Issue 49

O. Y. Smetannikov et alii, Frattura ed Integrità Strutturale, 49 (2019) 140-155; DOI: 10.3221/IGF-ESIS.49.16

The maximum of incremental stresses   is located on the walls of the well. Then, the stresses in the rock formation rapidly decrease in the radial direction until they are stabilized at a distance of 2-3 m from the wellbore, namely, at 20 30 c r    , where  is the current radius and c r is the radius of the wellbore (Fig. 3).

  in the isotropic field of horizontal stresses ( H h

   ) at the width of

Figure 3 : Radial distribution of incremental stresses

 , 2 — in the direction of h  .

20 w  mm. 1 — in the direction of H

opening of the primary HF

 we can conclude that in the isotropic field

Based on the characteristic features of the incremental stress distribution 

of horizontal stresses H h    it is more probable that re-fracturing will occur in the direction perpendicular to the primary HF, i.e., the secondary HF in the isotropic field of horizontal stresses is orthogonal to the primary fracture. In this case, the pressure increase can be observed at the moment of fracture initiation. Thus, at the pressure of the primary fracture of 80.0 MPa and the width of the primary fracture opening near the wellbore equal to 20 mm the secondary fracture pressure is 92.2 MPa. Essentially, the situation changes in the case when the minimum stress h  is about 70–95% of the maximum stress 47 H   MPa. Irrespective of the appearance of incremental stresses   , in all examined cases the most likely direction of the secondary HF is the direction parallel to the primary fracture path. As in the first case, the pressure begins to grow at the time of initiation of the secondary fracture and the primary fracture opening increases. The degree of pressure growth depends on the anisotropy of the stress field. Thus, the width of the primary fracture opening 20 w  mm at 47 H h     MPa is responsible for a maximal increase of the secondary fracture pressure by a factor of 1.15, whereas at 47 H   MPa, 32.9 h   MPa by a factor of 1.37. The deformation of porous liquid-saturated rocks is a complicated process, during which the deformation of the mineral matrix of rocks (under the action of varying effective stresses and fluid pressure gradients) occurs simultaneously with fluid filtration in pores (under the action of the fluid pressure gradients and bulk deformation of the skeleton). In the following, a drop of oil pressure due to extraction of fluid from the well with the primary HF causes a change in the stress state of the rock formation in the vicinity of the well. Therefore, an adequate treatment of the processes accompanied the deformation of a porous liquid-saturated medium requires that the system of differential equations describing the deformation of the rock skeleton and fluid filtration should be considered simultaneously. The solution of this coupled problem was found numerically using the ANSYS software package. To simulate the process of liquid removal from the wellbore, we performed coupled calculations for SSS and liquid filtration at the prescribed bottom-hole pressure. The finite element mesh was built using the CPT213 element, which makes it possible to implement numerically the filtration consolidation model [6]. By virtue of the problem symmetry we performed

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