investigation ON fracture initiation modes OF HORIZONTAL WELLS based on laboratory hydraulic fracturing test
WANG Lei1，YANG Chunhe1，2，GUO Yintong1，HENG Shuai1，WEI Yuanlong2，HOU Zhenkun2，LI Dan3
(1. State Key Laboratory of Geomechanics and Geotechnical Engineering，Institute of Rock and Soil Mechanics，Chinese Academy of Sciences，Wuhan，Hubei 430071，China；2. State Key Laboratory for Coal Mine Disaster Dynamics and Control，Chongqing University，Chongqing 400041，China；3. College of Urban Construction，Wuhan University of Science and Technology，Wuhan，Hubei 430065，China)
In order to investigate the initiation modes of hydro-fractures in horizontal wells，Laboratory hydraulic fracturing test was carried out using outcrop shelly limestone to simulate horizontal well hydraulic fracturing. The phenomena of fracture initiation and propagation around the horizontal wellbore were described and analysed by adding tracer to the fracturing fluid，analysing pumping curve，arranging AE monitoring system and splitting sample after test. Conclusions are as follows：(1) The initiation modes of hydro-fractures could be divided into three types：one transverse fracture(mode I)，transverse multi-fractures(Mode II)，transverse-longitudinal crossed fractures(Mode III). (2) Each hydro-fracture initiation mode has its certain pumping curves. Mode I had a low breakdown pressure followed by a sharp drop. Mode IIhad a relatively high breakdown pressure，then the pressure went up and fluctuated dramatically. Mode III also had a high breakdown pressure，but the pressure fluctuated slightly. (3) Among the specimens，the three initiation modes occurred randomly and the breakdown pressures were dispersed. (4) The AE activity could reflect the initiation and propagation of hydro-fractures and reached its peak value at the fracture initiation moment. (5) The area in which AE events concentrated was in agreement with the actual location of the hydro-fractures. AE locating results could be used to preliminary judge the distribution of hydro-fractures. (6) One transverse hydro-fracture was more likely to be formed at relatively high pumping rate，while the fracture geometry tended to be complex at relatively low pumping rate. Pumping rate is a key factor to determine the initiation mode. (7) It may be feasible to conjecture the initiation and propagation of the hydro-fractures using site fracturing pumping curves.
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2015, Vol. 34 
