So far in China, unconventionally low-permeability oil sandstone resource has become the first contributor of proved reserve, and is occupying the most important section of oil production in near future. The low-permeability reservoirs have been primarily developed by waterflood, and therefore display a series of new and specific features. Among them, dynamic fracture, an important factor influencing waterflood performance, was studied in this paper.
This paper is a 5-step study of low-permeability oil sandstone reservoirs (average permeability between 0.3mD and 10mD) in Ordos basin, which is famous for its richness in low-permeability sandstone resources. First, dynamic fractures were identified by production data, injection profile, tracer monitoring and well test data. Secondly, mechanical experiment and discrete numerical simulation were integrated to analyze the forming mechanism of dynamic fractures. Third, remaining oil saturation observation of densely-spaced inspection well cores and unstructured-grid simulations were used in combination to obtain a 3-dimensional sweep scenario influenced by both dynamic fracture and matrix. Based on study above, the impact of dynamic fracture on waterflood was then analyzed. Finally, how to curb and use dynamic fractures to maximize recovery according to their different forming mechanisms were suggested.
The dynamic fracture is defined as fracture that reopen, propagate or close influenced by injection pressure. It can be identified by simultaneous features of drastic watercut increasing, sharp spike in GR curve of injection profile, fast breakthrough of tracer and high permeability anomaly by well test. Core sample mechanical experiment and theory calculation support dynamic fractures propagating on the basis of pre-existence fractures as the predominant forming mechanism. Fresh core observation of newly drilled inspection wells after 20y's waterflood indicates a higher percentage of vertical sweep by up to 30% in the direction of dynamic fracture than that perpendicular to the direction of dynamic fracture. Also dynamic fracture simulations show fast growth of dynamic fractures apparently narrows horizontal sweep and causes earlier drastic watercut increasing by years. A fast growing dynamic fracture deteriorates horizontal sweep, while dynamic fractures of a certain height increase vertical sweep. Moderate injection and stimulated fractures with enough height are conducive to enhance sweep and therefore recovery. If dynamic fractures connecting injectors and producers are unavoidable, a line injection pattern is then proposed.
This paper conducted a systematic study on dynamic fracture, which is a dominant feature and geological challenge for waterflood of unconventional tight oil sandstone reservoirs. It offers a guidance for future waterflood development of similar reservoirs.