In the last few years, the oil industry has turned its focus from shale gas exploration to shale oil/tight oil. Some of the important plays under development include the Bakken, Eagle Ford, and Niobrara. New decline curve methods have been developed as possible alternatives to the traditional Arps model for use in shale gas wells, but much less study has been done to verify the accuracy of these methods in shale oil wells. There is a great amount of uncertainty about how to calculate reserves in shale reservoirs with long multi-fractured horizontals, since most of these wells of have not yet been produced to abandonment. Although the Arps model can reliably describe conventional reservoir production decline, it is still uncertain which empirical decline curve method best describes a shale oil well to get a rapid assessment of expected recovery.
The models on which we concentrated were Arps with a 5% minimum decline rate, the stretched exponential model (SEPD), and the Duong model.
Our focus began in the oil window of the Eagle Ford, but we ultimately chose to study the Elm Coulee field (Bakken formation) instead to see what lessons an older tight oil play could lend to newer plays such as the Eagle Ford. Contrary to the assumption that these wells were still in linear flow, we found evidence from diagnostic plots that many horizontal wells in the Elm Coulee that began producing in 2006 and 2007 have entered boundary-dominated flow. In order to accommodate boundary flow we modified the Duong and SEPD methods such that once boundary-dominated flow begins the decline is described by an Arps curve with a b-value of 0.3.
We found from hindcasting that using early production history, up to the first six months, is generally detrimental to accurate forecasting in the Elm Coulee. This was particularly true for the Arps with 5% minimum decline or the Duong method. Early production history often contains apparent bilinear flow or no discernible trend. There are many possible reasons for this, such as the rapid decrease in bottomhole pressure and production of fracture fluid.