effort, 4D seismic makes qualitative contributions to the on-going field development.

In the second case study, the field is characterized by complex geological settings. 4D seismic is impacted by overburden with strong lateral variations in velocity and steeply dipping structure (up to 40o). PSDM 4D seismic is used in a more qualitative manner to monitor gas injection, validate the geologic/reservoir models, optimize infill injector placement and consequently, enhance field development economics.

The third case study presents a deep offshore field characterized by a complex depositional system for some reservoirs. In this example, good 4D seismic quality is achieved, leading to an increased quantitative use of 4D monitoring for the assessment of sand–sand communication, mapping of oil-water (OWC) front, pressure evolution, dynamic calibration of petro-elastic model (PEM) and serves as a pseudo production-logging tool. Additionally, 4D is used to update seismic interpretation, provides a better understanding of internal architecture of the reservoirs and thereby, yielding a more robust reservoir model. 4D seismic on this field is a key tool for field development optimization and reservoir management. The last case study illustrates the need for seismic feasibility studies to detect 4D effects related to production. In addition to assessing the impact of the field environment on the 4D seismic signal, these studies also help in choosing the optimum seismic survey type, design and acquisition parameters. These studies would possibly lead to the adoption of new technologies such as broad-band streamer or nodes acquisition.

Introduction

Time-lapse (4D) seismic provides a vital tool for reservoir monitoring and also serves as a multi-disciplinary platform for efficient reservoir development. Reservoir management requires an all-round identification, quantification and management of reservoir uncertainties, which stem from limited and sparse reservoir data and the use of models, analogues, band-limited seismic and 3D reservoir model infilling based on statistical distribution of well logs. Identifying and quantifying the various uncertainties and their impacts are critical to optimizing reservoir development.

Field development in the Gulf of Guinea is largely seismic driven. Seismic amplitudes from horizons and time slices coupled with spectral decomposition and pseudo reservoir properties volumes (VCL, NTG, Porosity, etc.) derived from seismic inversion are integrated to identify and delineate the various architectural elements (AEs) in the field. 3D property fillings (facies, porosity, static permeability, Net-Gross (NTG)) of these elements are done using guided statistical distribution of reservoir properties obtained at well locations.