Differential Cased Hole Sonic Anisotropy for Evaluation of Propped Fracture Geometry in Western Siberia, Russia

Abstract

Abstract In a waterflooded reservoir, hydrocarbon recovery optimization is impacted by well spacing and hydraulic fracture extent. An excessive fracture length may lead to an earlier than desired increase in water cut. Uncertainty in propped fracture dimension is related to the distribution of stresses and elastic properties, as well as fluid leak off. Those factors have strong implication on proppant distribution, especially when larger size proppant are used. Although the latter could lead to more conductive fractures, they could also bridge at the wellbore, impeding both lateral and vertical extent. Differential cased hole sonic anisotropy (DCHSA) combines the use of cross-dipole shear sonic analysis carried out before and after hydraulic fracturing and adequately supported by other logs (ultra-sonic cement evaluation) to infer the change in anisotropy; the latter anisotropy includes the creation of a propped width. While the methodology has been used in carbonates, very few cases of its application in turbidites have been documented. Differential cased hole sonic anisotropy (DCHSA) can be determined from analysing cross-dipole shear sonic data acquired before and after hydraulic fracturing. From analysis of other log data, such as the ultra-sonic cement evaluation tool, it is possible to infer change in anisotropy, which can indicate the creation of a propped fracture. In this paper, the results from a DCHSA and hydraulic fracture campaign in a Western Siberian oil field are presented. In this field, the common well development practice consists of multiple, large job fracturing treatments, ranging from 100 to 300 tons, using large sized proppant. It is generally believed that a high stress barrier, above and below reservoir interval, will facilitate the bridging of proppants, and will cause an artificial barrier. Planar 3D models that incorporate stress detail, moduli width calculation, 2D flow proppant tracking and bridging criteria are available to the industry, but do need extensive set of accurate parameters. These models will be greatly enhanced by calibration from direct measurements of the fracture geometry. DCHSA has proved instrumental in detecting fracture induced anisotropy. Fracture heights were modeled with the expected treatment pressures. The outcome proved a valuable methodology that decreased the operational and production risks that are associated with hydraulic fracturing using large proppant sizes.