Fracture effectiveness evaluation in ultra-deep reservoirs based on geomechanical method, Kuqa Depression, Tarim Basin, NW China

Abstract

Onshore ultra-deep oil and gas resource is abundant, which has become the main area for exploration and development in the future. Fracture is an important factor affecting the development of ultra-deep reservoir, and its effectiveness, as a significant parameter for quantitative evaluation of the reservoir, determines the production. Taking the DB-4 ultra-deep gas reservoir in Kuqa Depression as an example, through geomechanical methods, the in situ stress distribution of the wellbore is determined by using drilling data and logging data. Based on the identification and quantitative characterization of fracture parameters, the stress state and shear slip trend of fractures are analyzed by three-dimensional Mohr's circle analysis technique and Mohr Coulomb criterion, so as to quantitatively evaluate the effectiveness of ultra-deep fractures. Based on above analysis, the fracture opening under the condition of fluid injection is simulated to establish the relationship between fracture effectiveness and oil and gas production capacity. The results show that the gas reservoir where Well X4 of DB-4 gas reservoir has a burial depth of more than 8,000m, a typical onshore ultra-deep gas reservoir, and it is in an environment with extreme strong in situ stress. For the Cretaceous target reservoir, the horizontal minimum principal stress ranges between 165–180 MPa and the horizontal maximum principal stress generally exceeds 200 MPa. Natural fractures of Well X4 are largely developed, and the wellbore fracture parameters show strong heterogeneity. The Well X4 has low fracture opening pressure and high fracture effectiveness on the whole. The production capacity of gas wells in the ultra-deep reservoir shows extremely obvious positively correlation with the effectiveness of fractures under the influence of in situ stress, while it has little influence on petrophysical properties such as porosity and reservoir thickness. Both the fracture effectiveness evaluation method and results of DB-4 ultra-deep gas reservoir is helpful to provide geological reference for the efficient development of ultra-deep oil and gas resources in Kuqa Depression, and it is of significance for similar ultra-deep fractured reservoirs in the world.