Abstract
Producing oil from liquid-rich shale fields requires well completions with multi-stage hydraulic fracturing to enhance well productivity. Good design of transverse fractures is a key to achieving successful well completion and field-development economics. Using large fracture spacing will limit the number of fractures along the horizontal wellbore of given length, while using small fracture spacing can result in merging of fractures in the fracture-shadow area, both will reduce well productivity and oil recovery factor. There exists an optimum fracture spacing for given geological and fracturing conditions that allows the maximum oil recovery factory in shale oil reservoir to be achieved. The current practice of determining the optimal fracture spacing involves running numerical simulators to identify the fracture shadow. The numerical method is not only time-consuming but also inaccurate due to the procedure of numerical analyses and visual identification of fracture shadow. This paper presents a simple analytical method to determine the minimum fracture spacing required for preventing fracture-merging.
On the basis of assumption of constant-width fracture, an analytical solution was first obtained in this work to predict the stress distribution around an existing fracture. The solution was then used to predict the boundary of fracture-shadow area and identify the minimum distance of new fracture from the existing fracture to avoid fracture-merging. The result of the analytical method was compared with that of a numerical model of Finite Element Method (FEM) for a typical fracturing condition in a shale formation. The comparison indicates that the new analytical solution gives a shorter fracture spacing that will increase oil recovery factor. According to the analytical solution, the minimum required fracture spacing is nearly proportional to fracture width and insensitive to the fracture length. The minimum fracture spacing does not exist when the net closure pressure is lower than a critical value, depending on the in-situ stress contrast and Poisson's ratio of shale. This paper provides engineers a simple tool for optimizing their well completion in liquid-rich shale formations.
