Application of ultra-high pressure hydraulic slotting technology in medium hardness and low permeability coal seam

In order to improve the gas pre-drainage efficiency of medium hard coal seam with high gas and low permeability, the applicable conditions, advantages and disadvantages of hydraulic piercing, hydraulic fracturing and hydraulic cutting seam anti-permeability technology were discussed. Based on the principle of ultra-high pressure hydraulic slotting technology, a kind of ultra-high pressure hydraulic slotting device for through-layer drilling is developed, it is mainly composed of diamond hydraulic slotting bit, shallow spiral drill pipe, ultra-high pressure rotary joint, ultra-high pressure clean water pump, high-low pressure converter, ultra-high pressure rubber pipe, etc. The water pressure reaches 60−100 MPa, which can realize the integration of drilling and cutting, and is simple and convenient to use. The device was used to carry out field tests in the pre-drainage boreholes of 11-2 coal seam through the floor roadway of 1361(1) haulage gateway in Dingji coal mine. The coal seam gas pressure was 1.43 MPa, the gas content was 8.05 m3/t, and the gas permeability coefficient was 0.013 m2/ (MPa2·d), the coal seam firmness coefficient is 0.79; 1361(1) transportation roadway floor No.11−No.15 drilling area unit length 227 meters, using high-pressure hydraulic slotting anti-permeability measures, 1361(1) transportation roadway floor No.6—No.10 drilling area unit length 213 meters, anti-permeability measures of low-pressure water punching in coal mine. The results show that the average single-knife slitting time of ultra-high pressure hydraulic slitting drilling is 10.7 min, the single-knife coal output is 0.31 t, the equivalent slitting radius is 1.38 m, the slitting density of the coal hole section is 1 knife/m, and the average cutting rate per hole is The average gas drainage concentration of ultra-high pressure hydraulic slotted holes is 56.97%, which is 2.37 times that of low-pressure punching; The time to reach the standard is about 23 days, which is 74.4% and 54.9% shorter than that of ordinary drilling and hydraulic punching technology respectively. Compared with ordinary drilling and hydraulic punching technology, ultra-high pressure hydraulic slotting technology is more ideal for gas drainage in low permeability medium-hard coal seams

Enhanced Gas Production from Class II Gas Hydrate Reservoirs by the Multistage Fractured Horizontal Well

In the two test productions that have been conducted in the hydrate reservoir test development zone in the South China Sea, the gas production capacity of single wells is low and the exploitation difficulty with the cost is too high for commercial demand economically. The low permeability of the hydrate-bearing layer (HBL) acts as the major barrier for pressure propagation during depressurization. Hydraulic fracturing by the combined depressurization is considered a promising hydrate production enhancement technology that can effectively improve the seepage state in the reservoir. In this study, to evaluate the effectiveness of the development methods association with fracturing, we established an idealized Class II hydrate reservoir and studied it using a multi-stage fractured horizontal well to assist in depressurization extraction. In order to evaluate the production enhancement effect of this method, we compared the gas production results of four methods, including single vertical well, vertical fractured well, horizontal well, and multistage fractured horizontal well through numerical simulation. In order to investigate the influence of key fracture parameters on the production enhancement effect, a sensitivity analysis of the production effect of Class II hydrate reservoirs with different fracture spacing, number of fractures, fracture conductivity, and fracture length was conducted, and the results were analyzed in terms of gas production and water production behavior curves as well as physical field evolution over time. The simulation results show that the multi-stage fractured horizontal wells have the best production increase in the comparison of well types. In the analysis of fracture parameters, it can be found that the selection of proper fracture spacing and dimensionless fracture conductivity can lead to a significant increase in gas production