Identification of High Residual Gas Saturation Zone and Evaluation of its Impact on the Development of the Central-C Gas Condensate Field- Oman

Abstract

The active government gas exploration programme in Oman has recently added another sizeable gas-condensate discovery that was drilled and tested by Central-C-1. Destined to unlock the first CA sandstone gas-condensate field, the well results indicated a very prolific reservoir. In the process of data gathering, a number of challenging aspects were identified and assessed. Residual gas saturation is the most interesting and potentially high impacting aspect. This paper addresses variations on the gas saturation profile as calculated from the petrophysical logs, the identification from the special core analysis measurement of high residual gas saturation rocks, and all relevant findings inferred from the production testing and the static pressure gradient measurement. Despite cautioned applicability of the SCAL measurement results to reservoir conditions, a potential evidence for part of the CA reservoir accommodating high residual gas saturation was recorded. Number of scenarios were modelled to assess the impact of high residual gas saturation in relation with other reservoir uncertainties normally associated with exploration reserve bookings. These scenarios ultimately studied the impact of the residual gas saturation uncertainty on the development economics of the field. This paper explains why major economic measures, such as gas and the condensate recovery efficiencies, well placements and aquifer influx were seen sensitive to this uncertainty. For a comprehensive evaluation of its impact, a relationship between rock properties distribution and residual gas saturation was invisaged as a comprehensive evaluation method. In a deeper concept this translates into a relationship between the interpreted original depositional model and the adopted charge model. This interesting relationship was evaluated in the simulator using a special saturation-porosity dependence that was primarily based on the SCAL measurements. This paper details the analytical and the numerical approach used to evaluate the impact of the uncertainty in residual gas saturation. It highlights how this uncertainty features within the appraisal data gathering targets and within the surveillance strategy of this field. Introduction Central-C-1 well was drilled in 2000 that resulted in discovering a sizable gas-condensate field. The well was successfully production tested. However, high residual gas saturation was interpreted in some of the penetrated reservoir zones. This posed a challenge to model particularly with good production test results. Reservoir simulation was used to assess impact of such high values on recovery efficiencies.