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

Abstract

Abstract The Saih Rawl field (SR) is the biggest gas field in Oman, containing condensate rich gas in the Barik sandstone reservoir and dry gas in the deeper Miqrat sandstone reservoir. Production to date was mostly from the Barik reservoir, at which most wells are completed, resulting in the reservoir pressure dropped to a current average of 260 to 470 bar form the initial pressure of 513 bar. 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) of the Barik reservoir has dropped from 480 m3/MMm3 to 300 m3/MMm3 as a consequence of liquid drop out resulting the Barik reservoir pressure dropping to below the dew point pressure, and the increasing dry gas production from Miqrat reservoir through commingled production from some of the wells. Well performance in the Saih Rawl field 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 of Barik and Miqrat. This makes predicting well performance a very complex undertaking. The limited worldwide experience in developing tight gas-condnesate reservoirs possed a genuine challenge for the initial development planning of this field in the mid-1990s. In particular the prediction of the long term production behaviour. Production of gas/condensate wells in tight reservoirs with liquid drop-out is still one of the most challenging subjects in reservoir engineering today. The Saih Rawl field provides one of the first sets of comprehensive data on this phenomenon. The paper reviews actual reservoir performance in the Saih Rawl field over the first 40 months of its production life. 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. Dynamic models, calibrated by historic production data, are used to assess these effects and to predict future production performance. Introduction The Saih Rawl field (SR), discovered in 1991, is the biggest gas field in Oman with total reserves (1.1.2002) of over 15 Tscf. The field contains condensate rich gas in the Barik sandstone reservoir and dry gas in the Miqrat sandstone reservoir The datum depths of these reservoirs are 4370 and 4850 mss respectively. The field is a layer-cake type with 8 and 6 gas bearing geological units in Barik and Miqrat respectively, separated by laterally extensive pressure sealing heteroliths. The Barik reservoir has a 220m gas column with average porosity of 7.6%. The Miqrat reservoir has a 100m gas column with an average porosity of 6.8%. Permeabilities of both reservoirs are in the range of 0.02 - 10 mD. All the production wells to date have been hydraulically fractured to increase flow rates (See Figure A). The initial reservoir pressure was 513 bar for the Barik and 581 bar for the Miqrat reservoir. The dew point pressure is 425 bar for the Barik rich gas. The Saih Rawl field is produced to meet a gas demand that is varying throughout the year, while ensuring a production capacity to meet a contractual maximum annual daily load (MADL) demand, this production philosophy also has the objective to maximize early condensate production without affecting the fields ultimate gas condensate recovery.