The Challenges of Predicting Commingled Production of Rich and Dry Gas Reservoirs of the Saih Rawl Field, Oman

摘要

The Saih Rawl field (SR) is the biggest gas field in Oman,containing condensate rich gas in the Barik sandstonereservoir and dry gas in the deeper Miqrat sandstone reservoir.Production to date was mostly from the Barik reservoir, atwhich most wells are completed, resulting in the reservoirpressure dropped to a current average of 260 to 470 bar formthe initial pressure of 513 bar. Furthermore the layered natureof the reservoirs and the varying hydraulic fracturing of thedifferent units, production from the various units differedsignificantly resulting in significant vertical differentialdepletion of the reservoir. The field’s CGR (Condensate GasRatio) of the Barik reservoir has dropped from 480m3/MMm3 to 300 m3/MMm3 as a consequence of liquid dropout resulting the Barik reservoir pressure dropping to belowthe dew point pressure, and the increasing dry gas productionfrom Miqrat reservoir through commingled production fromsome of the wells.Well performance in the Saih Rawl field is affected by theflow impairment caused by the condensate dropout, the qualityof the placed hydraulic fractures in terms of their efficiencyand geometry, and completion type, single reservoir orcommingled in the two reservoirs of Barik and Miqrat. Thismakes predicting well performance a verycomplex undertaking.The limited worldwide experience in developing tight gascondnesatereservoirs possed a genuine challenge for theinitial development planning of this field in the mid-1990s. Inparticular the prediction of the long termproduction behaviour.Production of gas/condensate wells in tight reservoirs withliquid drop-out is still one of the most challenging subjects in reservoir engineering today. The Saih Rawl field provides oneof the first sets of comprehensive data on this phenomenon.The paper reviews actual reservoir performance in the SaihRawl field over the first 40 months of its production life. Forexample, 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 crossflowof the dry gas to the rich gas as was indicated bycondensate production and the well capacity. Dynamicmodels, calibrated by historic production data, are used toassess these effects and to predict futureproduction performance.