4D (or Time-lapse) seismic study is based on repeating 2D or 3D seismic surveys over the same area with the same acquisition parameters, in different times and measuring the difference in seismic data in terms of both amplitude and reflection time. This technique has been increasingly utilized to monitor the fluid flow during the hydrocarbon production, or during the enhanced oil recovery to assess unswept target zones through miscible or immiscible flooding. An overview of reservoir properties such as porosity, pore pressure, temperature, and water/oil/gas saturation changes as a result of depletion or injection could be investigated through 4D seismic analyses. Time lapse studies are usually subject of investigation on reflection seismic data. However, this technique could be applied to any other seismic techniques such as Vertical Seismic Profiling (VSP). In 4D seismic studies, subtle changes in the reservoir properties could be studied via change in seismic wave properties, if good quality seismic data is available. A feasibility study is a prerequisite in 4D seismic acquisition, due to high cost of repeating a seismic survey (especially in a vast area with 3D coverage, such as oil and gas fields in Iran).
Gassmann rock physics model is widely in use for fluid replacement to predict seismic properties in porous rocks such as sandstones with high porosity and permeability. Carbonate rocks usually have a higher density and elastic modulus than sandstones, and include less porosity and permeability. Also the pore shapes and their connections in carbonates are more complex and different from sandstones (penny shapes rather than spherical). Since the Gassmann model does not count for the pore shape type and geometry, other rock physics models such as Kuster-Toksoz model is required to be utilized to study fluid replacement effect on seismic wave parameters in carbonate rocks. Kuster-Toksoz model is also considering the effect of mineralogy as well as pore shape and geometry.
In this research, as a 4D feasibility study, the change in seismic wave parameters due to hydrocarbon production (fluid replacement) in a well of carbonate oil reservoir located in South-West of Iran has been investigated. Based on inverse Kuster-Toksoz rock physics model, percentage of different porosity types (spherical, disk, and needles) in the considered well is modeled. As a result, however, the spherical porosity in the well was dominant (45% of total pores), other porosities included a considerable share (55% of total pores). Considering the pore shape and geometry and the proportional porosity percentages via Kuster-Toksoz rock physics model, the average change in P and S velocity, bulk modulus and density due to hydrocarbon production (replacing oil by gas) was about 380 m/s, 63 m/s, 2.6 Gpa and 78 kg/m3 respectively; while using Gassmann rock physics model which did not include the pore shape and geometry and the proportional porosity percentages, the primary wave velocity and the bulk modulus were 163 m/s, 1.35 Gpa respectively. This suggests that the field could be subject of 4D seismic study for fluid flow detection, if high repeatability of the seismic survey could be achieved and seismic data could be considered as high quality data