Mehdi Rafiee

Hydraulic Fracturing Design and Optimization: A Modification to Zipper Frac

In recent years, new fracturing designs and techniques have been developed to enhance production of trapped hydrocarbons. The new techniques focus on reducing stress contrast during fracture propagation while enhancing far field complexity and maximizing the stimulated reservoir volume. Zipper frac is one of these techniques, which involves simultaneous stimulation of two parallel horizontal wells from toe to heel. In this technique, created fractures in each cluster propagate toward each other so that the induced stresses near the tips force fracture propagation to a direction perpendicular to the main fracture. The effectiveness of zipper frac has been approved by the industry; however, the treatment’s optimization is still under discussion. In this paper, we present a new design to optimize fracturing of two laterals from both rock mechanic and fluid production aspects. The new design is a modification to zipper frac, where fractures are initiated in a staggered pattern. The effect of well spacing on the changes in normal stress has been evaluated analytically to optimize the design. Results demonstrate that the modified zipper frac improves the performance of fracturing treatment when compared to the original zipper frac by means of increasing contact area and eventually enhancing fluid production. Introduction Hydraulic fracturing is a stimulation technique used to extract trapped hydrocarbon. Fracturing vertical wells was used for variety of reservoir conditions varying from tight gas formations to high permeability formations implementing the FracPac applications. Fracturing horizontal wells started in the late 80’s for stimulation of tight gas formation. The use of fracturing horizontal wells proved to a key technology in the development of unconventional reservoirs. The technique has been widely used with the development of Barnett shale in the late 90s (Navigant Consulting, 2008). While the existence of natural fractures in shale oil and gas plays make them good candidates for hydraulic fracturing, the key in a successful treatment is creating a complex network that connects created hydraulic fractures with pre-existing natural fractures. This network of fractures, which consist of hydraulic fractures, primary and secondary natural fractures, are highly desired in low permeability reservoirs where higher conductive connectivity can be achieved as opposed to connectivity created by planar fractures (Soliman et al. 2010). Numerical simulations (Mayerhofer et al. (2008); Nagel and Sanchez-Nagel (2011); Warpinski et al. (2009); Cipolla et al. (2009) show that creating an interconnected network of fractures in nano-permeable reservoirs is a major factor in economic production. Various methods have been applied to create this complex network and ultimately maximize the total Stimulated Reservoir Volume (SRV). Creating secondary fractures is a vital occurrence in increasing the reservoir contact. Secondary fractures can be created by multistage fracturing along a horizontal wellbore in a naturally fractured reservoir. Different design parameters including the number of perforation clusters per stage, the spacing between stages, the length of the horizontal well, the sequence of fracturing operations, and the type and quantity of proppant should be optimized to create secondary fractures and a complex network of fractures (Mayerhofer et al. 2010). Among these parameters, spacing between perforation clusters as well as fracturing stages play major roles in fracture propagation and geometry. As noted by Soliman et al. (2008), the spacing between fractures is limited by the stress perturbation caused by the opening of propped fractures. However, fracturing designs can be optimized if the original stress anisotropy is known and the stress perturbation can be predicted (Soliman et al. 2010). Recent advances in fracturing design (East et al. 2010; Cipolla et al. 2010; Roussel and Sharma 2011; Waters et al. 2009) offer techniques for creating far field fracture complexity to enhance the SRV. Zipper frac is one of these techniques in which two horizontal wellbores are fractured simultaneously to maximize stress perturbation near the tips of each fracture. The