Hydraulic fracturing is particularly suited for natural gas wells. For low permeability reservoirs the need is obvious: it is the only means to monetize a huge number of gas wells. In addition, in recent years horizontal wells with multiple transverse fractures have emerged as the configuration of choice for many low permeability or unconventional reservoirs such as shale gas.
However, because of increasing demand in the world economy, previously stranded gas of much higher permeability is rapidly becoming target for new field developments. Fracturing, which for similar-permeability oil reservoirs may be abandoned in favor of horizontal or complex well architecture, is a must for higher permeability gas wells because of considerably enhanced turbulence effects.
But turbulence in the fracture itself creates the need for design adjustments because of substantial reduction in the effective proppant-pack permeability.
We present a physical optimization scheme for fracturing natural gas wells and extend it to the fracturing of multiple treatments in horizontal wells. The latter, because of enhanced turbulence effects in the fracture, has an upper limit of application, about 0.5 md. For higher permeability, the multiple fractured horizontal well has an unacceptable reduction in well performance, and vertical wells with fractures are indicated instead.
In this paper, physical optimization is complemented by economic optimization by comparing vertical wells with and without fractures and horizontal wells with multiple fractures in a range of permeabilities. Of importance are the geographic location (e.g., North America vs international) and the markedly different costs associated with them. For lower permeability reservoirs, economic considerations that would make multiple fractured horizontal wells attractive in e.g., North America, render them uneconomic in many other parts of the world. This, along with the physical constraint outlined above, creates a narrow range of permeability where the configuration is applicable.
Several recent publications have addressed the application of hydraulic fracturing in gas wells to reduce turbulence and maximize productivity (Economides et al., 2002; Wang and Economides, 2004; Economides and Martin, 2007; Marongiu- Porcu et al., 2008). The conclusion is that hydraulic fracturing should be adopted as standard completion for virtually any gas well, not only because it leads to stimulation but it also reduces turbulence effects. This can be seen from Figure 1 (Marongiu-Porcu et al., 2008).
Figure 1 shows calculated folds of increase (FOI) between fractured and non-fractured wells for both oil and gas reservoirs within a permeability range between 0.05 to 100 md. Figure 1 represents the results of physical optimization of hydraulic fracture treatments using the Unified Fracture Design (UFD) approach (Economides and Valkó, 2002). As the reservoir permeability increases, the FOI for oil wells declines relatively smoothly (from over 10 at 0.05 md to about 2 at 100 md).
For natural gas wells the behavior of the FOI trends at low reservoir permeability mimics that of oil wells but as the permeability increases, the trend diverges: a fractured gas well starts to perform far better than a non-fractured high-permeability well because of the considerable reduction in turbulence effects that adversely affect well performance and dominate radial flow.