Rate-Decline Analysis for Fracture-Dominated Shale Reservoirs

Abstract

Traditional decline methods such as Arps' rate/time relations and their variations do not work for wells producing from supertight or shale reservoirs in which fracture flow is dominant. Most of the production data from these wells exhibit fracture-dominated flow regimes and rarely reach late-time flow regimes, even over several years of production. Without the presence of pseudoradial and boundary-dominated flows (BDFs), neither matrix permeability nor drainage area can be established. This indicates that matrix contribution is negligible compared with fracture contribution, and the expected ultimate recovery (EUR) cannot be based on a traditional concept of drainage area. An alternative approach is proposed to estimate EUR from wells in which fracture flow is dominant and matrix contribution is negligible. To support these fracture flows, the connected fracture density of the fractured area must increase over time. This increase is possible because of local stress changes under fracture depletion. Pressure depletion within fracture networks would reactivate the existing faults or fractures, which may breach the hydraulic integrity of the shale that seals these features. If these faults or fractures are reactivated, their permeabilities will increase, facilitating enhanced fluid migration. For fracture flows at a constant flowing bottomhole pressure, a log-log plot of rate over cumulative production vs. time will yield a straight line with a unity slope regardless of fracture types. In practice, a slope of greater than unity is normally observed because of actual field operations, data approximation, and flow-regime changes. A rate/ time or cumulative production/time relationship can be established on the basis of the intercept and slope values of this log-log plot and initial gas rate. Field examples from several supertight and shale gas plays for both dry and high-liquid gas production, and for oil production were used to test the new model. All display the predicted straightline trend, with its slope and intercept related to reservoir types. In other words, a certain fractured flow regime or a combination of flow types that dominate a given area or play because of its reservoir-rock characteristics and/or fracture-stimulation practices all produce a narrow range of intercepts and slopes. An individualwell performance or EUR can be derived that is based on this range if the best 3-month average or the initial production rate of the well is already known or estimated. The results show that this alternative approach is easier to use, gives a reliable EUR, and can be used to replace the traditional decline methods for unconventional reservoirs. The new approach is also able to provide statistical methods to analyze production forecasts of resource plays and to establish a range of results of these forecasts, including probability distributions of reserves in terms of P90 (lower side) to P10 (higher side).

Production Patterns of Eagle Ford Shale Gas: Decline Curve Analysis Using 1084 Wells

“…Such as the Stretched Exponential (SE) decline model [19], the Power Law (PL) model [16], and Duong's model [20].…”

“…In the stretched exponential method, q i , D i , and n are undetermined parameters; τ is equivalent to (n/D i ) 1/n [20].…”

An approach to modeling production decline in unconventional reservoirs

“…Ilk (2010), after analyzing production (rate) data from shale gas wells, developed a power-law model and a method of applying the model that arbitrarily switched from the power-law decline behavior to an exponential decline behavior. Duong (2011) also observed that a loglog plot of q/N p versus time was a straight line. Based on this observation, he developed a power-law type model and a procedure for its application.…”

Characteristic Production Decline Patterns for Shale Gas Wells in Barnett

“…As the importance of shale gas has increased over the last decade new methods have been proposed to model the behavior exhibited by long horizontal wells with multistage hydraulic fractures in shale reservoirs. These methods include, but are not limited to, the Stretched Exponential (SE) model [25,26], the Power Law model [24], and Duong's model [27]. Two decline curve analysis models are used in the study -the Arps Hyperbolic model and the Stretched Exponential model.…”