research Research programme Module 2

Module 2

This is a well-recognized, popular topic of industry and academic research. Despite its maturity however, much remains to be achieved in terms of obtaining a robust and high resolution estimation of changes in the reservoir pressure and saturation (seis2PS) directly from 4D seismic signatures that satisfies the known fluid flow physics. This is especially true in complex reservoir settings. Pressure-saturation separation is also one of the keys to understanding the reservoir connectivity and dynamics, and forms invaluable input into a seismic history match. It cannot currently however be used in this way (if at all). This advanced level quantitative tool for reservoir engineering use is not yet within our grasp. This module also includes the familiar pressure change inversion from overburden strain which permits assessment of connectivity parameters such as transmissibility. Also identified is the need to use additional engineering data for connectivity studies, and so 4D seismic is integrated with production data for connectivity assessment. Finally, the topic of seismic geomechanics consolidates all of our recent advances under one sub-topic. The challenge here is to improve our current definition of the stress/strain field developed by reservoir production by careful inversion and modelling, and study of several geomechanically active datasets. Latest developments in the understanding of the R factor, or in time-shift interpretation also play a role in this sub-module.

Improvements to pressure and saturation inversion (seis2PS)

Here we continue to develop methods for the extraction of pressure and saturation changes from seismic data for clastic, chalk and hard-rock reservoirs. This will involve work with inversion products and rely on previous comprehensive assessments of our ability to detect pressure and saturation changes in the data. A primary goal of this work is the evaluation of changes for thick stacked reservoirs, where seismic resolution is challenged by production in the overlying reservoirs. In a second element, we take a closer look at gas saturation again (due to gas injection and liberated) with a view to accurate quantification. Of particular interest also is the estimation of pressure and saturation signals for complex recovery mechanisms such as water alternating gas, where pressure and saturation overlap. The main issue is the vertical resolution of water and gas, detection and separation of the pressure and saturation signals. The signals will get particularly complicated in stacked turbidites with miscible gas injection. Here 4D integrates with PLTs, well performance, open hole saturation logs and simulation model predictions. The aim is to improve areal and vertical sweep efficiency by monitoring using the seismic data. The analysis will be complemented by sim2seis (simulator to seismic modelling) scenario analysis followed by seis2sim (inversion to simulator properties) and seis2imp inversion (inversion to impedances).

The new sub-topics are:

• Pressure and saturation – thick, stacked reservoirs
• Engineering constraints imposed on inversion
• Integration of 4D seismic with well production data for thick reservoir, especially to analyse inter-layer connectivity
• Quantification of gas saturation
• Quantification of WAG
• Emphasis on speed of solution

Schematic of the double displacement process induced by water alternating gas recovery (Lawrence et al. 2013).

Production (and non-seismic data) analysis

Previous work has used the capacitance model to determine well to well correlations with the aim of integrating these results with 4D seismic data to enable pathways of communication to be defined for model update. These analyses also combine favourably with the highly successful well2seis approach developed by Huang and MacBeth (2007) and further extended by Yin and MacBeth (2015). Our aim here is to continue to explore joint and synergistic interpretations using the 4D seismic and engineering data. In this respect, machine learning may be used as a way of exploring the data correlations. Thus additional well and non-seismic data are sought for analysis such as: pressure data, tracer data, production and injection data for correlation analyses, logging data (PLTs), 4D gravity data, subsidence data, InSAR data and simulation predictions. The relationship to the connectivity defined in the geological model is also to be considered.

The following are the main points

• Analysis of production data and simulation studies
• Joint integrated interpretations between 4D seismic and production data e.g.
seismic PLT, 4D seismic – pressure transient analysis
• Analysis of well data: well2well data, tracers, produced and injected
volumes, pressure data, simulation, geological assessment of connectivity,
production logging tool data
• Link back to the well2seis technique

Reservoir connectivity and quality from a Carbonate field in the Campos
basin (Wong and Amini 2017)

Seismic Geomechanics

The incorporation of geomechanical principles into 4D seismic analysis has become increasingly popular in the industry from its origin (for example, Bourne and Hatchell 2005). Here we continue our own specific research thread on this topic extending back to previous phases (for example, Hall and MacBeth 2001, Hodgson and MacBeth 2006, Corzo et al. 2010, Garcia and MacBeth 2012, Wong et al. 2017). The aim in this new work is to investigate how stress/strain may be better defined using the 4D seismic data. In general, measuring, modelling and compensating for time shifts in 4D data is a commonly used interpretation tool. This will include looking at better ways to estimate post-stack time-shifts and invert for the underlying velocity changes, density changes, and physical subsurface displacements. We also intend to make use of pre-stack time-shift, and also 4D AVO in this work. An important element in this work is the accurate measurement of time-shifts themselves as methods to date do not routinely take into account the imaged wave. How can we do this better, particularly for complex overburdens, fractured reservoirs, and small time-shift signals? A final important aspect is the possibility of using additional non-seismic data in support of geomechanical measurements such as subsidence, 4D gravity or microseismic.

Key features of this sub-module are therefore:

• Time shift measurement:
– Measuring and modelling time-shifts or velocity changes, horizontal shifts, pre-stack data
– Time-shift analysis/warping/compensation for complex overburdens
– Possible de- and re-migration schemes
• Time shift interpretation:
– Interpretation of reservoir/underburden/overburden time-shifts – to include saturation and stress effects
– Salt overburden and stiff carbonate reservoirs
– Estimation of pressure using overburden geomechanics
• Closing the loop using geomechanics and the 4D seismic
• Links between velocity change and strain, and the ‘R’ factor
• Beyond Geertsma modelling?

It is anticipated that there will be applications to datasets from HPHT and chalk reservoirs, although other data must also be considered.

Different estimates of time-shifts for the Shearwater HPHT field using three well known techniques (Ji 2012).