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Coal Geology & Exploration

Abstract

Background In China, low-yield coalbed methane (CBM) wells represent a high proportion of approximately 57% (over 12400 wells), generally encountering challenges such as low pressure, dry reservoirs, and near-wellbore formation damage and blockage. Conventional hydraulic fracturing is prone to induce water blocking damage, while treatments such as acidification offer only limited blockage-removing radii. Therefore, existing techniques are insufficient to achieve blockage removal and permeability enhancement while avoiding reservoir damage. Exploring efficient waterless stimulation technologies suitable for these low-yield wells is of great significance for increasing CBM production and advancing the CBM industry. Methods This study investigated directional wells PXC-03 and PZC-15 in the Panzhuang block of the Qinshui Basin. First, using velocity sensitivity event analysis, combined with production performance data, this study determined the causes and types of the low productivity of both wells. Then, innovatively employing CO2 waterless dynamic-load fracturing technology, this study conducted field engineering tests of multi-interval fracturing stimulation in the two wells. Finally, the water and gas production performance of the two wells before and after stimulation was compared, followed by a comprehensive evaluation of stimulation effects. Results The calculation results of velocity sensitivity coefficients indicate that wells PXC-03 and PZC-15 experienced multiple velocity sensitivity events over 133 and 110 days of production, respectively. As a result, the reservoir permeability and gas production potential of the two wells were severely impaired, leading to a sharp decline in production. The two wells were classified as low-yield wells characterized by velocity sensitivity-induced near-wellbore blockage, both exhibiting low pressure and dry reservoirs. After fracturing stimulation, a large volume of gas-water mixture gushed out from well PXC-03, indicating that blockers were effectively removed through the scouring action. Concurrently, bottomhole flow pressure was restored, and stable water production was achieved. These findings confirm the effective removal of near-wellbore blockage and the successful reconstruction of seepage pathways in well PXC-03. Over subsequent 528 days of continuous production, the average daily gas production of this well increased from 318 m3 before stimulation to 1977 m3, and the peak daily gas production increased from 552 m3 to 3015 m3, representing 5.2- and 4.5-fold increases, respectively. Consequently, the cumulative gas production of well PXC-03 reached up to 1.044 × 106 m3. For well PZC-15, a low-permeability well with more complex geological conditions, the average daily gas production increased by 3.0 times after stimulation. These results demonstrate that the CO2 fracturing technology exhibits significant stimulation effects for distinct types of reservoirs.Conclusions The CO2 waterless dynamic-load fracturing technology employs CO2 as the sole working medium throughout the fracturing operation. Through dynamic-load fracture creation and the scouring action of high-pressure gas wedges, this technology can effectively reconstruct the near-wellbore fracture network and remove pulverized coal blockage while fundamentally avoiding water blocking damage in dry reservoirs. Therefore, this technology represents an effective approach to the efficient stimulation and production growth of low-yield wells characterized by low pressure, dry reservoirs, and near-wellbore blockage. Most especially, this technology holds broad prospects for widespread application in low-pressure and low-permeability CBM wells.

Keywords

coalbed methane (CBM), low-yield well, near-wellbore blockage, dry reservoir, CO2 fracturing, reservoir stimulation, production growth, Qinshui Basin

DOI

10.12363/issn.1001-1986.25.12.0928

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