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Integrating Surface and Subsurface Models for Commingled Production of Different Reservoir Fluids
Paper Abstract 5/21/2005

A deep Gas Condensate field in Oman, containing condensate rich gas in reservoir A (shallower) and dry gas in reservoir B (deeper). Production to date was mostly from reservoir A, at which all wells are completed, resulting in the reservoir pressure dropped by 60% in some specific locations from the initial pressure. Furthermore the layered nature of the reservoirs and the varying hydraulic fracturing of the different units, production from the various units differed significantly resulting in significant vertical differential depletion of the reservoir. The field’s CGR (Condensate Gas Ratio) has dropped by 50% as a consequence of liquid drop out resulting the reservoir A pressure dropping to below the dew point pressure, and the increasing dry gas production from reservoir B through commingled production from some of the wells.

The main challenges in the field is predicting the well performance which is affected by the flow impairment caused by the condensate dropout, the quality of the placed hydraulic fractures in terms of their efficiency and geometry, and completion type, single reservoir or commingled in the two reservoirs (A & B). Also, sealing and non sealing nature of faults play a big rule in well performance in long term. This makes predicting well performance a very complex undertaking.

As the field is very large in terms of area (20km x 30km) and due to the huge differential depletion areally and vertically; specific attention was paid to optimise the second phase development, such that the contracted gas volume and heating value, high level of oil gas ratio, optimum operating philosophy, optimum timing for compression, level of compressions, optimum number of wells and their completion and high ultimate recovery are achieved.

Integrating Surface and Subsurface models of gas/condensate wells in tight reservoirs with liquid drop-out is still one of the most challenging subjects in reservoir engineering today, especially when it is used as a tool for improving operating philosophy. This integrated project provides an overview of actual reservoir performance in the field over the first 5 years of its production life and the impacts of surface/subsurface Integration. For example, high well declines have been observed (upto 50% per annum), the effect of fluid mixture of the rich and dry gas, resulting from the commingled production, the potential cross-flow of the dry gas to the rich gas as was indicated by condensate production and the well capacity, Compression size and timing and operating philosophy. Dynamic models, calibrated by historic production data, are used to assess these effects and to predict future production performance.

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