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.