Compositional Reservoir Simulation of Adhi Field with Sharp Changes In Oil Gravity with Production
Authors: Siddiqui Fareed , Khan M. Noman , Rehman Zaghum Ur. , Bandal Malvinder 
Published: November 2011

Adhi field is located in Punjab province of Pakistan in Potwar region and was discovered in 1978. Adhi structure consists of main northeast-southwest (NE-SW) trending popup structure with a relief of ~600m and extensions separated by faults in the NE and southern flanks. So far 10 wells have successfully penetrated Tobra & Khewra reservoirs with most of the wells drilled along the fold axis. The PVT analyses based on the initial wells show the reservoir to contain rich retrograde condensate fluid with an initial condensate to gas ratio of ~150 bbls/MMscfd. Production from the Tobra and Khewra reservoirs started in 1990 with the installation of 25 MMscfd LPG processing plant. The plant capacity was doubled in 2006. The field presents complex geology because of the presence of extensive faulting and high relief structure (600-700m). The field’s production performance is complicated by the variation in the PVT behavior of the produced fluids with production and the presence of possible oil rim. The field is estimated to contain ~1120 Bcf of wet gas and more than 160 MMstb of Liquid Hydrocarbons (Condensate and Oil). The reservoir pressure dropped below the dewpoint in 1992-93 and the reservoirs have been producing below dewpoint for the last 18 years. The most challenging part in planning the further enhancement of production and maximization of recoveries from the reservoirs is the prediction of the composition of production stream in the future. Both the reservoirs are experiencing significant change in fluid properties for the last 5-6 years. The produced condensate used to be transparent, which has gradually turned black with API gravity decreasing in almost half of the wells from the initial 55° to 35°.

A three dimensional compositional reservoir simulation study was initiated to have more reliable prediction of the complex compositional behavior of field production. The study has completed the fluid characterization with matching of lab results for saturation pressure, Constant Composition Expansion (CCE) & Constant Volume Depletion (CVD). The tuned Equation of State (EOS) has been used in compositional reservoir simulation for history matching and predictions. However, for matching the high density fluid breakthrough, rigorous tuning of equation of state (EOS) was required. The sensitivity of critical condensate saturations Scc was also being attempted to achieve history match at well level.

Reasonable HM of reservoir pressure has been achieved; with change in oil API and CGR & Tubing Head Pressure (THP) with assigned uncertainty range of 10-20% due to the complex nature of reservoir behavior. This paper discusses in depth, the challenges of compositional reservoir simulation, describing formulation of EOS & its subsequent matching, establishing the reservoir fluid gradient & gravity segregation with their impact on HM. The future development strategies are also discussed with the stand point of reservoir management to increase recoveries from such challenging reservoirs.