Objective The southern Shenfu block along the eastern margin of the Ordos Basin hosts one of the rare gas fields with reserves of over 100 billion cubic meters (bcm) in China, serving as a pilot demonstration area for the exploration and production of deep coalbed methane (CBM) conducted by the China National Offshore Oil Corporation (CNOOC). In this block, the gas production of horizontal wells is 2-10 times that of vertical wells. However, the horizontal wells are challenged by significantly different and rapidly declining gas production. Determining dominant factors controlling the productivity of horizontal wells for deep CBM is the key to elevated overall production efficiency and commercial development.

Methods This study investigated 16 horizontal wells in the southern Shenfu block using the dynamic and static data of 17 CBM assessment wells. Specifically, fine-scale assessments of in-situ stress and coal brittleness index were conducted using the wellbore collapse method and acoustic logging. Accordingly, an assessment system for engineering sweet spots (ESSs) was developed, followed by the fracability assessment of coal seams. Following the principle of one strategy for one well, this study delved into the drilling quality of horizontal wells and the technical indicators for large-scale volume fracturing. Then, three drilling quality parameters and seven fracturing stimulation parameters were selected for assessment. Based on the fine-scale characterization of factors influencing geological and engineering dual sweet spots (GESSs) within the influence range of horizontal wells, this study proposed the concept of the GESS coefficient, using which the productivity of horizontal wells for deep CBM was predicted. Through a systematic assessment of parameters for well drilling and completion and large-scale volume fracturing, this study performed quantitative assessments of 16 geological and engineering factors extracted. Consequently, dominant factors controlling the productivity of horizontal wells for deep CBM in the southern Shenfu block were determined.

Results and Conclusions Dominant factors controlling geological sweet spots (GSSs) include the ash content of coals within the influence range of a horizontal well, gamma ray (GR) value while drilling, and peak gas-logging-derived value. Based on these factors, the assessment criteria for GSSs were developed. In the case where the horizontal segment of a horizontal well exhibits Class I GSS sections exceeding 500 m in length and a GSS coefficient of greater than 0.4, the peak and stable gas production can reach over 20 000 m³/d and 15 000 m³/d, respectively. Dominant factors controlling ESSs in a horizontal well include the horizontal in-situ stress difference and the brittleness index of coals. Given the weak heterogeneity of the horizontal in-situ stress differences in the study area (5.1 MPa to 5.5 MPa), this study constructed the GESS coefficient using the brittleness index. A GESS coefficient of greater than 0.52 suggests that the commercial development of horizontal wells for deep CBM can be achieved. Dominant factors controlling the production of horizontal wells include the length of GESS sections and the scale of fracturing stimulation. When the total length of Classes I and II GESS sections in the horizontal segment of a horizontal well accounts for 50% of the well’s total length, 80% of the horizontal well productivity can be achieved through large-scale volume fracturing. Dominant factors controlling the horizontal well productivity were quantitatively determined based on Kendall's rank correlation coefficients and Pearson correlation coefficients. The results indicate that the top six factors consist of the GSS coefficient, peak gas-logging-derived value, ESS coefficient, injection rate of fracturing fluids, fracturing pressure, and average proppant concentration. The results of this study will provide strong support for the differential design of both clusters in fracturing sections and fracturing scale for deep CBM production.