Most commercial oil sands exhibit shale-resistivity ratios (ratio of normal R_(shn) to observed R_(sho) of less than approximately 1.6 in adjacent shales, and can be reached without an expensive string of protection pipe. On the other hand, some experts claim that 'no commercial production is found when the shale resistivity ratio reaches and/or exceeds 3.5. This statement, however, needs further investigation. Such wells often are highly productive initially and are characterized by extremely fast pressure depletion. Based on extensive compaction studies of rocks, the authors argue that the latter is due to plastic deformation (irreversible compaction) in undercompacted overpressured rocks with increasing effective stress soon after production is initiated (or during well testing). Thus, well tests could be quite misleading, and many erroneously condemned overpressured reservoirs should be re-examined, re-evaluated, and strategies be developed to recover the oil and gas from these stress-sensitive reservoirs. (4 figures, 15 references)
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