The Keshen gas formation is located in the Tarim basin, western China, characterized with developed heterogeneous natural fractures, with most prospective tight gas opportunities. Though high initial gas production rate is commonly achieved, a rapid decline sometimes occurs during the long-term gas production when an unsuitable drawdown pressure is applied. This paper presents studies of the stress-dependent permeability of the core samples from the ultra-deep, naturally fractured tight gas reservoir in the Tarim basin, China and investigates its impact on the long-term gas recovery.
In this work, tight sandstone samples with different morphology of natural fractures were obtained from three exploration wells in the Tarim Basin. Permeability was measured using the pulse decay method by increasing the effective stress from 600psi to 6500psi, in steps of 1000psi, and repeating the same procedure in the reverse order. In addition, a numerical simulation model was established and validated by fitting the results of the experimental data to study the effect of natural fracture dip on the stress-dependent permeability.
Different permeability decline characteristics were observed for tight sandstone samples with different dominant flow channels (i.e., matrix or natural fractures). Due to the existence of natural fractures in the tight reservoir, the stress dependency and hysteresis of permeability is larger. Specifically, the hysteresis of stress dependent permeability is the largest if a natural fracture is through the whole sample and vertical to the end faces. Moreover, for the samples studied in this work, the permeability declines more as the effective stress reaches 3000psi if the natural fracture dip is large compared to that of the samples with a smaller natural fracture dip. Furthermore, a control strategy was proposed to address the stress sensitivity of permeability through the numerical simulation results.
Interpreting the permeability decline characteristics provides insights of the gas production behavior from tight sandstone reservoir while proper reservoir pressure needs to be maintained to mitigate the sharp production decline and enhance the ultimate gas recovery for the tight reservoirs with developed natural fractures.