The fracability evaluation of Dadaş Shale in Southeastern Turkey based on its geomechanical properties

Date

2023-9-08

Author

Cihaner, Oğuz

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In an unconventional shale reservoir system, the source rock, the reservoir rock, and the caprock are all represented by the same shale formation. Shale reservoirs have poor natural productivity due to their extremely heterogeneous structures with ultra-low permeability. Therefore, an overwhelming majority of shale oil/gas wells requires horizontal drilling technologies combined with multi-stage hydraulic fracturing. Hydraulic fracturing of horizontal wells has been commonly used worldwide for the last 25 years to increase the stimulated reservoir volume (SRV). The primary and sustained productivity of a shale reservoir is strongly dependent on geomechanical parameters. The fracability, which is stated as the most critical parameter in unconventional reservoir geomechanics, can be defined in its simplest form as the degree of a formation’s ability to be fractured effectively. Accordingly, the fracability index (FI) term can be used as a theoretical benchmark to mathematically calculate the tendency of rocks to be fractured. For hydraulic fracturing (HF) operations, FI can reflect a formation's tendency to initiate & propagate fractures and its ability to generate complex fracture network systems. Consequently, for unconventional reservoirs, fracability plays a crucial role in characterization of sweet spots and fracture barriers and in optimization of HF. This study mainly aims at calculating the fracability index (FI) of Dadaş shale as a function of mechanical brittleness index (BImech), fracture toughness (KC), minimum horizontal stress (σh), and differential horizontal stress (DHS, Δσ). To this end, firstly, a digitization study has been performed on the available logs (Gamma-ray log, Sonic (DT) log, and Density log) of Çalıktepe-2 well to obtain the crucial mechanical rock properties such as Young’s Modulus (YM, E) and Poisson Ratio (PR, ν), geological principal stresses, DHS (Δσ), fracture toughness (KC), unconfined compressive strength (UCS), tensile strength (To), internal friction angle (𝜑), and cohesion (CO). Then, these geomechanical parameters have been utilized to evaluate the fracability from deterministic and probabilistic aspects. In this context, four fracability models, namely Rickman’s et al.’s model (2008), Yuan et al.’s model (2017), Dou et al.’s model (2022), and newly proposed model in this study (2023) were implemented. This new fracability model obtained by modifying Dou et al.’s model includes mechanical brittleness index (BImech), mode-I and mode-II fracture toughness (KIC & KIIC), minimum horizontal stress (σh), and DHS (Δσ). To achieve abovementioned goals, a comparative analysis between Marcellus shale and Dadaş shale was performed by examining FI results, which deterministically estimated from all studied models. Additionally, using Proposed model, FI values of some other shale formations (Barnett, Haynesville, Bakken, and Eagle Ford) were obtained to observe the correlation between FI and BImech and to validate successful HF performances applied in the U.S.'s productive shale reservoirs. In stochastic process, firstly Monte Carlo simulation with 10,000 iterations was applied to perform probabilistic risk analysis for FI. Next, for all studied fracability models, the effect of fracability components on output data (FI) was examined by sensitivity analysis using tornado chart and spider chart, accordingly, critical input parameters were determined for each model. Finally, all FI results of Dadaş shale was analyzed as a whole to compare deterministic method and stochastic method. Interestingly, it was observed that the FI values obtained from Rickman et al.’s model (depends on normalized YM and PR) are significantly close to FI values of Yuan et al.’s model (depends on normalized YM and PR, KIC and KIIC, and minimum horizontal stress (σmin)). On the other hand, it was analyzed that values of Dou et al.’s model and Proposed model are obviously larger than those of two other models. Accordingly, this result may reflect that DHS (Δσ) has a strong effect on the fracability of Dadaş shale. Besides, it was found that the results of proposed model are highly close to results in Dou et al.’s model. From this point of view, it may be inferred that mode-II fracture toughness (KIIC) plays a noticeable but small role in FI evaluation for Dadaş shale. The low effect of KIIC on FI may be attributed to the fact that the initiation and propagation of fractures are more related to tensile fractures rather than shear fractures. All these findings mentioned above indicate that the Proposed model emphasizes the importance of differential horizontal stress and mode-II fracture toughness in fracability evaluation of shale reservoirs, which especially reside in strike-slip (SS) faulting and reverse faulting (RF) environments. The relatively high deterministic FI results of Marcellus shale may be used as a supportive argument to successful hydraulic fracturing (HF) operations applied in this formation. From a comparative aspect, Marcellus shale has larger FI values than Dadaş-I shale in all studied models (especially in Rickman et al.’s model and Yuan et al.’s model). This can be explained by that Marcellus shale has a much more desirable geomechanical structure compared to Dadaş-I shale. On the other hand, deterministic FI results of Marcellus shale indicated close values with those of Dadaş-I shale for Dou et al.'s model and the Proposed model. It was also observed in this study that all examined formations in the U.S. was graded as highly fracable according to Proposed model despite their relatively not much bigger BI values. By this way, Dou et al.’s fracability model was verified by Proposed model. Additionally, it was found that Dadaş Shale shows a similar tendency with Barnett Shale and Haynesville Shale in regards to FI and BImech. The results also showed that there is not always a positive correlation between FI and BI. In brief, according to Rickman et al.’s model and Yuan et al.’s model, it was identified that Dadaş shale is a low-level fracable formation, and it is difficult to obtain an effective HF performance from this formation. However, Rickman et al.’s model only contains the mechanical brittleness in FI equation. The reliability degree of Yuan et al.’s model is disputable due to the absence of DHS in fracability equation, and close FI values observed between this model and Rickman et al.'s model. On the other hand, Dou et al.’s model and the Proposed model showed in Dadaş shale that there is a high probability of effectually applying HF and a high tendency of obtaining complex fracture networks. Besides, compared to all other zones, L2 zone has more favorable petrophysical, geochemical (in terms of reservoir quality), and geomechanical properties (in terms of completion quality). From this viewpoint, it was concluded that L2 zone is the most likely ideal option in the matter of the effective stimulation of Dadaş shale by HF. In light of the findings above, the Proposed model may be presented as an alternative FI method to determine sweet spots in an HF operation; yet, the results of this study should be extended by experimental data and numerical modelings. In the future, this research is expected to serve as a geomechanical benchmark in HF optimization of Turkey’s pioneering unconventional shale resources.

Subject Keywords

Dadaş formation, Fracability, Fracability index, Geomechanical properties, Hydraulic fracturing

URI

https://hdl.handle.net/11511/105498

Collections

Graduate School of Natural and Applied Sciences, Thesis