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TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractAsymmetrical hydraulic fractures have been observed in the field to reduce the efficiency of such stimulation treatments. Efforts have been made to investigate asymmetrical hydraulic fractures by imaging their initiation using microseismic imaging technology developed in-house. Images of the fracture initiation were generated using the located sources of microseismic events. Analyses of these images indicate that the initiation of an asymmetrical hydraulic fracture is a complex process. While asymmetrical fractures were generated under asymmetrical stress fields along the length direction, selective propagation along the height direction was also observed, as well as nonalignment and reorientation of the fractures near the borehole. Scarce distribution of microseismic events near the borehole may indicate poor connection between the far-field fracture and the borehole. Dense distribution of microseismic events near the borehole probably results from the coupled effect of fluid leakoff and stress-induced tensile fracturing. Sample PreparationThree samples of different materials: cement, Tennessee sandstone (Tenn. S.S.), and Jackfork sandstone (Jackfork S.S.), were prepared and hydraulically fractured. They were prepared as follows: Cement Block. The cement block was made following Ong's recomendations: 16 API Class H cement, silicon flour, water, defoamer, and dispersant were mixed thoroughly for 30 minutes according to the weight ratio of 100: 39: 43.2: 0.1: 0.1. The slurry was then poured into a greased steel mold. An electrical vibrator was used to shake the slurry for 30 minutes to make the slurry fill all the space of the mold homogeneously, driving out the air bubbles. After setting for two days at room temperature in a 100% humidity environment under a steel cover, the cement block was unmolded and immersed into a water tank to mature for 28 days to avoid thermally induced microcracks. 2 0 J J J J 2 0 0 J J J