Technology of Fracturing Complex Lithologic Gas Reservoirs in High Temperature Deep Wells

The Shahejie Formation of the No.5 structure in Jidong Oilfield is a complex lithologic gas reservoir, the lithology of which is mainly basalt and volcaniclastic rocks. The reservoir, which has poor physical properties (porosity in a range of 0.04-0.4 mD) is buried in depths between 4,781.6 m and 5,200 m, with formation temperature between 170℃ and 180℃. The reservoir has previously undergone fracturing operation in the φ139.7 mm casing, with flowrate of 2 m³/min and pressure of 70 MPa. The pressure after stopping pumping was 61 MPa, and the operation was failed. From laboratory experiments (total analysis, Xray diffraction analysis, acid dissolution, ground stress and natural fracture identification) it was determined that natural fracture is developed in the reservoir and the fillings of the fractures are mainly acid-soluble minerals. Based on these analyses a technique of using pre-pad acid fracturing followed by fracturing operation was presented to improve the production of the reservoir. Using a high temperature organoboron low residue guar gum fracturing fluid (functioning at temperature up to 170℃), a thickened acid, combined with other measures such as optimized perforation method, increased compressive strengths of the surface equipment and plugging natural fractures with selfdegrading filter loss reducing materials etc., the fracturing operation was successfully performed in deep wells penetrating the fractured volcanic rock reservoirs in the No. 5 structures in Nanpu area. The well with failed fracturing operation previously was fractured again with flowrate of 5 m³/min and the maximum pressure of 74 MPa. Total amount of sand added was 60 m³, and the fracturing operation was successfully performed. The technology described above was used on 3 wells in Nanpu area.The maximum amount of sand added in a single well was 125 m³, and the maximum casing pressure was 85 MPa. The well NP5-A, after fracturing operation, was flowing with a φ6.35 mm choke, and the daily gas production output was 10.7×10⁴ m³.