Case Study: Analysis of Refracturing Crack Orientation-Angle and Extension-Length in Tight Gas Reservoir, Sulige Gasfield of China

Abstract

Attribute to the hydraulic fracturing technology, China has carried out commercial development of the low permeability and tight gas reservoirs in Sulige Gasfield, Ordos Basin. However, the practice indicates that the gas well with hydraulic fracturing performs rapid decline rate, which generally repeated fracturing technology is often adopted to enhance the economic benefits of gas field development. Therefore, the reservoir physical properties, pressure system, fluid properties, and formation parameters of fracturing engineering, such as rock characteristic parameters and original in-situ stress, are respectively summarized. Furthermore, compared with traditional hydraulic fracturing, the theory of refracturing and the simulation of crack extension are studied. This study starts from the geological characteristics, gas reservoir properties and rock physical properties of the main layer. Firstly, based on the theory of rock elasticity, the problem of crack induced stress field is analyzed. Then, combined with the rock media and mechanical environment around the initial artificial crack, the mathematical model of the induced stress field of the initial artificial crack is established. Meanwhile, the semi-inverse solution is applied to solve the mathematical model. Finally, the analytical formula of crack induced stress is obtained, by introducing Fourier transform, complex variable and Bessel function integral formula. Taking a fractured gas well in Sulige Gasfield as an example, only single-phase gas flowing is considered and depletion constant pressure production is adopted. The results show that: (a) The induced stress is mainly related to the net pressure on the crack wall, in which the induced stress in the direction of the original horizontal principal stress increases with the net pressure. (b) Through the simulation of tight gas reservoir performance, we found that the change of production induced stress is great with the longer production time, the lower bottom-hole flowing pressure and the more variable anisotropy of reservoir permeability. (c) The area of in-situ stress reorientation is also greater, and the new crack gets easy to change direction. (d) This simulation can help engineers realize that the initial artificial crack induced stress and gas well production induced stress all change the initial in-situ stress, thence, the new crack of refracturing will not fracture along the direction of the old crack. In this case, the Orientation-Angle and Extension-Length are recalculated, after calculating the current stress state in the direction of the original principal stress, and production time, bottom hole production pressure and others that affect the new crack are analyzed. More importantly, this research could be applied for other similar refracturing wells with vertical cracks in tight gas reservoirs worldwide and provides a research basis for the afterward study of the description of volumetric crack.