Causes for low CBM production of vertical wells and efficient development technology in northern Zhengzhuang Block in Qinshui Basin

In order to realize the efficient development of deep CBM in the north of Zhengzhuang block in Qinshui Basin, firstly, the geological conditions and development data of coal reservoir are deeply analyzed, and the development data of shallow CBM are compared. Secondly, based on the evaluation of gas content, the difficulty of coalbed methane recovery is evaluated by comprehensive evaluation of reservoir permeability, reservoir-desorption pressure difference, coal structure and in-situ stress state-Finally, the reasons for low production of CBM wells in the northern Zhengzhuang Block are clarified, and the adaptive technology for efficient development is put forward. The results show that the gas content in northern Zhengzhuang Block is generally higher than 20 m³/t. While the permeability is extremely low, with the fracture development index below 140. And the difference between reservoir pressure and desorption pressure(reservoir-desorption pressure difference) of CBM formation is greater than 6 MPa, resulting in small desorption range of CBM wells. In most areas, the ratio of the thickness of broken coal to that of whole coal seam is above 0.7, leading to shorter hydraulic fracturing fractures. The vertical stress, a geostatic-field-type stress, is greater than horizontal stress and the hydraulic fracturing is prone to form vertical fractures with shorter fracture extension. So the coupling of the extremely low permeability, relatively high reservoir-desorption pressure difference, high broken coal ratio and geostatic-field-type stress, all these factors result in poor production in northern Zhengzhuang Block. Artificial borehole is established by tree type horizontal well to realize high efficient stimulation of coal seam, and the coordinated depressurization is realized and the negative effects are overcome by shortening the seepage distance of CBM and water. At the same time, the artificial borehole is used instead of the relative shorter hydraulic fractures. The production of the tree type horizontal well is over 10 000 m³ per day. But the disadvantages of tree type horizontal well are risks such as unfavorable drainage and depressurization, easy hole collapse and high cost. Multi-stage hydraulic fracturing of L shaped horizontal well can overcome the disadvantages perfectly of tree type horizontal well. Compared with vertical well, the cost of L shaped horizontal well increases by 2 times while the production increases nearly by 100 times. The production of L-shaped horizontal well is as high as that of tree type horizontal well, while the cost of the L shaped horizontal well is only 40% of tree type horizontal well. In conclution, the L shaped horizontal well realize the efficient development of Northern Zhengzhuang Block.