Abstract
Tight sand and shale gas reservoirs are becoming bigger players in natural gas supply system as easily developed gas resources become scarcer, natural gas prices go higher, and more efficient technologies become available to develop these hard-to-access reservoirs. Although these reservoirs are hard to develop, recent technologies such as better hydraulic fracturing are making them more productive.
Production and development of these types of reservoirs depend heavily on the hydraulic fracturing and the existence of natural fractures in the formations. Since the permeability of the matrix is very low no commercially feasible gas production can be achieved without fracturing. Therefore, characterizing and understanding the fractures have become of great importance.
We have proposed using a combination of methodologies for fracture characterization and simulation in tight gas reservoirs. Dual Porosity Dual Permeability modeling has been used for simulating fractured reservoirs. For a commercial simulator to be able to simulate the fluid flow in a reservoir, properties such as permeability and porosity should be known which are, however, not adequately measured. The most readily available measurements are indirect observations such as bottom-hole pressures and production rates. Using indirect measurements to constrain the model parameters requires solving an inverse problem. In this study, Ensemble Kalman Filter (EnKF) is used for the estimation purposes. EnKF is a Monte Carlo based, minimum mean square error (MMSE) estimation tool that generates multiple realizations based on the prior knowledge of the reservoir, in which the model performance is improved via sequential assimilation of the observations.
Our proposed methodology is tested on several synthetic 2D reservoir models containing a number of fractures, some intersecting the wells. The results have proven the applicability and the advantage of this methodology. We have shown that EnKF can be used efficiently to characterize a fractured reservoir when the approximate orientation of the fractures is known which may be obtained from the stress-strain field data in the reservoir.
