research Research programme Module 1

Module 1

This theme continues the several major subject areas that underpin many of our preliminary quantitative 4D seismic analyses, now grouped under one sub-topic. It also includes two new sub-topics that have been initiated in response to research trends to date. The first topic is the analysis of pre-stack 4D seismic data, which opens up a range of studies for amplitudes and time-shifts, in addition to tomographic and inversion approaches. The second topic for consideration is the dynamic overburden, this time expanded to include many issues such as reservoir shales that are already a major research thread in ETLP. The final topic examines the feasibility of 4D seismic monitoring of IOR/EOR. A number of studies are already beginning to emerge in the literature, and we believe this is an important area to develop when expanding the applicability of future applications for 4D seismic surveillance.

Pre-stack 4D analysis

Restricted offset stacks have been used for some time to separate pressure and saturation, and post-stack time-shifts have been employed with much success to delineate geomechanical effects and saturation. Pre-stack time-shifts are less well researched, although there is a growing awareness that extra accuracy may be gained by working in this domain. In particular, there is a need to know how best to capture and utilise the variations that are observed in our data. One approach is to work with 4D pre-stack tomography. Also required is a way to disentangle the amplitudes, time and phase shifts in regions of complex geology or production signatures.
Sub-topics include:

• Independent measurement of time-shift and amplitude changes
• Interpretation of pre-stack data, possible azimuthal variations
• 4D seismic tomography
• 4D pre-stack inversion – low frequency update using time-shifts
• Shear wave data, anisotropy?
• Use of spectral decomposition, and the Q factor
• Full wave/waveform methods, least squares migration or fast-track variants
• Near-surface TL effects, new acquisitions

It is our intention to package the above into a series of practical projects that apply directly to several field datasets and our PhD students.

4D tomography results from applying a recursive scheme to limited offset data from the Shearwater field (Nguyen et al. 2017).

Dynamic overburden: impact on imaging

This second major topic for study is one that we have investigated in detail in the past and has grown to become a major part of our programme. Typically in any given phase we create one or two PhD projects around this area. For Phase VII stress sensitivity and geomechnical issues are continued to be explored, and the impact on 4D seismic interpretation at the producing reservoir level is further investigated. This is particularly important for geological settings with thick sequences of stacked reservoirs with varying degrees of connectivity and complex 4D signals due to overlapping pressure and saturation effects in variable geology. The impact of overlying reservoirs on producing reservoirs below must also be addressed. Also of interest here are pre-salt reservoirs with complex imaging and difficult 4D signatures. Finally, this module continues to support our shale project which aims to categorise the geomechanical response of overburden shales. The impact of shales and shaley sequences on reservoir connectivity is analysed in depth for several field datasets in an integrated study. An old theme now revisited as part of the above, is the effect of the overburden on sim2seis, which has been shown to be significant in past work by a number of authors (Doyen et al. 2005 in inversion, Domes et al. 2010 in modelling). We do need to incorporate the effects of the overburden in general as these influence the changes we see at the reservoir level (in amplitude, phase or time-shift), particularly as we push for greater quantitative perfection. Finally, the role of the velocity model and time to depth issues are considered, as these are in practice critical to the success of any quantitative analysis scheme.
Specific sub-topics include:

• Geomechanical effects in the overburden
• Stacked reservoirs
• Complex 4D signals
• Ray-bending effects
• Salt geomechanics, salt overburden
• Reservoir and overburden shales

Sections along the wells showing predicted and observed time strain after 3 years
of production in the Shearwater field for different scenarios of shale activation (Rangel,

Feasibility and assessment of IOR/EOR

In this work we examine the viability of monitoring a range of specific IOR and EOR situations, following on from our formative work on water flooding and polymer flooding in Phase V (three years ago). The main aim here is to establish how 4D seismic can be of value in these particular cases, which in turn demands that we investigate the magnitude of the 4D signal and the timing of reservoir changes relative to the possible seismic acquisitions. Here, in addition to an accurate understanding of the physics of the fluid flow and property changes, scale and representation of the recovery mechanisms will be important. This adds to material that has already been published in the literature (for example, Ghazali et al. 2017). This will require searching for appropriate datasets within the sponsorship group. It is anticipated that several case studies will be established to emphasise the development made with this understanding.

The following points are to be incorporated:

• Acoustic properties of EOR fluids
• Reservoir fluid distribution modelling, properties, scale and geological variation:
– Compositional versus black oil, fluid properties in a reservoir undergoing production and recovery
– Gas saturation, distribution and migration; fine-scale geology from outcrops; pressure sensitivity effects
– Thin interbedded reservoir areas with stratigraphic complexity – shale/sand, laterally extensive cemented sands
– Transition zones, geological gradations
• Secondary and tertiary recovery processes – injection/displacement: gas, WAG, steam, SAGD, air, nitrogen, CO2, miscible flooding, polymer floods, CO2/LPG injection, low salinity flood, heavy oil.
• CO2 fluid-rock interactions within carbonate rocks, impact on the 4D response
• Changes to our modelling codes to accommodate the above developments (this will feed into our simulation models and sim2seis)