Hydraulic fracturing effect of CBM vertical well and its impact on productivity: a case study of Yaojie Mining Area

CBM reservoir is a typical fracture-controlled gas reservoir, and the scale of fracturing and fracture-filling effect have decisive control on gas well productivity. In order to reveal the influence mechanism of the hydraulic fracturing effect of CBM vertical wells on the productivity effect of gas wells, taking the Haishiwan Mining Area of Yaojie, Gansu as an example, this paper focuses on reporting the fracturing effect of coal reservoirs under the form of vertical well development and its understanding of the constraints on productivity. the result shows: ① The fracture pressure of CBM well mainly feeds back the characteristics of the wellbore cement sheath rather than the mechanical properties of the coal reservoir. From the fracturing curve of this well, the fracture pressure is not obvious, indicating that the thickness of the cement sheath in the wellbore annulus is moderate, and the cement sheath is relatively easy to rupture during hydraulic fracturing. The energy of the injected fracturing fluid is mainly used to prop up the coal cracks; ② It can be seen from the fracturing curve that the propagation pressure of the hydraulic fractures(HF) in this well is relatively high, indicating that the structure of coal reservoir is relatively fragmented and coal fines are developed. In addition, there are multiple wave morphological features in the fracture propagation stage of the fracturing curve, indicating that multiple secondary fractures are propped up. On the whole, the HF in this well are relatively complex, and it is speculated that branched secondary fractures develop on both sides of the main HF. ③ Serious sand plugging occurred after sand injection in this well. The main reason was that the fracturing fluid in the coal reservoir was lost due to the loss of the fracturing fluid in the near-wellbore area, resulting in the formation of wedges by proppant de-sanding in the HF, which made subsequent proppant injection difficult. It is related to the coal fines of primary fractures and the development of a small amount of tectonic coal fines; ④ The fracturing microseismic monitoring data show that the main HF of the coal reservoir in this well are NE 50°, and the coal rock microseismic events are more active in the northeast direction, indicating that the natural fracture (NF) of the coal reservoir is more developed in this direction; Based on the observation results of fracturing and mineback, this paper proposes a division plan for the effective propped area, the actual fracture area and the rock mass disturbance area of HF. It is believed that the actual half-length of the HF in the coal reservoir of this well is less than 20 m, the half-length of the effectively propped HF is less than 5 m, and The small amount of fracturing fluid injected, the formation of wedges by proppant de-sanding induced by fracturing fluid-loss, and the sand plugging caused by coal fines accumulation are the main factors for the short fractures in the coal reservoir of this well; ⑤ Combined with the law of coalbed methane productivity, it is considered that the gas reservoir management and drainage system in this well is reasonable,However, the effective propped HF in the coal reservoir are too short, and the coal fines migration in the coal reservoir leads to the rapid decline of the conductivity of the near-well HF, which is the key restricting mechanism for the productivity of gas wells.It is recommended to increase the injection volume of fracturing fluid, appropriately increase the flow rate of fracturing fluid, reduce the proppant-sand ratio, and focus on controlling fracturing fluid leak-off and the formation of screenout wedges in the hydraulic fracturing of coal reservoirs in the later stage.The coal seam in this area has a large burial depth and high stress, and the realization of large-scale fracture propped is the key to coal reservoir fracturing. Meanwhile, the reservoir is dry and the fracturing fluid is seriously leak-off, the CO₂ pre-fracturing test can be carried out.