Calculation model for borehole collapse volume in horizontal openhole in formation with multiple weak planes

In view of borehole collapse in testing of horizontal wells, a calculation model for borehole collapse volume of horizontal open-hole in multiple-weak-plane formation was set up to analyze the factors affecting collapse volume. It was compared with the single-weak-plane model by field application. After analyzing the influences of fracture occurrence, well-bore azimuth, in-situ stress anisotropy, number of weak planes and fluid density on well-bore collapse volume, this study reveals: When the fracture dip is small, the volume of well-bore collapse is big and the well-bore stability is poor; When the fracture dip is big, the collapse volume varies greatly with the change of well-bore azimuth and fracture trend; The maximum collapse volume appears with different well-bore azimuth if fracture trend is different; The greater the in-situ stress difference is, the greater the well-bore collapse volume will be; The collapse volume decreases with the increase of borehole fluid density, and is more sensitive to the change of fluid density when fluid density is near the equivalent density of pore pressure. Field application results show that, compared with the single-weak-plane model, the multiple-weak- plane model gets big collapse volume in most cases so it can assess the collapse risk more reliably.