Analysis of key elements affecting production and technical countermeasures to increase production of CBM fractured single horizontal wells: A case study of Zhengzhuang Block

As the main technology of CBM development in the Qinshui CBM field, single-branch fractured horizontal wells have supported the breakthrough of 2 billion m³/year production of gas field. However, in the process of its large-scale application, there have been a series of problems, such as large difference in production among wells and poor adaptability in local areas, which seriously restrict the further improvement of CBM production in gas fields. On the one hand, although many key elements affecting the production of single-branch fractured horizontal wells have been identified, the core key elements are not clear. On the other hand, the understanding of the change rule of key elements on the plane is generally simple mathematical extrapolation or pure mathematical inversion, which lacks geological constraints and corrections, and has low reliability and resolution. Therefore, it is urgent to clear the core key elements that affect the production, and then establish targeted technical countermeasures on this basis. First of all, based on the objective geological conditions and the understanding of development and production practice in the Zhengzhuang block, the preliminary screening principle of the core key elements affecting the production of single−branch fractured horizontal wells was proposed, and nine elements were selected, including drilling length in pure coal seam, horizontal section inclination, total hydrocarbon, GR, number of fracturing stages, average sand gradient, average displacement gradient, average fluid volume gradient and desorption pressure gradient. Then, grey correlation analysis and single element analysis were used to determine the correlation degree between the nine elements of the preliminary screening and the production of single−branch fractured horizontal wells, and determine the internal mechanism of the elements' influence on the production. Secondly, from the three perspectives of resource base, reserve control and reconstruction, combined with the specific process of a single-branch fractured horizontal well deployment, reconstruction design and operation, nine elements were classified, and targeted technical countermeasures for production enhancement were established. In the stage of well location deployment, extrapolation data based on evaluation wells were used to initially identify the relatively enriched area, and the seismic data were processed by structure-oriented filtering + high−frequency coherence technology to delineate the local area for clearing hidden destructive structures in the area, and then straight/inclined wells were drilled to obtain GR and total hydrocarbons in the coal seam segment to determine the resource base of the well location deployment area. In the stage of well trajectory design and drilling, the waveform clustering technology was used to predict the distribution law of strongly deformed coal in the well location deployment area. The pre−stack depth migration of seismic data was processed to predict the thickness and fluctuation change law of coal seam in the well location deployment area, the well group mode was used to drill in batches, and the near−bit geological guidance technology was combined with the real drilling data to correct the local coal seam understanding in real time, to increase drilling length in pure coal seam, optimize well trajectory, ensure more high−quality fracturing stages, and improve reservoir control. In the reservoir reconstruction stage, GR (Gamma Ray), total hydrocarbon, well trajectory conditions and real drilling data were used to select fracturing stages, and the joint operation techniques of bridge plug setting, shooting and fracturing was used as a new fracturing technology to increase the displacement, sand and fluid volume, increase the reconstruction scale and strengthen the reconstruction, and improve resource utilization. Finally, relevant tests were carried out in the Zhengzhuang block by using the above countermeasures. A well group with 7 wells was deployed in the north of Zhengzhuang. Compared with the early single-branch fractured horizontal wells around the well group, the desorption pressure of the test wells is about 1-1.5 MPa higher than before, and the stable gas production is increased by nearly 5 000-10 000 m³/d. According to field practice tests, the core key elements affecting the production of single-branch fractured horizontal wells and the targeted production technical countermeasures established on this basis can greatly improve the stable gas production and desorption pressure of coalbed methane, which can help improve the production of the Qinshui coalbed methane field and provide important production practice experience and successful cases that can be learned from for peers who adopt the same main development technology.