This paper presents a method to estimate induced fracture properties and reservoir geometry in multilayered gas reservoirs similar to the Travis Peak formation in East Texas. The Travis Peak is characterized by commingled production from small, high-permeability channel sands and blanket low permeability formations. Our specific objectives are to estimate fracture half-length, fracture conductivity, and area of the small, high-permeability layer in a two-layer reservoir system similar to the Travis Peak.
In this study, we developed constant rate drawdown type curves applicable for analyzing pressure buildup tests which follow production at constant bottomhole pressure. The results obtained from our type curves are used only as a starting point for a more rigorous numerical simulation (history-matching) study.
Our type curves assume the small, high permeability sand is a "channel" deposit, which is modeled with a 2x1 rectangle, whereas we assume the larger, lower permeability layer is a blanket-like deposit which can be modeled as a 160-acre square. A single vertical fracture of finite conductivity intercepts the wellbore and completely penetrates each layer, extending down the short side of the 2x1 rectangle.
Even though this paper presents a specific solution for a specific type of layered reservoir, it illustrates a general approach for analyzing other layered reservoirs with induced fractures and known geometry in the layers.
The objective of this study is to develop a method to estimate fracture half-length, fracture conductivity, and area of the small layer in multilayer formations where production from small, high permeability channel sands is commingled with production from larger, low-permeability sands with blanket-like deposition. This reservoir description has been encountered during the course of the Gas Research Institute's Tight Gas Sands project in the Travis Peak formation. However, it is also encountered in many low permeability formations throughout the world in addition to the Travis Peak.
The GRI project has concentrated its efforts in East Texas where Travis Peak gas production is established. Comprehensive studies have shown the Travis Peak formation is a fluvial-deltaic depositional system comprised of a middle sand-rich fluvial sequence overlain and underlain by marine-influenced delta fringe zones. The middle sand rich fluvial sequence can be further classified by several sets of distinct sand packages. These sand packages are where routine completions are being performed in the GRI project. A typical sand package consist of upper fluvial sands and a lower succession of interbedded sandstones and mudstones. The upper sandstones are fining upward lenticular channel deposits. These moderate sinuosity channels are relatively thin (15 ft thick) although they are quite permeable due to relatively clean quartz content. The lower sandstones are actually a succession of blocky low sinuosity braided channels. They are generally 30-40 ft thick and have been deposited in blankets of fairly large areal extent (160 ac or more). Due to the higher clay content, these sandstones are much less permeable than their fluvial counterparts.