Practical Application of Variable Rate / Variable Pressure Drop Decline Curve Analysis: Valdemar Lower Cretaceous Case History

Abstract

Abstract Application of conventional decline curve analysis following Arps' method is limited to situations where flowing bottom-hole pressure is constant. This paper describes the application of Blasingame decline curve analysis in a low permeability reservoir where flowing bottom-hole pressures are reduced in frequent and managed steps. The Valdemar Lower Cretaceous Field is a low effective permeability (from 0.1 to 1 mD) chalk reservoir developed offshore in the Danish North Sea through long horizontal wells (horizontal sections of approximately 10,000 ft) with up to 16 sand-propped fractures. Pressure drawdown in Valdemar wells is managed by controlled lowering of the flowing bottom-hole pressure over time in order to avoid destabilization of the formation and well failure due to chalk production. These changes in operating conditions, in addition to reservoir complexity and challenging architecture of the wells make conventional methods (simulation, Arps' decline curve analysis and rate transient analysis) for production forecasting and well performance analysis either not adequate or difficult to implement. These issues have been resolved by applying advanced decline curve analysis to the variable rate / variable pressure drop theory. Application of the Blasingame Theory for the Valdemar Lower Cretaceous Field using available commercial software is complicated as it requires input parameters which are not very well known such as fracture half-length, fracture conductivity and number of producing fractures. In addition, uncertainty in fluid and reservoir properties can make the task even more challenging, since a non-unique history match often results with subsequent predictions outside a reasonable range. This paper presents a clear step by step, performance-based methodology of how to apply the Blasingame Theory to decline curve analysis. The methodology does not depend on detailed knowledge of well and fracture dimensions and it only requires the initial reservoir pressure, historical oil rates and flowing bottom-hole pressures to be known for production forecasting. This paper also presents the practical application to the Valdemar Lower Cretaceous Field. This methodology has been demonstrated to work in the Valdemar Lower Cretaceous Field and it is currently the key tool used to evaluate well performance and to estimate reserves and resources associated with different production strategies (drawdown, gas lift, etc.).