The hydraulic fracturing method (HF) has become widespread to solve various mining problems in underground conditions. An impact on the rock mass is performed by supplying of pressurized working fluid in the interval sealed between two packers. The problem of poor isolation of the selected interval is particularly relevant for open-hole wells with excessive cross-section variability along the borehole axis, e.g. in-seam degassing boreholes in coal. In such conditions, inflatable packers made of reinforced rubber sleeves may be ineffective. To solve this problem, it is proposed to use unremovable bridges consisting of a solidified material. The unremovable bridges are created by polymerization of the two-component mixture of polyurethane or organomineral resins with high adhesion strength to enclosing rocks.
In this paper, the adhesive interaction that occurs between polymer bridges and the borehole wall was numerically investigated. This interaction allows bridges to withstand a certain pressure in the sealed interval and do not move along the borehole. A special layer with Cohesive Zone Model (CZM) approach was used to simulate its destruction. The aim of the study was to determine the peculiarities of the stress distribution in the vicinity of polymer bridges and its effect on the failure behavior of the layer. It was found that at low values of critical shear stresses, the length of the affected part of the layer in the case of bridges with a higher modulus of elasticity is higher than in the case of a low modulus. We also determined how the stresses on the borehole wall vary at different degrees of layer destruction described by the critical shear strength. It is established that with an increase in the degree of layer destruction, the values of axial and circumferential stresses in the vicinity of the sealed interval are decreasing.
A high-quality sealing of open-hole wells is an important part of the interval hydraulic fracturing method performed for coal degassing. The strong variability of the cross section along well's axis (the maximum and minimum value can differ by 3 times) does not provide the opportunity to use standard inflatable packers, since their sleeves reinforced with cord are not able to completely isolate the part of the borehole. An increase of the packers’ length to several meters complicates the underground activity and does not guarantee reliable sealing of the fracturing interval, where the pressure of the working fluid can reach several hundred bars.