The Effects of Hydraulic Fracture Reorientation

Abstract

Abstract The basic concept of hydraulic fracture reorientation involves inducing a second artificial fracture into the producing zone, with this secondary fracture propagating in a different direction from the original. For reorientation to occur, the near wellbore stress has to have altered in orientation from the time the original fracture was created. To investigate the effects of this action on reserve recoveries in tight gas lenticular reservoirs, a series of simulations were run in a reservoir modeling program where orientation was assumed to occur at various given angles. The reservoir and fracture properties that were manipulated in order to run the model under different scenarios included the following: fracture orientation, fracture half-length, fracture conductivity, reservoir area, permeability anisotropy, and geologic aspect ratios. For each scenario, production of the field was then simulated over a period of time to study sensitivities of the parameters. The research presented in this paper led to the following main conclusions:refracture reorientation can be effectively studied using a reservoir simulator through manipulation of the fracture and reservoir parameters over time;incremental gains in production and pressure responses were observed with the variance of these various reservoir and fracture properties that were consistent with the possibility of hydraulic fracture reorientation; andresults indicate that even assuming refracture reorientation is possible, it would not be economical under typical conditions in most tight gas lenticular reservoirs due to their limited volumes. Introduction Historically, the aim of refracturing was to repair or replace an ineffective initial fracture treatment, thereby increasing production rates and associated reserves. Reorientation theory provides another reason for refracturing by providing the means to encounter and produce unstimulated pay (Elbel and Mack, 1993; Wright and Conant, 1995; Weng and Siebrits, 2007). In tight gas reservoirs, conventional radial drainage patterns are rarely exhibited but instead are replaced by a more elliptical drainage pattern around the fracture area (Wolhart et al, 2007). In such tight gas situations, a second fracture treatment will frequently propagate in a new direction until it reaches a virgin pressure zone which exhibits the original stress field (Siebrits et al, 2000). When the fracture reaches this point in the reservoir rock, it will realign itself parallel to the original in-situ horizontal stress field once again (Fig. 1). The idea of refracture reorientation, though not new, is still not completely understood. Although reorientation isn't always seen, refracturing wells has become normal practice in many areas including the Barnett Shale, where it is not uncommon to hydraulically fracture a well two or even three times (Moore and Ramakrishnan, 2006). For reorientation to occur, the near wellbore stress has to be different in orientation than it was at the time the original fracture was created. This is theorized to happen by altering the effective stress near the wellbore and original hydraulic fracture plane in one of two ways - either through depletion of the reservoir pore pressure or increase of pore pressure in the stimulated area.