This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper URTeC 2023-4026063, “Numerical Rate Transient Analysis for Dry Gas Wells,” by Mathias L. Carlsen, SPE, Whitson; Curtis H. Whitson, SPE, Whitson and the Norwegian University of Science and Technology; and Mohamad M. Dahouk, SPE, Whitson. The paper has not been peer reviewed.
The complete paper outlines the importance of numerical rate transient analysis (RTA) for dry gas wells. The authors write that the simple, fully penetrating planar fracture model proposed is a useful numerical symmetry element model that provides the basis for the work presented. Synthetic and field examples are used in the complete paper to illustrate the application of numerical simulation to perform rigorous RTA.
RTA methods to correct for superposition and multiphase flow effects have been studied extensively. Analytically (In this context, “analytical” refers to linearization of partial differential equations) correcting for the combined effect of both superposition and multiphase flow at the same time is not possible because one needs to know saturations and pressure/volume/temperature (PVT) properties as a function of space for all times. Multiple cycles of hysteresis in a multiphase, complex PVT and relative permeability system are impossible to handle analytically.
The complete paper aims to achieve the following goals:
- Outline the importance of numerical RTA for dry gas wells
- Generalize the workflow for dry gas systems as opposed to only liquid-rich or oil reservoirs
- Incorporate geomechanical effects (pressure-dependent permeability) in the workflow
- Validate the workflow with both dry gas simulated data and real field data
The theory presented in this paper assumes a “symmetry element” model, in which the underlying numerical model represents one-quarter of a fracture (Fig. 1). The model is 1D in that no flow contributions exist beyond the fracture tips or beyond the fracture height. Thus, there is only one no-flow boundary, resulting in two dominant flow regimes over time—infinite acting linear flow, followed by boundary-dominated flow. In this study, only one permeability zone is assumed. Nevertheless, most of the underlying concepts presented in the complete paper also are relevant for more-complex reservoir geometries.
The authors devote a subsection of the complete paper to a discussion of RTA fundamentals before describing their own work. They move on to subsections describing important aspects of numerically assisted RTA, including key geometric variables; relationships between modified liner flow parameter, original gas in place, and well performance; a modification of a numerically assisted workflow well-known in the literature; and geomechanical effects.
