Hamed Amini

4D Seismic Feasibility Study for Enhanced Oil Recovery (EOR) with CO2 Injection in a Mature North Sea Field

A mature North Sea field with a long history of water flooding has been modelled to predict the potential for enhanced oil recovery (EOR) with CO2 injection. This technique has the potential to further extend the life of the field by extracting more oil and to store the injected CO2 in the reservoir. A feasibility study was conducted to assess whether 4D (time-lapse) seismic could be used as a monitoring tool during CO2 injection. Different approaches were employed in this project; the initial examination using a 4D risk assessment table (Lumley et al., 1997) suggests that 4D seismic can be applied. For a more quantitative analysis, a petro-elastic model was designed to capture both the saturation and pressure signatures from the injected CO2. 1D modelling using fluid substitution at well locations, and simulator to seismic modelling in conjunction with the compositional simulation model was performed to predict the three-dimensional 4D signatures within the simulation model grid. Based on this analysis a seismic impedance change of up to 9% is predicted which is above the limit that is typically considered for 4D applications. The discrimination between pressure and saturation signals is also discussed and the possibility to use 4D seismic to detect pressure changes was evaluated. This study makes suggestions regarding the seismic survey characteristics and appropriate timing to acquire the monitor surveys.

Calibration of Simulator to Seismic Modeling for Quantitative 4D Seismic Interpretation

Realistic quantitative match between the results of simulator to seismic modelling and observed seismic can be used to update the reservoir model. For this purpose, it is vital to accurately adjust the parameters involved in petro-elastic model (PEM) and seismic modelling as the two controlling components of Sim2seis analysis. The PEM parameters are calibrated from a Sim2seis perspective using well log data. It is important to adapt the PEM parameters according to the lithology definition at the simulation cell scale, which is different from the log scale. The convolutional model (CM) is calibrated by comparison of the results with a full waveform pre-stack finite difference (FD) seismic modelling approach. The results show that if enough care is taken, CM can produce reliable results for 4D analysis. CM amplitudes are consistent with FD, while for time shift, in the case of small time shifts the error between the two methods can be of the same magnitude as the 4D signature.

A review of post-stack 4D seismic time-shifts, part 1: values and interpretation

A review and analysis of post-stack time-lapse time-shifts has been carried out that covers published literature supplemented by in-house datasets available to the authors. Time-shift data are classified into those originating from geomechanical effects and those due to fluid saturation changes. From these data, conclusions are drawn regarding the effectiveness of post-stack time-shifts for overburden and reservoir monitoring purposes. A variety of field examples are shown that display the range and magnitude of variation for each class of application. The underlying physical mechanisms creating these time-shifts are then described, and linked to a series of generic and field-specific rock physics calculations that predict their magnitudes. These calculations serve as a guide for practitioners wishing to utilize this information on their own datasets. Conclusions are drawn regarding the reliability of this attribute for monitoring purposes, and the extent to which further development is required and how it should be reported by authors.

Calibration of Rock Stress-sensitivity Using 4D Seismic Data

Rock stress-sensitivity is a key aspect of 4D seismic studies, particularly when discriminating the effects of pressure and saturation change. Typically, laboratory measurements on core samples are used to address the rock stress-sensitivity in petro-elastic models. Such measurements are subject to uncertainties and their applicability to the in-situ field-scale reservoir response is questionable. Here, as an alternative to laboratory measurements, a technique is presented to calibrate the rock stress-sensitivity based on the 4D seismic data alone. After years of injection, the variation of the 4D signal in the fully-flooded zone in the vicinity of the injectors can be solely attributed to pore pressure variations. The proposed method utilises comparison of the observed with synthetic 4D seismic response in this fully water flooded zone for a range of rock stress-sensitivity models. The results of the modelling around two injectors, for a deep-water turbidite reservoir in the West of Shetland on the UKCS, suggest higher rock stress-sensitivity compared to the stress-sensitivity determined from laboratory measurements.

Calibration of the Petro-elastic Model (PEM) for 4D Seismic Studies in Multimineral Rocks

We propose a method to calibrate the petro-elastic model in a multi-well fashion, applicable to multimineral,multi-fluid rocks. We use a model that combines Gassmann’s equations (1951) with the modified MacBeth (2004) rock stress sensitivity model (Alvarez & MacBeth, 2013) through a linear search optimisation algorithm, which allows modelling of the variations of the dry-frame elastic moduli with changes in porosity and mineralogy. The importance of the rock framework characterisation on PEM predictions is highlighted, and it is concluded that an over-simplification of the dry-frame characterisation in 4D seismic-related studies can lead to errors of the same magnitude as the 4D seismic signal and therefore these effects cannot be ignored.

Blake Field Simulator to Seismic Modelling Study

Simulator to seismic modelling (sim2seis) was applied to the Blake Field to understand the controlling parameters of the 4D signal in a three phase system. This was necessary as it is a significant challenge to distinguish simultaneous uneven movement of the GOC and OWC around horizontal producers based on 4D seismic. Here, 4D amplitude and time-shift maps were produced via sim2seis to illustrate the effect of 1) reservoir structure, 2) relative movement of the contacts during production, 3) immediate overburden properties, and 4) seismic wavelet on the 4D signature. The outcome was an improved comprehension of the reservoir for future management.

Porosity Concepts in Gassmann Fluid Substitution: A Simulator to Seismic Modelling Perspective

In this study, porosity concepts in Gassmann’s fluid substitution model are examined and their implications for quantitative 4D seismic studies are discussed. Three porosity models (total porosity, effective porosity, and a movable fluid model) are compared from a simulator to seismic (sim2seis) modelling perspective. From the three selected models, total porosity predicts the largest softening effect due to gas breakout and the smallest hardening effect from water-flooding; whereas the movable fluid model predicts the least softening due to gas breakout and the largest effects for water-flooding. Effective porosity predictions lie between the total porosity and movable fluid models. The differences between these models are due to the proportion of fluids in the mixture which are input into Gassmann’s equations. Sim2seis results based on different porosity models were evaluated against the observed 4D seismic. This comparison shows that the magnitude of the saturation-induced hardening and softening signals due to the movable fluid model is closer to the observed seismic. The total porosity model is in least agreement with the observed 4D seismic.

Finding a Petro-elastic Model Suitable for Sim2seis Calculation

The petro-elastic model (PEM) is a necessary step in simulator to seismic modelling, intended to close the loop between the seismic and engineering domains. In this work, we discuss some fundamental issues within the conventional PEM algorithm, not commonly covered by published literature. Firstly, we explain the importance of the porosity rock model for the PEM. It is shown that both total/effective porosity models are able to generate satisfactory seismic results, provided that the density and bulk/shear moduli of the solid components are set correctly using an optimisation problem. We find the underlying connections between the simulation model parameterisation and the effective porosity model from the petrophysical domain. Finally, we discuss the effect of vertical upscaling on the seismic domain. We highlight the differences between property upscaling and reflectivity upscaling, and challenge the idea of developing a scale-dependent PEM based on Backus averaging. In addition to a sim2seis analysis, the results of this work have direct impact on seismic inversion via the PEM for pressure and saturation change or impedance change onto the reservoir grid. Keywords: PEM, rock porosity model, simulation model, effective porosity, total porosity, upscaling, Backus averaging, reflectivity upscaling.

Application of Compositional Simulation in Seismic Modeling and Numerical Well Testing for Gas Condensate Reservoirs

Recognition of condensate blockage is important in reservoir monitoring and management since the secondary operations such as gas cycling might need to be triggered to re-pressurize the reservoir. This study addresses the monitoring of a realistic gas condensate reservoir by time-lapse seismic data and transient well test analysis. A compositional reservoir modelling is employed to perform the numerical well test simulation. However, implementing a compositional fluid flow simulation highlights the limitations of the current petro-elastic model (PEM). Therefore we have developed an approach to consider the compositional changes of the fluid in the petro-elastic modelling. The equivalent black-oil model results are cross-validated against the compositional PEM as well. Our results show that the original widely used Batzle & Wang approach should be modified for a gas/condensate system. We show that the response strength of the well testing and the 4D seismic are complementary in each particular flow period (drawdown and build-up) which can be coupled to give useful information for reservoir monitoring purposes.