Vol.:(0123456789) 1 3 Acta Geophysica (2019) 67:141–148 https://doi.org/10.1007/s11600-019-00246-w RESEARCH ARTICLE - ANTHROPOGENIC HAZARD Seismic anisotropy of a fractured rock during CO 2 injection: a feasibility study Shib Sankar Ganguli 1,2  · Prakash Kumar 1  · V. P. Dimri 1 Received: 18 June 2018 / Accepted: 11 January 2019 / Published online: 2 February 2019 © Institute of Geophysics, Polish Academy of Sciences & Polish Academy of Sciences 2019 Abstract Fluid substitution plays the key role in reservoir characterization, leading to enhance understanding of the infuence of fu- ids on seismic parameters. In general, fuid substitution tool assumes that the Earth is as an isotropic medium, which may not represent the practical feld situation. Nevertheless, anisotropic fuid substitution provides important insights into the processes that control the anisotropic seismic response of a fractured rock when subjected to CO 2 injection for enhanced oil recovery and its geological sequestration. Here, we examine the infuence of fuid substitution in a porous yet fractured res- ervoir for quantitative interpretation of seismic data. This investigation involves anisotropic Gassmann’s equation and linear slip theory for fuid substitution in a transversely isotropic media with a horizontal axis of symmetry (HTI). We present a synthetic case by conceptualizing a double-layered half-space model with upper layer as shale and bottom layer as HTI sand- stone, representing an Indian mature reservoir. The efects of variation in background porosity and fracture weaknesses on anisotropic (Thomsen’s) parameters, acoustic parameters including amplitude variation with angle have also been discussed. We observe that brine and oil sands to be associated with the highest elastic moduli, while CO 2 sands exhibit contrasting trend. It is noteworthy that CO 2 is more sensitive to fracture weakness when compared to the other reservoir fuids such as hydrocarbons and brines, as P-wave moduli (as much as 37.1%) and velocity (as high as 12.2%) reduces signifcantly with the increase in fracture weakness. Further, Gassmann’s assumption is validated as we noticed unchanged values in shear-wave moduli and shear-wave splitting parameter (γ) for various fuid types. Keywords Anisotropy · Seismic, CO 2 -EOR and sequestration · Reservoir characterization Introduction In the recent years, reservoir characterization for fractured porous media has captivated increasing interest as this aid in enhanced reservoir assessment with less risk. However, one of the fundamental issues in reservoir characterization is the ability to predict the behaviour of fuid on seismic char- acteristics (Batzle and Wang 1992; Gurevich 2003; Huang et al. 2015; Ganguli et al. 2016a, b; Ganguli 2017). This demands knowledge of fuid substitution on seismic wave propagation that has numerous applications in exploration and production geophysics including the estimation of in situ hydrocarbon saturation in time-lapse seismic, assessment of the CO 2 -enhanced oil recovery (EOR) and sequestra- tion potential of a reservoir, etc. In general, for an isotropic reservoir conditions, the efect of fuid substitution can be realized using the classic equations by Gassmann (1951). These equations are relatively simple, yet most commonly used fuid substitution tool in the industry as well as in aca- demia since these connects the elastic moduli of a rock to its porosity, fuid and rock matrix properties. Nevertheless, these simple expressions may not be valid for anisotropic medium in terms of fractured reservoir as the fractures sig- nifcantly afect fuid fow characteristics within the reservoir by stimulating permeability anisotropy. To date, applica- tion of anisotropic fuid substitution to envisage the seismic properties is still in its infancy. Most of the studies on fuid substitution consider the earth to be an isotropic medium, which may not represent geologically realistic situation since majority of the reservoirs possess oriented lithologies, com- plex stress pattern, fractures, hence, seismically anisotropic * Shib Sankar Ganguli ganguli.ism@gmail.com 1 CSIR- National Geophysical Research Institute (NGRI), Uppal Road, Hyderabad 500 007, India 2 Department of Earth Sciences, Indian Institute of Technology Kanpur, Kanpur, U.P 208016, India