Abstract Horizontal well technology has been widely used in developing gas fields. Very commonly, these wells are hydraulically fractured to improve productivity in low permeability reservoirs. A previously developed method, the Distributed Volumetric Source method (DVS), was applied to horizontal gas wells with or without fractures to predict well performance. The method is flexible and can be easily applied. The method provides an effective tool to evaluate horizontal well design and well stimulation design for gas wells.
In this paper, we conducted a well performance study by applying the DVS method to typical gas formations in East Texas Basin, San Juan Basin, and Appalachian Basin.. The objective is to determine the best practice to produce from horizontal gas wells. With the transient flow feature of the DVS method, well placement for multiple horizontal wells in a defined drainage area can be studied, and the limit of well spacing and wellbore length is identified. For fractured wells, well performance of a single fracture and multiple fractures are compared, and the effect of the number of fractures on productivity of the well is presented. Realizing that reservoir permeability and anisotropy ratio are the critical parameters in developing low-permeability gas fields, the effect of permeability on well performance, well placement and fracture treatment design is addressed in the paper.
Introduction Development of low permeability tight gas reservoirs is becoming attractive to the energy supply problem we are facing today. The lack of flow path for gas is the biggest limitation for tight gas formations. In order to overcome that limitation, horizontal wells have been drilled, and many of them were furthermore fractured to expand the contact between the well and the formation.
To study the effects of reservoir properties, well structures, and fracture treatment design, on well performance in tight gas formations, we need a simple but robust method to predict the performance of horizontal wells, with or without fractures. Horizontal well models have been presented in many literatures in the past. In order to arrive at an analytical solution, different boundary conditions have to be assumed. Thus, the models have been divided to steady-state models (Butler 2000, Furui et al 2003, and Zhu 2006) pseudo-steady state models (Babu and Odeh, 1988 and 1989), and transient flow models (Goode and Thambynayagam, 1987, Ozkan 1988 and 1989). For low permeability formations, transient flow period for a horizontal well may be significantly longer than for conventional formations. A model that can handle both transient and pseudo-steady state flow conditions will be convenient.
A previous study presented a Distributed Volume Source method (Valko and Amini, 2007). The method solves the flow problem in a box-shaped reservoir with a box-shaped volumetric source. The shape of the sources is flexible, easily portraying a horizontal well with or without fractures. A smooth transition between transient and pseudo steady state flow regions was achieved by the method. The main concept of the method was to find the analytical solution for the response of a closed rectilinear system to an instantaneous volumetric source. This solution is then integrated over the time to provide the response for a continuous volumetric source. Application of the principle of superposition was used to simulate multiple fractures along a horizontal well. The method is developed for a source with uniform flux over its volume. The extension of method to cases with infinite conductivity is made possible by dividing the source into segments of uniform-flux sources.
The Distributed Volumetric Source (DVS) method was extended to gas wells (Zhu et. al, 2007), and that will be the approach used in this study.
Application of DVS method to Case Studies In this paper, we apply the DVS method to several typical tight gas fields in the US in different basins to study the effect of designing horizontal wells and fracture treatments on well performance. Fig. 1 shows all the tight gas basins in the USA. This paper uses data from East Texas Basin, San Juan Basin, and Appalachian Basin. It is important to point out that all of these basins have their unique characteristics, and the results and conclusions made from the study are based on each individual basin, and may not be used as general conclusion.