Technology of Soak After Hydraulic Fracturing to Increase Production in Tight Carbonate Gas Reservoirs

The lower Paleozoic Majiagou Formation reservoir, located in the southern area of the Ordos Basin, is a typical tight carbonate gas reservoir with an average burial depth of about 3000 m, a formation pressure coefficient of 0.8, a porosity of 3.16%, and a permeability of 0.13 mD. The gas production was generally low after hydraulic fracturing at early-stage development, and the proportion of wells with industrial gas capacity is less than 25%. During the fluid discharge phase, the flowback rate was generally low (less than 30%), the bottom hole flow pressure was small (less than 7 MPa), and the flowback pressure difference was large (more than 15 MPa). Because the initial water saturation of tight reservoirs is usually less than the irreducible water saturation, the retention of a large amount of undischarged fracturing fluid in the formation causes the increase of water saturation in the near-well area, resulting in damage to gas permeability. The ‘water trapping effect’ is serious in the near-well area, and no liquid was produced during the later stage of drainage, making it impossible to obtain the true production capacity of the reservoir. In order to improve the effectiveness of fracturing, a soak test after hydraulic fracturing was carried out in the tight carbonate gas reservoir. After 50 days of soaking and pressure diffusion, the fracturing fluid in the near-wellbore area was dispersed to the deep formation by capillary force. As the diffusion area increased, the water saturation in the near-wellbore area was reduced, and the gas permeability was effectively restored. The daily gas production increased from 4,678 m3 to 8,424 m3 after soaking under pressure. The successful trial of this technology has a significant reference for improving the effectiveness of hydraulic fracturing and understanding the true production capacity of tight gas reservoirs .