The seventh member of the Yanchang Formation, Upper Triassic black shale deposited in the deep and semideep lacustrine environment, is located in the southeast area of the Ordos Basin. It is 1200–1800 m in buried depth and 30–60 m in thickness. As the main target of lacustrine shale gas in this region, the seventh member of the Yanchang Formation usually uses fracturing technology as the major stimulation treatment. Slick-water fracturing with a high-pumping rate has been successful early on, but the low reservoir pressure of this formation resulted in a large volume of fracturing fluid invasion. As a consequence, the shale-gas well production has been postponed owing to the low rate of fracturing-fluid flowback and a long cleanup period. Liquid-[Formula: see text] fracturing has caused almost no damage to the formation, and the volumetric expansion of [Formula: see text] after gasification will increase formation energy and improve the fluid cleanup rate. I have developed a liquid-CO2/slick-water hybrid fracturing technology that combined the advantages of liquid-[Formula: see text] and high-pumping-rate slick-water fracturing. Sixteen vertical wells and one horizontal well have been tested in the Ganquan Area, Shaanxi Province, China. As a result, the fluid cleanup rate improved 30% in comparison with regular slick-water fracturing. Moreover, the fracturing-fluid cleanup period of vertical wells decreased from 45 to 25 days. A field test indicated that because more liquid [Formula: see text] was used (i.e., the volume ratio of liquid [Formula: see text] to slick water increases), the performance of reservoir energizing and flowback result improved.
Geological features of Mesozoic lacustrine shale gas in south of Ordos Basin, NW China
