AVO Application in a Carbonate Offshore Oil Field, U.A.E.

Abstract

Abstract Amplitude Variation with Offset (AVO) is a well known technique as a direct hydro-carbon indicator in clastic gas fields. Carbonate reservoirs usually have a more rigid rock framework making them more difficult to see the changes in elastic parameters based on differing types of fluid saturation. An AVO Pilot Study has been conducted in 2007 to investigate the ability to detect fluid fronts in a producing Lower Cretaceous carbonate oil field, using pre-stack seismic data acquired in 2001. Assessing the position of injected water is a critical factor for future in-fill well location optimization and provides additional support for reservoir fluid movement prediction when coupled with reservoir surveillance and simulation data. The study was conducted in a small pilot area that includes a recently drilled well, where necessary data were available for a modeling. The modeling study was performed utilizing log data to assess the degree of seismic change due to fluid substitution based on Biot-Gassmann equation and synthetics have been generated to compare with actual seismic response. In addition to the conventional AVO technique, other techniques have also been tested such as Elastic Impedance and simultaneous AVO inversion followed by Lambda Rho / Mu Rho(LMR) transforms. The results of the pilot study were validated with the known well fluid status in the pilot area. Mapping of the different water-oil-gas fluid response using the LMR technique shows a reasonably good agreement with currently known well saturations. It is, however, difficult to discriminate reservoir sub-layer water-cut percentages due to data resolution limitations. Generally, the results of the analysis indicate potential to identify hydrocarbon to water fluid contents, with the above constraints. Introduction One of the largest Ocean Bottom Cable 3D Seismic data volumes was acquired in a Middle East carbonate offshore oil field in 2001 and 2002. Structural and property modeling work has been conducted for better understanding of the reservoir. The field is a very gently dipping (2–3 degree) east-west elongated domal structure with an extensional fault system developed at crest. There is an original gas cap at several of the multi-reservoir levels in the crestal part of the field. In the reservoirs of interest, the field has a peripheral injection development scheme, complimented with crestal gas injection to support reservoir pressure. Water is being injected below expected OOWC (Original Oil-Water Contact) around the structure. In addition, a historical water dump flood from an overburden acquifer existed for 11 years (1972–1982) between the current crestal gas injectors and the peripheral water injectors. Producer well spacing is about 1–1.5km but the distance between peripheral water injectors and producers varies from 2–8 kilometers. Therefore, one of the most important factors for future infill well plannning is to capture the fluid distribution trend in the reservoir. AVO techniques have been developed as direct hydro-carbon indicaters for clastic gas reservoir environments. Some successful examples of AVO application to carbonate fields exist for chalk reservoirs and gas carbonate reserovirs (Ex. Li et al., 2003), however, it remains a challenge for most of "stiff" high velocity carbonate reservoirs where the fluid effect is small. In this paper, we show the results of conventional AVO and other techniques such as Elastic Impedance and Simultaneous Inversion followed by ???-µ?? (lambda-rho mu-rho) techniques, which are evaluated with available log and seismic data in a pilot study area of interest (AOI) plus some laboratory core measument results. Approximately 36 km2 of pre-stack time migrated data were used in the analysis, utilizing seismic data that was reprocessed primaly for seismic anisotropy analysis. The AOI includes one key well (Well A), where cross-dipole sonic, a multi-azimuth walkaway vertical seismic profile (VSP) and conventional log data were available. In addition, cross-dipole and conventional log data was acquired in a recent well (Well B) located near the flooded downdip flank of the field which could also be utilized to confirm fluid substitution results, although it is located at outside of AOI.