This paper consideres how the initial hydraulic fractures and the oil and gas wells production/injection impact on initial stress field, and forms the stress field prediction software. Applied the software in Xinzhan Oilfield, 5 layers of 4 wells which might be reorient were selected from 19 layers of 7 wells. In the treatment of these wells, tiltmeter was used to monitor how the fractures propagation. After refracturing, the production histories of the wells were matched. The results show that the reorientation occurred in the refracturing of experiment wells, and so the validation of the software was verified.
Reorientation refracturing is that some measures are taken in refracturing process to make the refracturing reorientation. At present, more and more fields and wells need to be refractured. Refracturing has two ways: 1.fractures spreading along original fracture azimuth, 2. fractures spreading along a new azimuth different from original azimuth, reorientation refracturing belongs to the latter. As we know, fractures always spread along the azimuth vertical to the minimum principal stress, so the stress field after the first hydraulic fracturing determines the fractures breakdown and spreading. The research on change trend of stress field could direct the refracturing, predict fractures azimuth and decide the best refracturing opportunity.1–2
No matter what refracturing reorientation induced by principal stress reorientation or that induced by shear stress change making fractures spread along shear plane, the original stress field change is the basic dominating factor.
The existence of the first hydraulic fractures and the production of wells, the anisotropy of reservoir, the fractures and injection or production of neighboring wells cause the change of original stress field, and may make the stress reorientation. In these factors, the existence of the first hydraulic fracture and the production of wells are the most important ones.3 The physical models (Fig. 1) are used to describe stress induced by the existence of the first hydraulic fracturing:4 a plate with a 2 a long linear fracture, the fracture has the same thickness with the plate, and the tension on the fracture is - P. The principal stress and shear stress in x, y, z direction induced by the first hydraulic fractures can be described by the following formulas: