# Study on the Mechanism of Stress Sensitivity Changes in Ultra-Deep Carbonate Reservoirs
## 1 Introduction  
Since 2000, China has been exploring ultra-deep (with a depth of burial over 6000 m) carbonate reservoirs [1]. According to data from the China National Petroleum Corporation (CNPC), as of the end of 2021 [2], China has identified oil and gas reserves of more than 100 billion barrels in ultra-deep carbonate reservoirs, of which the oil and gas resource-rich areas are mainly concentrated in the Tarim Basin, Sichuan Basin, and Ordos Basin [3; 4; 5]. With the continuous advancement of exploration and extraction technology, the exploration and development capabilities of ultra-deep carbonate reservoirs have been significantly improved [6; 7; 8; 9]. The vast reserves of oil and gas in ultra-deep carbonate reservoirs can provide important support for China's future energy supply. Ultra-deep carbonate reservoirs are complex storage media for oil and gas, with highly heterogeneous reservoirs and complex spatial structures [10]. During the extraction process,as the remaining reserves continuously decrease, the underground pressures gradually decrease, causing the deformation of the rock skeleton. The increasing effective stress causes the reservoir's internal pore structure to deform, along with the formation of micro-fractures and changes in the mechanical properties of rocks [11]. The effective stress to which it is subjected increases, causing the deformation of the internal pore structure of the reservoir, micro-fractures and other deformations, and ultimately the phenomenon of the loss of permeability and decline in production capacity, which is also known as the stress sensitivity of carbonate rock. The stress sensitivity of carbonate reservoirs not only affects the reserve evaluation in the development of carbonate oil and gas fields, but also has important implications for the formulation of development technology indicators for carbonate oil and gas fields First, the stress sensitivity of carbonate reservoirs directly affects the size of the storage space, which in turn affects the calculation of the true reserve of oil and gas reservoirs by petroleum engineers during the development process. In addition, with the deepening of the exploitation degree, the stress sensitivity of the carbonate reservoir will directly change the pore structure of the reservoir, which will affect the process of oil and gas seepage and influence the final development effect. Therefore, stress-sensitive parameters are important parameters that affect the oilfield development process. Therefore, how to scientifically and accurately evaluate the stress sensitivity of carbonate reservoirs has become a difficult problem in the research of carbonate reservoirs. Researchers have conducted extensive research on the stress sensitivity of reservoirs; [PERSON] [12] conducted stress sensitivity experiments on fractured carbonate reservoirs. By measuring the permeability of core samples at different confining pressures, [PERSON] [12] found that there is a linear relationship between rock confining pressure and permeability. An empirical formula was derived from the experimental data; [PERSON] [13] successfully solved the flow equations of the system considering stress sensitivity, and [PERSON] [14] made further improvements based on [PERSON]'s work. [PERSON] [15] analytically studied the effect of stress sensitivity on reservoir permeability in oil reservoir flow problems and defined the stress parameter alpha. They also analyzed several typical pressure curve graphs using numerical simulation methods. [PERSON] [16] defined the stress sensitivity index as the percentage loss of reservoir properties when the formation pressure drops a certain value. They derived the stress sensitivity index from the relationship between effective stress and the rate of permeability loss. Numerous scholars have conducted laboratory experiments to study the relationship between reservoir permeability and effective stress [17; 18; 19; 20].