Variation rules of fracture initiation pressure and fracture starting point of hydraulic fracture in radial well

• Fracture initiation model with seepage-stress-damage coupling is developed and coded.
• Considering local stress redistribution, hydraulic fracturing model is established.
• Influence of factors on initiation pressure and fracture starting point was analyzed.
• The region susceptible to fracture initiation of radial well was studied.

Radial well technology in combination with hydraulic fracturing technology has gained encouraging achievements as a new method of increasing production in oil fields. Compared with conventional perforation fracturing, radial well fracturing has an obvious advantage in breaking through polluted borehole areas, and it is superior to horizontal well fracturing because of the shorter construction period, less consumption of fracturing fluid and lower damage to the reservoir. Currently, in China, the study on the position of fracture starting point and fracture propagation of radial well is still at the preliminary stage and the fracture initiation pressure and position of fracture starting point remain unclear. Consequently, it's difficult to design radial well completion parameters (length of radial well, diameter of radial well and fracturing truck units) precisely and implement the technology more efficiently. Based on fluid-solid coupling effect and the maximum tensile-stress criterion, this paper follows the concept of dynamic analysis and analyzes the influence rule of the length, diameter and azimuth of radial well, horizontal in-situ stress and natural fracture on fracture initiation pressure and fracture starting point under the stress of strike-slip fault by using ABAQUS to simulate and study local stress accumulation situation caused by drilling vertical well section, radial well section and fracturing section through finite element method. The result shows that initiation pressure and distance between well and fracture starting point increases as the length, diameter and azimuth of radial well section rise. Azimuth is most influenced, followed by length, and lastly diameter of radial well section. When the horizontal in-situ stress ratio (σH:σh) is decreased from 1.9 to 1.1, if the azimuth is 0°, the initiation pressure increases by 41.35%; if the azimuth is 90°, the initiation pressure declines 0.8%. However, the positions of these two fracture starting point remain unchanged. The permeability increases by four orders of magnitudes and the fracture initiation pressure goes up 34.5%, with no influence on the fracture starting point. When there existed natural fractures in the reservoir, the intersection between fracture section and radial well section firstly shows fracturing. Besides, the fracture initiation pressure and the hydrostatic fluid column pressure in vertical wellbore are equivalent. Reduced length and diameter of radial well section as well as reservoir permeability and properly increased azimuth of radial well are conducive to fracture at the toe end of the radial well section. On the contrary, fracture at shaft linings of vertical well section is easily occurred. The research result can be used to predict the direction of fracture propagation to some extent and is favorable for designing parameters of radial well completion and fracturing operation.