Microfracturing mechanisms and techniques in unconsolidated sandstone formations
1 State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum-Beijing, Beijing 102249, China 2 College of Petroleum Engineering, China University of Petroleum-Beijing, Beijing 102249, China 3 College of Artificial Intelligence, China University of Petroleum-Beijing, Beijing 102249, China
Unconsolidated sandstone formations display specific mechanical deformation behavior in their stimulation or injection, which is distinct from that of tight formations upon hydraulic fracturing. In this regard, the mechanical mechanisms must be
revealed to interpret the macro and micro-scale deformation, therefore providing theoretical guidance for field practices. Given the characteristics of an unconsolidated sandstone formation such as soft texture, great plasticity, and high permeability, this article brings in soil mechanics to introduce the formation stimulation and injection augmentation technique known as microfracturing, then delves into the dilation and microfracture initiation mechanisms during microfracturing in such a type of formation. Based on the microfracturing mechanisms, assessment methods of microfracturing efficiency, including laboratory experiments, analytical solutions, and numerical approaches, are discussed, and summarized on their necessity as well as advantages and disadvantages. Based on the mechanical mechanisms and the calculation methods, the applications of microfracturing in the stimulation of ultra-heavy oil formation, plugging-removal in wellbores and artificial interference of the geostress field, are discussed with respect to the detailed approaches and successful case studies. Finally, the automatic evaluation technique of microfracturing efficiency is proposed using ultra-heavy oil formation stimulation as an example. It is found that microfracturing is a complex mechanism that combines the theories and techniques of poroelasticity, plasticity, and fracture mechanics. It demonstrates various functionalities in different engineering scenarios, but generally shows a series of features of the process from dilation to microfracture initiation. It is believed that the analysis of its stimulation efficiency in realistic practices must consider multiscale effects and use multiple numerical approaches. Microfracturing in an ultra-heavy oil formation is mainly characterized by the microfracture generation surrounding the wellbores and the porosity dilation in between the dual wells, which lead to an enhanced effect of thermal convection and subsequently a fast interwell hydraulic communication. Microfracturing in the plug removal of a wellbore results in a back flush of the sand grains, interwell porosity dilation and flow recovery through microfractures, which would extend the efficiency of the removal attempt. Moreover, microfracturing can also provide a means of artificial geostress field interference for the near wellbore, interwell, and inter-stage or inter-cluster regions, which favorites the predesigned fracturing goal. Future investigation shall be dedicated to improving the automatic implementation of the microfracturing technique, and to further developing an intelligent decision making and control approach when applying the technique, eventually reducing the need for man-made work efforts.