Abstract

A field in Oman provides a good example of compaction and subsidence due to a pore-collapsing carbonate formation. The field is comprised of a gas reservoir in the Natih formation and an oil reservoir in the deeper Shuaiba formation. Both reservoirs are formed by high porosity carbonates that can display a highly compressible "pore collapse" response with depletion. Both reservoirs are in production. Gas production from the Natih reservoir started in the early 70's, and continues at present from a dozen wells producing from the center of the field. Oil production from the underlying Shuaiba reservoir began in the late 60's, and the field has been under waterflood since the early 70's. All 450+ Shuaiba producers/injectors are drilled through the compacting Natih reservoir. Compaction damage to these oil wells is a major incentive for understanding the compaction and subsidence in this field. This paper presents the results of a detailed geomechanical study done to predict subsidence and compaction for the field. Core, log and field data are all incorporated to create a calibrated 3D geomechanical model for predicting ongoing field subsidence and estimating compaction potential. Key to this is an understanding of the field geology and diagenetic controls on rock strength and compaction in the Natih reservoir. Core compaction tests show the reservoirs are highly compressible pore collapsing carbonate formations. Although this is highly non-linear compaction behavior, in-situ field compaction measurements show an averaged, more linear response This fact advocates against using a 'pore collapse trend' to predict in-situ compaction. The measured field subsidence in the late 1990's & early 2000's is successfully predicted assuming an average linear material response, provided the variable diagenetic character of the field is honored for the pore-collapsing facies. Extrapolation of the model indicates compaction strains at the end of field life in the order of 5% in the crest of the field and a maximum subsidence of about 2 meters.

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