Time-lapse joint facies and impedance inversion applied for production and CO₂ applications

Time-lapse inversion of seismic data has become a mainstay of O&G reservoir surveillance and management in the last 20-30 years and is certain to play a significant role in CO2 storage programs in coming years. At present, most time-lapse inversion approaches work with seismic difference data, and operate in a cascading approach to inference where baseline decisions and models are frozen throughout the forward litany of monitor inversions. Conventional time-lapse inversion does not leverage the significant advantages of joint elastic and facies inference in single-vintage inversions, nor the possibility of simultaneous inversion of multiple vintages to apply the statistical power of monitor data to the baseline inferences. It is well known that facies and their permeability contrasts are the key to mapping fluid pathways, and also that certain kinds of fluid movement can lead to strong seismic responses that may ‘illuminate the geology’ of subsurface regions where low elastic contrasts may have led to poor baseline interpretation. This widely accepted wisdom is a strong encouragement towards trying to frame the multi-vintage inversion problem in a way that the full statistical power of seismic can be exerted on facies and elastic inversion.In this work, extending (Gunning and Sams, 2018), we describe a joint elastic+facies, Bayesian approach to multivintage seismic inversion, modelling the fluid movement by latent, discrete lithofluid variables, and seismic data by a temporally consistent forward elastic model dependent on the joint elastic and facies model state. The depth-based voxellised model framework enables both reflection and kinematic effects in the seismic to be modelled. Representation of the fluid state by discrete variables gives modelling facility for spatial ordering of facies, density ordering, and temporally sensible transition rules.We present a case study on real datasets using a CO2-EOR scenario from the Bell Creek Field, onshore US.