# Study on the Mechanism of Stress Sensitivity Changes in Ultra-Deep Carbonate Reservoirs
## 6 Conclusions  
By fully considering the characteristics of carbonate reservoirs and exploring the mechanical properties of rocks, such as Young's modulus and Poisson's ratio, under different stress conditions, and through utilizing classic Griffith mechanics solutions, conventional Darcy's law, sedimentary structures, and layer sequence stratigraphic features, quantitative evaluation formulas for the stress sensitivity of matrix and fractured carbonate reservoirs were derived. These formulas were applied in the Shunbei area of the Tarim Basin and the following main conclusions were reached:  
1. Under the same effective stress, the stress sensitivity of matrix- and fracture-type carbonate reservoirs is related to Young's modulus of the rock framework. In matrix-type carbonate reservoirs, rocks with a higher Young's modulus exhibit lower stiffness and are more susceptible to both elastic and plastic deformation, resulting in a higher level of stress sensitivity. Matrix-type carbonate reservoirs with a lower Young's modulus typically exhibit higher levels of stiffness, and thus may not readily undergo plastic deformation, resulting in a lower level of stress sensitivity. For the fracture-type carbonate reservoirs, those with a lower Young's modulus possess a higher resilience and capacity for elastic deformation, making them more likely to undergo fracture closure and less susceptible to significant deformation and stress sensitivity. When Young's modulus of fracture-type carbonate reservoirs is higher, the stiffness of the rock is large, making it easy for external stress changes to bring about the crack extension, resulting in a higher level of stress sensitivity.
2. Under a confining pressure of 87 MPa, a flow pressure of 50 MPa, and at 120 \({}^{\circ}\)C, it was determined that the effective stress of the matrix-type carbonate reservoir and the permeability of the reservoir exhibit an exponential relationship, with a stress sensitivity index \(\sigma\) of 0.009324. Based on rock mechanics parameters, with a matrix Young's modulus of 53.41 GPa and a Poisson's ratio of 0.201, and a fracture Young's modulus of 5.041 GPa and a Poisson's ratio of 0.33, the stress sensitivity of the fracture type carbonate reservoir is generally larger than that of the matrix-type carbonate reservoir, and the smaller the Young's modulus, the greater the difference in stress sensitivity between the two types of rocks.
3. By establishing a dual-porosity model based on the initiation pressure gradient, stress sensitivity simulations were conducted for typical ultra-deep carbonate formations in the Shunbei area of the Tarim Basin. The results indicated that these ultra-deep formations are characterized by high levels of stress sensitivity, providing new evidence for the high-stress sensitivity characteristics of ultra-deep carbonate reservoirs. The impact of stress sensitivity on the calculation of reserves and the efficient development of ultra-deep carbonate reservoirs must be given considerable attention during practical mining processes.  
Conceptualization, W.C. and S.J.; data curation, W.C.; formal analysis, S.J. and H.L.; funding acquisition, S.J.; methodology, W.C. and H.L.; project administration, H.L. and S.J.; software, W.C.; writing--original draft, W.C.; writing--review and editing, S.J. and H.L.; visualization, W.C.; validation, S.J. and H.L. All authors have read and agreed to the published version of the manuscript.  
This research is funded by the Open Project of State Key Laboratory of Shale Oil and Gas Enrichment Mechanisms and Effective Development (GSYKY-B09-33).  
Not applicable.  
All data and materials are available on request from the corresponding author. The data are not publicly available due to ongoing research using a part of the data.  
The authors declare no conflicts of interest.