The Onnagawa Formation is a Miocene biosiliceous shale and is the main source rock in the western Tohoku region of Japan. Acid stimulation was tested at a well drilled in the source rock in 2012. The result shows significant improvement of oil production from 1.5kl/d to 50kl/d. A possible phenomenon having caused the exceptional improvement is considered to be dissolution of dolomite veins. It is, therefore, a key issue for future well operation in the Onnagawa Formation to characterize the dolomite veins in relation with estimation of appropriate acid volume and post-acidizing fracture permeability.

This paper focuses on the scale problem of the vein size, and has the potential to increase the accuracy of vein characterization. The dolomite veins were studied in both core and image log over a12 meter interval at a well one kilometer away from the tested well. Total numbers of veins interpreted in the core and the image log are 1181 and 117, respectively. Comparison of the two data sets suggests that many small veins shorter than 10 centimeters are out of resolution in the image log. In order to investigate effects of the small veins on the reservoir properties, dolomite veins in the core were separated into two groups: Group-1 comprises veins shorter than 10 centimeters; Group-2veins are greater than 10 centimeters. Subsequently, post-acidizing fracture intensity, fracture porosity and fracture permeability were examined in the two groups by assuming all the dolomite veins became open fractures.

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