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Title: Altering Reservoir Wettability to Improve Production from Single Wells

Many carbonate reservoirs are naturally fractured and typically produce less than 10% original oil in place during primary recovery. Spontaneous imbibition has proven an important mechanism for oil recovery from fractured reservoirs, which are usually weak waterflood candidates. In some situations, chemical stimulation can promote imbibition of water to alter the reservoir wettability toward water-wetness such that oil is produced at an economic rate from the rock matrix into fractures. In this project, cores and fluids from five reservoirs were used in laboratory tests: the San Andres formation (Fuhrman Masho and Eagle Creek fields) in the Permian Basin of Texas and New Mexico; and the Interlake, Stony Mountain, and Red River formations from the Cedar Creek Anticline in Montana and South Dakota. Solutions of nonionic, anionic, and amphoteric surfactants with formation water were used to promote waterwetness. Some Fuhrman Masho cores soaked in surfactant solution had improved oil recovery up to 38%. Most Eagle Creek cores did not respond to any of the tested surfactants. Some Cedar Creek anticline cores had good response to two anionic surfactants (CD 128 and A246L). The results indicate that cores with higher permeability responded better to the surfactants. The increased recovery is mainly ascribedmore » to increased water-wetness. It is suspected that rock mineralogy is also an important factor. The laboratory work generated three field tests of the surfactant soak process in the West Fuhrman Masho San Andres Unit. The flawlessly designed tests included mechanical well clean out, installation of new pumps, and daily well tests before and after the treatments. Treatments were designed using artificial intelligence (AI) correlations developed from 23 previous surfactant soak treatments. The treatments were conducted during the last quarter of 2006. One of the wells produced a marginal volume of incremental oil through October. It is interesting to note that the field tests were conducted in an area of the field that has not met production expectations. The dataset on the 23 Phosphoria well surfactant soaks was updated. An analysis of the oil decline curves indicted that 4.5 lb of chemical produced a barrel of incremental oil. The AI analysis supports the adage 'good wells are the best candidates.' The generally better performance of surfactant in the high permeability core laboratory tests supports this observation. AI correlations were developed to predict the response to water-frac stimulations in a tight San Andres reservoir. The correlations maybe useful in the design of Cedar Creek Anticline surfactant soak treatments planned for next year. Nuclear Magnetic Resonance scans of dolomite cores to measure porosity and saturation during the high temperature laboratory work were acquired. The scans could not be correlated with physical measurement using either conventional or AI methods.« less
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Technical Report
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Correlations Company
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United States