Analysis of time-lapse logs to determine shale-related R factor

Time-lapse changes in the overburden can be related to pore pressure variations in the underlying reservoir. The geomechanical changes observed are independent of fluid flow given the impermeable nature of the caprock, however, such deformation has the potential of causing a significant impact on the 4D signal. A physical model widely used to couple geomechanics and time-lapse seismic signatures, relates the fractional change in velocity and the vertical strain of reservoir and surrounding rocks via a constant factor R. This study presents improvements in understanding and predictability of the overburden R factors for future seismic interpretation. Here, we compare two different methods to investigate the complexity of the R factors using the well log data of two Jurassic shales from Central North Sea. A time-lapse analysis on repeated well logs is carried out to reveal the velocity response to porosity and pressure change that results into dilation in the overburden. The results are compared to estimates from a theoretical model that describes the unloading process in shales.