In time lapse (4D) seismic studies, the impact of effective stress on the dry rock frame is difficult to constrain. The relationship between rock moduli and effective stress is dependent on rock properties, initial effective stress and the change in effective stress with time. A workflow is outlined to: 1) constrain laboratory and analogue data where significant variability is observed through use of well data; 2) provide initial effective stress and a time sensitive effective stress taking in to account the effects of pore pressure-stress coupling, where the stress state is not extensional. A time lapse feasibility case study in the Taranaki Basin, New Zealand is shown to demonstrate the sensitivity of these parameters, and indicate how uncertainties in bulk moduli and effective stress can then be quantified in the amplitude domain through modelling.
Introduction
Time lapse (4D) seismic describes the detection of changes in a reservoir through use of seismic data. In order for seismic data to have a time lapse response, variation in the elastic properties of the reservoir must be significant enough to be detected. Key reservoir factors include state of consolidation, porosity, stress sensitivity of the dry frame modulus, pore fluid phase and compressibility and reactivity of the minerals and fluids. Time lapse seismic feasibility analysis aims to quantitatively assess the potential for seismic detection of the reservoir based on rock physics principals. While not discussed further, equally important in producing a viable time lapse response are seismic attributes.
A number of authors have demonstrated the benefits of using time lapse seismic to highlight a number of reservoir phenomena, including water flood, bubble point dissolution and carbon sequestration. Such case studies are often associated with porosity reduction in compacted reservoirs or high porosity unconsolidated reservoirs. This study focuses on the impact of stress sensitivity on the dry rock frame moduli. In contrast to many published examples, feasibility is considered marginal, as porosities are moderate, the dominant principal stress is not vertical (i.e. not a continental margin) and stress-strain calibration data is not available.
