Abstract
Steam assisted gravity drainage (SAGD) is a widely used thermal recovery method. During steam injection in interbedded sands, the portion of the deposits above the vertical permeability barriers is likely not recoverable with current practices. This paper examines the reservoir engineering aspects of using multistage fracturing in SAGD operations in interbedded sands such as IHS, where the vertical permeability barriers impede the gravity drainage process.
Assuming that the permeability barriers can be targeted and broken with vertical hydraulic fracturing treatments before steaming, it is very likely that the steam can pass through the shales, effectively heat the otherwise isolated oil, and eventually drain it. The flow mechanism and the recovery response of a fractured SAGD well are discussed and it is shown that the suggested process has a great potential to improve production rate and recovery factor. The desired fracture properties and expected production rates are obtained by using both analytical and numerical reservoir engineering techniques. Some quick assessment criteria are also introduced.
Analytical equations to estimate the SAGD peak rate were derived for various sand-shale sequences and fracture configurations. The analytical equations can be used for quick evaluation of the fracturing potential in such SAGD wells. A correction factor was suggested to compensate for simplifying assumptions.
Through numerical simulation, a matrix of sensitivity cases was developed with variation of key parameters such as fracture permeability, fracture half-length, fracture spacing and orientation. It was shown that fractures with short half-length but high conductivity are required for the process to work. Using the sensitivity results, an optimum range for key fracturing parameters was determined. Furthermore, a dimensionless fracture conductivity criterion was used as a general design criterion. The issues around fracturing design and sand-shale geomechanics are presented in a separate paper.
The multistage fracturing technique in horizontal wells was first used in tight and shale gas plays, and different variations of the field execution of this technique are now widely available. To our knowledge, this work is the first detailed examination of the potential of multistage fracturing for improving SAGD in interbedded pays. It is shown that, if this technique can be combined with in-situ steam operations, some of the problems with steaming interbedded pays can be addressed. In particular, it is shown that the recovery can be substantially improved to be comparable to clean sands. However, the use of multistage fracturing in SAGD process also introduces unique execution challenges that require thorough analysis and design, which were the focus of the companion paper (Saeedi and Settari, 2016).
