Prediction of mechanical parameters for low-permeability gas reservoirs in the Tazhong Block and its applications

Rock mechanics parameters control the distribution of in situ stress and natural fractures, which is the key to sweet spot evaluation in reservoir engineering. Combined with the distribution of in situ stress, an experimental scheme of stress on rock physical parameters was designed. The results show that rock sonic velocity is extremely sensitive to water saturation under overburden pressure. At ultrasonic frequencies, when the water saturation increases from 0% to 80%, the P-wave velocity increases first and then decreases. When the water saturation continues to increase to 100%, the P-wave velocity increases. This is due to the effect of water saturation on the shear modulus. Saturation is negatively correlated with shear wave velocity and resistivity. Different minerals have different control effects on the rock P-S wave velocity ratio. Quartz content plays a dominant role, and the two are negatively correlated, followed by feldspar and clay, and the two are positively correlated with the P-S wave ratio. The confining pressure, axial compression, stress ratio and burial depth are positively correlated with the P-S wave and negatively correlated with the P-S wave ratio; in descending order, the influencing factors of stress on the petrophysical parameters are maximum stress ratio > confining pressure > axial pressure.

Strike-slip faults and their associated fractures in the ultra-deep marine carbonate reservoirs of the Fuman Oilfield, Tarim Basin, hold significant petroleum geological importance, with the developmental characteristics of fractures being a key factor controlling reservoir productivity. This study targets the F_I17 strike-slip fault zone within the oilfield, where a comprehensive evaluation of fracture effectiveness was performed by integrating geological methods, including core and thin section observation, fluid inclusion thermometry, geophysical fracture identification approaches using imaging logging and seismic data, and geomechanical simulations. The results showed that: (1) structural fractures were developed in at least three stages, predominantly high-angle fractures with their strikes obliquely intersecting the main fault at a small angle, and were affected by multiple episodes of fluid activity, while early-phase fractures exhibited severe filling whereas late-phase fractures had good effectiveness; (2) ultra-deep carbonate rocks contained well-developed stylolites, with extensive horizontal stylolites reducing fracture effectiveness; (3) mechanical effectiveness evaluation parameters were proposed by integrating normal stress, shear stress, and formation pressure, with slip tendency as the dominant indicator, and referenced to the leakage factor and dilation tendency to characterize fracture effectiveness; (4) dynamic effectiveness was assessed using closure/opening pressures, defining a reasonable formation pressure range for hydrocarbon exploitation. The findings of this study can provide theoretical guidance for the further exploration and development of ultra-deep reservoirs in the Fuman Oilfield.

Rapid encroachment of edge and bottom water in ultra-deep gas reservoirs often leads to water breakthrough in gas wells and productivity decline, which has become a major engineering issue restricting the long-term stable development of the Keshen Block in the Tarim Basin. Reservoir pressure depletion caused by continuous gas production and local pore-pressure redistribution induced by water encroachment alter the effective stress state of the reservoir, thereby causing rock deformation and the evolution of mechanical parameters. However, the coupled evolution of the effective stress coefficient b, elastic parameters, and mechanical parameters under high in situ stress and high pore pressure remains unclear. Therefore, effective-stress-coefficient measurements and triaxial compression tests were conducted on Bashijiqike Formation outcrop sandstone from the Kuqa Depression, Tarim Basin, under combined high in situ stress and high pore pressure conditions representative of 6000–8000 m burial; the evolution of the effective stress coefficient, Young’s modulus, and Poisson’s ratio was jointly characterized, and a unified stress-dependent framework linking the three parameters was proposed. The results show that, with increasing in situ stress, the effective stress coefficient and Poisson’s ratio decrease, whereas the elastic modulus increases. Increasing pore pressure produces the opposite trends, and the pore-pressure sensitivity of each parameter weakens with increasing in situ stress. Under a given in situ stress, the elastic modulus and Poisson’s ratio exhibit linear relationships with pore pressure. The effective stress coefficient b decreases nonlinearly with increasing simple effective stress and stabilizes at 0.67–0.68 when the simple effective stress exceeds 120 MPa. After incorporating the stress-dependent b, the elastic modulus and Poisson’s ratio can be uniformly expressed as functions of effective stress, exhibiting a sigmoidal saturating increase and an exponential decay, respectively. Validation using Well Keshen 2-1-14 shows that the average relative errors between the calculated and log-interpreted values are 7.42% for the elastic modulus and 12.38% for Poisson’s ratio.