Predicting the “sweet spot” of deep coalbed methane based on resource conditions and fracability

Objective China has made breakthroughs in the exploration of large-scale deep coalbed methane (CBM) reservoirs along the eastern margin of the Ordos Basin. However, effectively converting the reserves of these reservoirs into production and achieving their efficient exploitation remain unclear.

Methods Based on the geological, tests, fracturing, and production performance data, this study conducted research into the assessment of geological-engineering dual “sweet spots” of deep CBM in the southern Shenfu block by focusing on the resource conditions and fracability of deep CBM reservoirs. Results and Conclusions Key findings are as follows: (1) The study area hosts suitable coal-forming facies zones dominated by deltaic sediments. In this block, the Nos. 8 and 9 coal seams exhibit substantial thicknesses, stable distributions, and thermal maturity ranging from 0.7% to 1.5%, suggesting favorable hydrocarbon generation conditions. Furthermore, both coal seams feature well-developed pore-fracture systems, providing superior reservoir space for CBM. The roofs of both coal seams, composed primarily of mudstones with a high sealing capacity, are located in groundwater stagnation zones, creating favorable conditions for CBM preservation in the southern Shenfu block. With excellent gas-bearing properties, both coal seams show the coexistence of free gas and adsorbed gas. (2) This study characterizes the distributions of the brittleness, horizontal principal stress differences, and fractures of the coal seams using geophysical techniques that integrate logs and seismic data. Based on this, it constructs an indicator system for identifying the geological-engineering dual “sweet spots” of deep CBM, and accordingly, identifies three types of geological-engineering dual “sweet spots” in the southern Shenfu block: type I located in the eastern portion, type II in the west-central portion, and type III in the northwestern portion. (3) Considering the substantial burial depths, low porosity and permeability, and well-developed cleats and fractures of coal seams in the study area, this study develops a volume fracturing technique that centers on injection of a large volume of fracturing fluids at high rates, fracturing and proppant transport via slickwater of varying viscosities, high- intensity proppant injection, and propping of fractures on various scales. As indicated by the assessment of post-fracturing production tests, wells put into production in type I “sweet spots” have witnessed a rapid increase in CBM production, with stable production ranging from 7 000 to 8 000m³/d, peaking at approximately 8 000 m³/d. The results of this study have effectively guided and advanced both the selection of the optimal areas for the exploration and production of deep CBM in the Shenfu block and the implementation of plans for pilot test areas, thereby holding positive significance for the large-scale production capacity construction of deep CBM in the block.