Horizontal well technology is now considered a standard completion practice in unconventional gas reservoirs. With significant improvements in the drilling and completion technology, many tight gas and shale gas prospects have become economically viable. Optimizing location, distribution and the number of stages of hydraulic fractures is an important issue in tight gas reservoir completions, particularly for horizontal and complex wells.
The paper presents a fast approach to optimizing well completions in tight gas reservoirs using a rigorous semi-analytic computation of well drainage volumes in the presence of multiple stages of hydraulic fractures. Our approach relies on a high frequency asymptotic solution of the diffusivity equation and emulates the propagation of a ‘pressure front’ in the reservoir along gas streamlines. The proposed approach is a generalization of the radius of drainage concept in well test analysis (Lee, 1982). The streamlines are computed from the pressure and velocity distribution derived from finite difference simulation. This makes the approach completely general and capable of handling complex spatial reservoir heterogeneity and arbitrary well conditions. Furthermore, the streamline approach is visual, intuitive and allows us to examine the interactions between the hydraulic fractures, reservoir heterogeneity and the implications on the drainage volumes and EUR calculations.
A field example is presented to demonstrate the application of our approach by optimizing well completions in a horizontal well recently drilled in the Cotton Valley formation. We first apply the proposed drainage volume calculations to an existing vertical well and identify its ‘region of influence’ along with the potential interference from the proposed horizontal well. We then apply the drainage volume concept to the proposed horizontal well and examine the effects of different number of hydraulic fracture stages. The combined drainage volumes from the vertical and horizontal well are calculated as a function of the number of fracture stages to determine the point of diminishing return and to optimize the number of fracture stages. The results are found to be consistent with independent analysis based on rate profiles from numerical simulation and NPV calculations.