1.5.1 Thesis outline
The remainder of this thesis is divided into seven chapters as follows:
Chapter 2 uses rock-physics modelling to understand the sensitivity of outcrop sandstones and a recovered reservoir sandstone to reservoir changes in pressure and fluid saturation. Zero-offset 4D amplitude and time-shifts are modelled for independent effects of pore pressure increase, pore pressure decrease, gas saturation and water saturation changes.
Chapter 3 implements an interpretation based method that calibrates maps of 4D amplitudes from multiple monitor 4D seismic data (full-offset post stack) and engineering data to quantify the in-situ pressure, water and gas saturation sensitivity of the reservoir, along with its uncertainties. The pressure or saturation sensitivity is expressed in relative amplitude terms as a function of the change in pressure (in MPa) or the change in saturation (in fraction). The method is applied to four producing North Sea clastic fields with different geological environment, production mechanisms, rock properties and simple to complex 4D seismic responses. A different more volumetric 4D seismic attribute, time-shifts is also used. The analysis is done using intra-reservoir time-shifts extracted from the computed 4D time-shift volumes of multiple monitors (full-offset post stack seismic data). Time-shift sensitivity is quantified as the change in two-way-time relative to the reservoir thickness as a function of the change in pressure (in MPa) or the change in saturation (in fraction). This is applied to a geomechanically active high-pressure-high-temperature (HPHT) clastic field and a normally-pressured clastic field.
Chapter 4 addresses the issue of time scales when relating the acquired 4D seismic data to the reservoir engineering data. It examines the spatiotemporal relationship between physical processes in the reservoir such as pressure, water and gas saturation as controlled by field/well operations and the time sequence of shooting of real field North Sea acquisitions from a PRM and towed streamer survey. It suggests that intra-survey reservoir fluctuations (that is, production fluctuations that occur during the shooting of
monitor surveys) might prevent accurate quantitative measurement of the reservoir change using post-stack 4D seismic data.
Chapter 5 assesses the impact of the above intra-survey reservoir fluctuations in the quantification of pressure and saturation changes from 4D seismic data. It reveals that the intra-survey reservoir fluctuations create a complicated spatio-temporal imprint on the post-stack data, and adds to the lack of accuracy in the measurement of reservoir changes using offset stacks from the 4D seismic data, especially pressure changes. It is then recommended that the shot timestamps of the acquisition is used to sort the seismic data immediately after pre-stack migration and before any stacking. The seismic data should also be shot quickly in a consistent pattern to optimise time and fold coverage. Chapter 6 presents an engineering-constrained, data-driven, map-based inversion scheme to estimate pressure and water saturation changes from 4D seismic data in clastic reservoirs. It is a deterministic least squares inversion that quantifies and uses the uncertainty in the 4D seismic data, as well as the engineering data. Multiple monitor 4D seismic data are combined to estimate the spatially-varying reservoir sensitivity for each sub-offset stack (near, mid and far), which are then used to invert for pressure and water saturation changes at a specific monitor time. The fluid flow simulation model is included to provide soft dynamic constraints. To account for uncertainties with using a single model and lessons learnt from Chapter 4 and 5, multiple realisations of geologically-consistent and history-matched reservoir simulation models are generated, and embedded within the inversion. The added benefit is that the scheme automatically presents the reservoir model that best honours the 4D seismic signals.
Chapter 7 applies the inversion scheme discussed in Chapter 6 on the Heidrun field which has five repeated 4D seismic monitor data. Realistic synthetic data is modelled based on the Heidrun field’s reservoir properties, and by extracting the observed 4D seismic noise and adding it to the synthetic data obtained by simulator-to-seismic modelling.
Chapter 8 summarizes the conclusions of the work from previous chapters, and in addition, recommendations are put forward for further future research.
1.5.2 Publications
Parts of this thesis have been independently presented in the following publications:
Omofoma, V. and MacBeth, C. (2015). Intra-survey Pressure Variations- Implications for 4D Seismic Interpretation. Paper presented at the 77th EAGE Conference and Exhibition-Workshops, Madrid, Spain, 1 – 4 June, 2015.
--- Chapter 4
Omofoma, V. and MacBeth, C. (2016). Quantification of Reservoir Pressure-
sensitivity Using Multiple Monitor 4D Seismic Data. Paper presented at the 78th
EAGE Conference and Exhibition, Vienna, Austria, 30 May – 2 June, 2016. --- Chapter 3
Omofoma, V., MacBeth, C. and Amini, H. (2017). Intra-survey Reservoir Fluctuations – Implications for quantitative 4D seismic analysis. Geophysical Prospecting (accepted with corrections underway, May 2017)
2 Chapter 2
A rock-physics understanding for the quantification
of pressure and saturation sensitivity in sandstones
using zero-offset 4D amplitudes and time-shifts
Key to quantitative interpretation of 4D seismic data for the separation of pressure and saturation effects is accurate knowledge of their individual contributions to the 4D seismic signatures. In this chapter I use known rock-physics equations to model zero- offset 4D amplitudes and intra-reservoir time-shift responses in a producing black-oil reservoir. Of particular concern is the sensitivity of normally-pressured sandstones to independent changes caused by pore pressure increase, pore pressure decrease, and water and gas saturation increase. The sensitivity to the reservoir dynamic changes are quantified using the modelled 4D amplitudes and time-shifts, which are found to be complementary attributes. A generalised understanding of the dependence of the time- shift and amplitude sensitivity to lithology variations is also provided by analysing three sandstone samples which ranged from high porosity (18-27%) unconsolidated sandstones to low porosity (5%) cemented sandstones. The modelling also provides insights into the specific nature of sensitivity at various magnitudes of dynamic changes, and the imbalance of sensitivity between pressure and saturation effects.