Dynamic characterisation of non-reservoir rocks from time-lapse seismic data

Abstract

Production-related activities in oil and gas fields lead to deformation of the reservoir and its surroundings, where this deformation is pronounced the fields are coined ‘geomechanically active’ fields. The stress and strain changes cause seismic velocity changes, and potentially rock failure that in turn, would affect the field development plan. For more than a decade, time-lapse seismic time-shifts have been effectively used to understand and monitor geomechanical processes outside depleted reservoirs, suggesting that the nature of these time-shifts is linked to reservoir compaction. Nonetheless, there remain multiple challenges confronted at different geological settings, production stages and reservoir conditions, implying that the accuracy of the 4D seismic interpretation of geomechanics events largely varies from one field case to another. In particular, high-pressure and high-temperature fields can exhibit a complex 4D seismic signal and deformational behaviour, questioning the ability to establish an effective link between geomechanics and time-lapse seismic than can be quantitatively assessed under these extreme conditions. This thesis offers a novel approach that pursues to highlight issues and identify potential pitfalls with current state-of-the-art methods for analysing 4D seismic and geomechanical modeling. It provides technical improvements and recommendations for the application of these methods to enable quantitative improvement in the analysis of geomechanics, following an interdisciplinary approach that integrates geological characterisation and 4D seismic observations of non-reservoir rocks. The case study corresponds to the high-pressure and high-temperature Shearwater Field, located in the Central North Sea. Therefore, it is intended to find answers to unsolved questions related to this field that help increase the predictability of the geomechanical model and the interpretation of the time-lapse response. The study has been conducted in three stages. The first stage deals with the geological characterisation and rock physics modeling of the rocks immediately above and below the reservoir. The second stage involves the construction of a comprehensive geomechanical model, following a system-based approach rather than an isolated reservoir-centred approach. In the third stage, time-lapse attributes are used in combination with the geomechanical model and repeated well logs to explain the nature of the time-shift observations as a result of reservoir depletion. Following this approach, this research has contributed to: (i) Introduce a method to reduce 4D noise by time-shift iteration, (ii) Propose an interval method to derive R factors across the reservoir, (iii) Establish a new practice for improving consistency between 4D seismic and geomechanics by including the aquifer pore pressure explicitly in the geomechanical modeling, (iv) Introduce a new repeated well log analysis methodology to calculate lithology-dependent R factors from macro and micromechanical modeling and (v) Develop a new methodology to assess the effect of lithology fluctuations and internal multiples in the development of underburden time-shifts.