A zero offset VSP survey was acquired over a thick carbonate interval. The objectives of the repeat survey were to assess repeatability of the measurement, robustness of the time lapse VSP processing mainly to account for time lapse noise and quantification of the time lapse signal in the reservoir due replacement of water by gas caused by gas injection that occurred between the baseline and repeat surveys.
A long side of the two VSP's, an array sonic (SDT) and the sonic scanner tools were run. The SDT was run to recover the monopole compresssional and shear waves, while sonic scanner to acquire both the dipole and monopole shear and compressional.
Data processing was focused on isolating the time lapse signal in the reservoir section by comparing the baseline and repeat VSP surveys and sonic logs. Both the baseline and repeat surveys were processed with identical parameters to ensure that the datasets are comparable at every processing step. The processed data shows a good degree of repeatability, and deterministic deconvolution as expected was enough to take care of source wavelet variations between the two surveys.
The time lapse VSP and sonic logs were run in carbonate reservoirs where gas is being injected to assess the effectiveness of recovering more oil as part of Enhanced Oil Recovery (EOR). The current recovery mechanism in the reservoirs is to inject water to enhance pressure support. The gas pilot objective is to increase the recovery factor to recover more oil and enhance productivity.
In the past ten years, time-lapse (4D) seismic has evolved from an academic research topic to a standard way of monitoring reservoir performance. The method is now being used as good reservoir management practice to provide evidence of saturation changes within the reservoir at field scale. 4D provides a new piece of data describing the dynamic behavior of the reservoir fluids between the wells, often limited to small scale monitoring at the borehole scale. Thus, it provides sophisticated techniques of reservoir monitoring and management relying on the integration of geological models, static and dynamic properties of the reservoir rock, and detailed production and pressure field data.
While 4D seismic data has been very successful in monitoring hydrocarbon production in clastic reservoirs, there is still no consensus on its applicability to carbonate fields. The main difficulty is the well-known fact that the acoustic velocities of carbonates are insensitive to saturation and pressure changes, relative to the clastics. Beside the geological processes such as production induced compaction which has large impact on porosity, density and permeability variation during the life of a 4D surveys, the complexity and heterogeneity of carbonate pore geometry and network further aggravate the difficulty of 4D applications. Although the geological characteristics may not change at small time scales but they are linked to fluid flow and distribution in the reservoir.
An Integrated approach of 4D seismic analysis using all historical production data along with open/cased hole logs and simulation models has demonstrated its viability to understand saturation changes in heterogeneous reservoirs. Thus using this approach as reservoir management tool to improve sweep efficiency and enhance field development plans.