Nodal Analysis for Unconventional Reservoirs—Principles and Application

Abstract

Summary Nodal analysis is the standard technique used to evaluate the performance of integrated production systems. Two curves represent the capacities of the inflow and of the outflow, and the intersection of the two curves gives the solution operating point. Limitations of traditional nodal analysis include: Results are offered only at a snapshot, not as a function of time. Inflow-performance-relationship (IPR) models are limited, with none available for shale gas wells. Analysis is performed on a well-by-well basis, with no account of multiwell interference. We propose a new nodal-analysis method that enables the study of transient production systems, such as unconventional reservoirs, with IPR models generated from a high-speed semianalytical reservoir simulator and outflow curves generated from a steady-state pipeline simulator. The use of analytical reservoir simulation allows accurate, reliable modeling of the real inflow system. The new approach studies the time-lapse behavior of the system, with consideration of production history and neighboring-well interference. This new method enables the study of transient deliverability at the wellhead, where the measurement is usually available, and shows the time-lapse relationship between wellhead pressure and production rate. We provide examples of wellhead deliverability and choke management and explain advantages of the method with case studies involving tight and shale wells. The method is also applied to design and optimize artificial lift in unconventional wells and to study the method's validity over time. In addition, we discuss an example of operational well dynamics with time-lapse nodal analysis. Furthermore, this new method generates discussion about some concepts that are often taken for granted—What should be the definition of IPR in a transient production system? On the IPR curve, is the zero-rate-pressure the reservoir pressure? Can IPR curves at two different timesteps cross each other? Finding the answers to these questions will help us better understand production systems. The commonly used productivity-index (PI) method is reviewed and compared with the new method. Results show that one should not use the PI method when well operational conditions change.