THE EVOLUTION AND MECHANISM OF EARTHQUAKE SEQUENCES INDUCED BY WATER INJECTION IN JIAO 7 OIL WELL OF SHENGLI OIL FIELD, SHANDONG PROVINCE

The earthquake sequences,occurring from December 1985 to January 1986,induced by water injection into Jiao 7 oil well of Shengli oil field,Shandong Province. The earthquakes have undergone a particular evolutional process,the whole sequence can be divided into four sub-sequences,and the sub-sequence transiting gradually from the earthquake swarm type to the main shock-after shock type; the early earthquakes occurring before the largest earthquake showing a tendency of increase in magnitude,the earthquake b-value being higher in the early period than in the late period,the fractal dimension,D.,of the time composition of earthquakes being lower in the early period than the in late period. The lg(△t)-lg(t) of the major earthquakes being more disorderly in the early period,and showing linear dependence in the late period. The pore pressure is larger than the critical pore pressure in the rock after water injection and therefore inducing premature release of the initial stress existing in the rock of shallow crust,but the osmosis of the water causes transform of the rock pore pressure from highly inhomogeneous distribution into relatively homogeneous distribution with time. The evolution of the earthquake sequences induced by water injection in Jiao 7 oil well is related to this kind of physical process.