Scaling and blockage in deep coalbed methane wells: Mechanisms and countermeasures

Authors

• ZHANG Lei, China United Coalbed Methane National Engineering Research Center Co. Ltd., Beijing 100095, China; PetroChina Coalbed Methane Company Limited, Beijing 100028, ChinaFollow
• CHEN Ming, PetroChina Coalbed Methane Company Limited, Beijing 100028, China
• ZHEN Huaibin, China United Coalbed Methane National Engineering Research Center Co. Ltd., Beijing 100095, China; PetroChina Coalbed Methane Company Limited, Beijing 100028, China
• ZENG Wenting, China United Coalbed Methane National Engineering Research Center Co. Ltd., Beijing 100095, China; PetroChina Coalbed Methane Company Limited, Beijing 100028, ChinaFollow
• HAO Pengcheng, China United Coalbed Methane National Engineering Research Center Co. Ltd., Beijing 100095, China; PetroChina Coalbed Methane Company Limited, Beijing 100028, China
• HOU Wei, PetroChina Coalbed Methane Company Limited, Beijing 100028, China
• YIN Decai, China United Coalbed Methane National Engineering Research Center Co. Ltd., Beijing 100095, China; PetroChina Coalbed Methane Company Limited, Beijing 100028, China
• ZHANG Kang, PetroChina Coalbed Methane Company Limited, Beijing 100028, China
• LIU Chuanqing, PetroChina Coalbed Methane Company Limited, Beijing 100028, China
• TIAN Zhida, China United Coalbed Methane National Engineering Research Center Co. Ltd., Beijing 100095, China; PetroChina Coalbed Methane Company Limited, Beijing 100028, China
• LIU Qihu, PetroChina Coalbed Methane Company Limited, Beijing 100028, China
• WANG Qian, PetroChina Coalbed Methane Company Limited, Beijing 100028, China
• XIONG Wei, PetroChina Coalbed Methane Company Limited, Beijing 100028, China
• SUN Weiwei, PetroChina Coalbed Methane Company Limited, Beijing 100028, China
• XIA Fei, PetroChina Coalbed Methane Company Limited, Beijing 100028, China
• WANG Chunyan, China United Coalbed Methane National Engineering Research Center Co. Ltd., Beijing 100095, China; PetroChina Coalbed Methane Company Limited, Beijing 100028, China

Abstract

Background The exploitation of deep coalbed methane (CBM) is accompanied by continuous pressure drawdown. In this case, scaling and blockage are prone to occur in systems where formation water with high total dissolved solids (TDS) and high-concentration CO₂ coexist. In the middle to late production stage, the blockage of both wellbore and flow pathways in reservoirs frequently leads to abrupt declines in gas production and bottomhole pressure or even induces the shutdown of natural flowing production, severely restricting the stable production of deep CBM wells. Methods This study investigated the Daning-Jixian block in the eastern Ordos Basin. Using multiple techniques including inductively coupled plasma-optical emission spectrometry (ICP-OES), X-ray diffraction (XRD), and scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS), this study investigated the ionic composition of the water samples, as well as the mineral composition, microscopic morphologies, and element distributions of the scale samples. Accordingly, the mechanisms behind scaling in deep CBM wells were systematically explored. Furthermore, an effective acidic fluid system was developed based on experimental results, and a novel scheme for blockage removal was designed. Results The scale samples were dominated by calcium carbonate (> 90 wt%), and their precipitation was primarily governed by CO₂ solubility. Under a relatively constant temperature in horizontal sections of CBM wells, bottomhole pressure was identified as the dominant factor controlling the precipitation of calcium carbonate scale, while continuous pressure drawdown represented the direct cause. The scaling intensified significantly when CBM wells underwent gradual decline in gas production. Additionally, production parameter analysis and borehole televiewer (TV) monitoring revealed perforation hole blockage by scale in CBM wells and sand accumulation in their horizontal sections.Conclusions A novel philosophy for blockage removal via acidizing is proposed for deep CBM horizontal wells. This philosophy highlights three shifts: a shift from wellbore to reservoirs regarding the treatment target, a shift from a small-scale to a large-scale acidic fluid consumption, and a shift from blockage removal alone to the integration of blockage removal and sand carrying. A comprehensive acidic fluid system for blockage removal is developed, composed of 15% hydrochloric acid, 1% corrosion inhibitor, 1% iron stabilizer, and 0.3% flowback aid. Experimental results reveal that hydrochloric acid delivered high performance in dissolving scale samples, with the dissolution rates of calcium carbonate scale and MgCaCO₃ composite scale determined at 100% and 97.8%, respectively. This acidic fluid system effectively addresses the major challenges associated with efficient scale dissolution, corrosion protection, water blocking removal, and the prevention of secondary precipitation. To address heterogeneous blockage across the horizontal sections of CBM wells, a process integrating sand carrying via nitrogen foam with oil blockage removal through coiled tubing drag acidizing is designed, achieving efficient differential blockage removal. The developed process was applied to five wells (three horizontal and two vertical wells) in the Daning-Jixian block. Consequently, the single-well daily gas production increased by 1−5 times, and some wells exhibited distinct pressure recovery, with natural flowing production restored. The integrated process can be further applied to deep CBM wells with scaling risks in the Daning-Jixian block. The daily gas production is expected to increase by 80×104 m³, accounting for 26% of the total production of mature wells across the block. This establishes the process as an important approach to recreating the natural flowing period and achieving enhanced gas recovery of deep CBM horizontal wells in the block. Based on this process, a low-cost, weak acid-based blockage removal process has been developed through preliminary optimization based on the characteristics of medium-to-shallow CBM reservoirs. The resulting technique of differential blockage removal via acidizing will emerge as an effective countermeasure for production growth and potential tapping of mature wells in unconventional gas reservoirs.

Keywords

deep coalbed methane, scaling and blockage mechanism, horizontal well, acidizing plug removal, research and application

DOI

10.12363/issn.1001-1986.26.04.0262

Recommended Citation

ZHANG Lei, CHEN Ming, ZHEN Huaibin, et al. (2026) "Scaling and blockage in deep coalbed methane wells: Mechanisms and countermeasures," Coal Geology & Exploration: Vol. 54: Iss. 9, Article 18.
DOI: 10.12363/issn.1001-1986.26.04.0262
Available at: https://cge.researchcommons.org/journal/vol54/iss9/18

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