Time-lapse (4D) seismic in onshore Abu Dhabi is extremely challenging. Velocity changes due to fluid displacement are very small: time-lapse P-wave velocity changes after 3 years of CO2 injection are estimated −2% around injection wells. 4D noises are problematic too. Near surface heterogeneities hereafter investigated represent the most dramatic source of non-repeatable noise.
To investigate the behavior of non-repeatable noises seismic modeling has been performed. Surface and VSP seismic were both generated using a 2D Finite-Difference Time-Domain (FDTD) method. Expected 4D effects were modeled through simulation results before and after injection. P- and S-wave velocity, and density model within the reservoir, overburden, and near surface are derived from reservoir simulation models, cores, well log data, and uphole surveys. Time-lapse perturbations are added to near surface intervals to take into account possible variations.
Surface seismic gathers show predominant elastic scattering, hence near surface non-repeatable noises overwhelm time-lapse (4D) changes. On the other hand, VSP gathers exhibit clearer reservoir reflections and time-lapse. After reverse time migration 4D processing, VSP data indicate the main potential to detect 4D changes. Source and receiver positioning errors are also included, as such errors usually create strong bias. This study confirms that source position repeatability is critical and therefore a permanent seismic monitoring system would be required for successful detection in onshore Abu Dhabi.