Study on Reorientation Mechanism of Refracturing in Ordos Basin—A Case Study:Chang 6 Formation, Yanchang Group, Triassic System in Wangyao Section of Ansai Oil Field

摘要

The essence of fracture reorientation of refracturing is rock failures. Therefore, the study on reorientation mechanism should be started from studying rock failures, taking the influences of objective and subjective factors such as reservoir microfracture distribution, geostress field, rock mechanics characteristics, physical properties and the methods of wellcompletion, injection/production, hydraulic fracturing technology upon fracture reorientation into account. As a result, the conditions that rock failures occur were analyzed, thus the theory criteria for fracture reorientation was given, namely the critical bottomhole pressure required and the conditions which the geostress field and rock mechanics parameters should have. The new fractures are divided into shear fractures and tensile fractures, which correspond respectively to the two kinds of rock basic failure mechanisms-shear failure and tensile failure. The azimuths of these two kinds of fractures are both different from the initial hydraulic fracture (abbr.IHF), and the critical pressure is the function of the geostress field and rock mechanics parameters. Based on the above criteria, utilizing the refracturing data of Chang 6 formation, the feasibility of fracture reorientation for Wangyao section was investigated. And three typical wells were analyzed in details, which show the refracture characteristics we predict accord with that of microseismic monitoring or the curve of the treating pressure. It indicates that even if the geostress field keeps unchanged, fracture reorientations can be achieved by other ways. In the certain range of parameters for reservoir physical properties, if a proper fracturing technology, such as TSO (I.e. Tip Screen Out) with temporarily blocking treatment (abbr.TBT), can be applied so as to create the treating conditions that meet the demand of the above criteria, the refracturing can produce new fractures in Wangyao. And for refracturing treatments, with the shot holes along the IHF azimuth blocked effectively, multi-fractures and new fractures can be achieved by oriented perforations.