Abstract
Based on logging, seismic, fracturing and production data from 301 productive wells in Longmaxi Formation in Changning shale gas field of southern Sichuan basin, the various influence factors of shale gas stimulated potential have been analyzed to carry out the correlation study with estimated ultimate recovery (EUR) of the same fracturing operation intensity to fully clarify the main controlling factors for shale gas stimulated potential. The results show that matrix brittleness, fracture propagation and reservoir properties are the 3 key secondary potentials that control shale gas stimulation potential. The matrix brittleness is controlled by elastic modulus, Poisson’s ratio, Type I and II fracture toughness, which reflects the uniformity of hydraulic fracture propagation. Fracture propagation is controlled by critical net pressure, which reflects the scale of hydraulic fracture propagation. Reservoir properties are controlled by porosity, total organic carbon content (TOC) and horizontal interval differences, which reflect the enrichment conditions and dynamic production capacity of reservoir resources. The matrix brittleness index, fracture propagation index, reservoir properties index and their combined stimulated potential index were formed by using the above 7 parameters to verify and apply the wells in Changning shale gas field. Results show that the matrix brittleness index and length-width ratio of hydraulic fracture was significantly negative correlation, the fracture propagation index and stimulated reservoir volume (SRV) were significantly positive correlation, stimulated potential index was developed taking into account enrichment & exploitation potential. In areas where the value is greater than 0.5, increasing the fracturing scale can effectively improve the well productivity; in areas less than 0.5, increasing the fracturing scale has an upper limit on the increase of productivity, so the physical properties of the reservoir itself play a significant role in controlling the production of shale gas wells.
Highlights
For thick shales with stable sedimentary structures condition and continuous distribution in North America, such as Barnett, Utica, Marcellus and Eagle Ford, it is necessary to obtain large enough reservoir stimulated volume (SRV) through hydraulic fracturing of horizontal Wells to achieve resource production (Zhao et al, 2018)
The study first extracted the required source factors (SF) and their derived factors (DF) as much as possible through logging and seismic interpretation data, which can be divided into matrix brittleness (MB), fracture propagation (FP) and reservoir properties (RP) (Table 1)
We introduce a large number of parameters not involved in conventional evaluation: 1Generally, the azimuth angle β of natural fracture zone is of great significance for well trajectory design, but for drilled and fractured wells, the approach angle θ is more valuable for analysis; 2WNF and WLM reflect the development scale of micro-fractures and micro-bedding inside the matrix rock, which is controlled by sedimentary facies and is positively correlated with the content of mineral components, and can be obtained by multiple regression of VSi and VCa in the source
Summary
For thick shales with stable sedimentary structures condition and continuous distribution in North America, such as Barnett, Utica, Marcellus and Eagle Ford, it is necessary to obtain large enough reservoir stimulated volume (SRV) through hydraulic fracturing of horizontal Wells to achieve resource production (Zhao et al, 2018). Such as the Haynesville shale in North America and the southern Sichuan Basin in China with larger burial depth, more complex seepage law and large in-situ stress difference, only aiming to obtain larger SRV can no longer guarantee that the larger SRV in the region or area can obtain higher productivity (Shen et al, 2021). From the perspective of engineering, some scholars proposed that stage length, operation (net) pressure, well (segment, cluster) spacing, fracturing fluid scale and performance, pumping rate and proppant dosage can be used as evaluation indexes (Zhao et al, 2013; Huang et al, 2016; Liao and Lu, 2018)
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