Advanced Strategies for Optimizing Recoveries from Retrograde Gas Reservoir Development

Abstract The development of near-critical rich condensate reservoirs is often challenged with the requirement to optimize recovery from both the gas and associated condensates. This is further complicated for green fields as there is little to no information on aquifer size and response time, sand development, connected volumes, fault behavior and calibration for expected in-place volumes. Optimal facility sizing, offtake rates and well productivity are some of the other uncertainties. This work presents a classic example of the innovative workflow adopted to model field development scenarios, test condensate drop-out in an initially undersaturated near-critical retrograde condensate Niger Delta reservoir using available howbeit limited data from a static model, Modular Dynamics Tester, available bottom hole, BH and surface recombined, RSS fluid samples. Two PVT samples were available for this evaluation; one DST BHS Sample and an RSS sample which was acquired during the same DST. The PVT samples were quality checked against available information from the surface and downhole parameters which were recorded during the test. The BHS sample was adjudged to be representative of the reservoir fluid as the fluid was obtained in a monophasic state with the bottom hole flowing pressure above the dew point realized from the laboratory analysis of the fluid. Moreso, the resulting fluid reproduced the CGR recorded during the surface test when streamed to the test separator conditions. Next a Peng Robinson Corrected equation of state, EoS was tuned to the available laboratory experiments, CCE and CVD. The resulting 7 component EoS model was then exported for use in the Intersect Simulation Model. Relative permeabilities were generated using SHF for each rock classes and the average porosities of the facies in the static model and robust Niger delta correlations. Next, the model was quality checked to ensure consistency and robustness and initialized. The model was then calibrated using available MDT pressure vs depth data at a well location to ensure consistency with available pressure trend. A stability run was also carried out by running the model for a 10-year period with no production and the pressure and fluid saturation was checked against the preproduction information. Next, the model was run under different scenarios with the available well completed to develop the resources from this reservoir. Multiple offtake rate sensitivities, fault transmissibility and aquifer size sensitivities were carried out. For each case, condensate drop out was clearly observed in the production profile and in the ternary plot as the pressures dropped below the dew point. The results show that increasing the offtake rates with a base aquifer (aquifer captured in the static model grid cells), has an impact of increasing the UR of gas and condensate respectively. At lower offtake rates, water breakthrough is delayed. Whereas, with the higher rates, water breaks through quicker, there is significant acceleration of up to 5 years and gains in UR for both gas and condensate in the 70 mmscfd vs 20 mmscfd. This is most likely due to the concept of outrunning the aquifer. Furthermore, the impact of a completely sealing fault was tested by implementing a zero transmissibility across the main intra-reservoir fault and this resulted in ~2 MMstb and 2 Bscf reduction in condensate and gas recoveries respectively. The impact of increasing the aquifer size resulted in better pressure maintenance and significant acceleration of condensate due to plateau extension by about 8 months. However, as soon as the water arrived at the well, the water rate increased very rapidly and consequently caused the well to quit prematurely. Following the depletion by the completed well, three additional notional drainage points were tested to develop the remaining gas and associated condensates which were identified using net gas map evaluated pre- and post-production by the development well. An incremental of ~ 5 MMstb and 33 Bscf of condensate and gas were realized respectively.