Northside: Optimizing Fracture Spacing in Unconventional Oil & Gas Reservoirs
Fracture stimulation and production forecasting are two important processes for conventional low-permeability or unconventional reservoirs. Fracture stimulation of either type of reservoir helps assure commercial production rates or maximize exploitation, while proper production forecasting informs the operator in advance of the potential short- and long-term value of the well and the area.
To provide more accurate recovery forecasts for conventional low-permeability or unconventional reservoirs, one must consider the key parameters of the generated hydraulic fracture, including effective fracture length (affected by filtrate clean-up and fracture damage), relative differences in fracture and formation flow capacity (e.g., dimensionless fracture conductivity), proppant distribution, tapered fracture conductivity, and stress dependence of the fracture conductivity. In addition, stress dependence of reservoir permeabilityand reservoir fluid properties also have a major impact. Non-darcy and multiphase flow effects inside the fracture have been extensively discussed in the literature and will not be discussed in this paper but can be indirectly included when considering the discussion on reduced fracture conductivity.
For the horizontal wellbore architecture commonly used to exploit unconventional reservoirs, the knowledge of the optimum number of fractures to maximize the recovery is very important. This paper uses a numerical reservoir simulation study to develop simple correlations that quantify what fracture spacing is necessary to optimize recovery factors in unconventional gas reservoirs and how various hydraulic fracture parameters and non-ideal reservoir behaviors affect the horizontal well completion design.
Because the reservoir simulation process can take significant time and effort, and analytical solutions are sometimes very complex, a simple “back of the envelope” methodology to estimate the optimum fracture spacing can be advantageous for everyday use and a starting point for completion optimization. This paper also discusses how deviations from ideal behavior, in terms of fracture and reservoir properties, can affect the well design.
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