Dependence of Key Estimates From PTA on Flow Paths in Heterogeneous-Reservoir Systems

Abstract

Although parameter estimates from pressure-transient analysis (PTA) in homogeneous reservoir systems are well understood, such estimates from heterogeneous systems can be illusive if the flow paths of fluid movement in the reservoir are not captured well. This study brings in the perspectives of parameter estimations (for example, flow capacity, skin factor, and volume of the investigated region) based on the fluid-flow paths. The disseminated knowledge will help mitigate potential errors due to misunderstanding flow paths. A number of idealized multi-layer models, and geological sector models have been built to generate pressure responses, and to understand the flow paths towards wells. Flow paths are controlled through horizontal and vertical permeability, and skin factors in perforated and open intervals in the wells. In each case, the flow capacity, skin factor and volume investigated are estimated from the generated pressure responses to show the dependence of estimated parameters on the flow paths in the reservoirs. We will also determine the shape and size of the region investigated with changes in flow paths during the numerical experiments. In this paper, we highlight the pitfalls of interpreting pressure-transient data for layered, heterogeneous reservoir systems. The fluid-flow paths in complex reservoirs have direct impacts on the transient-pressure behavior, and on the estimates of key reservoir parameters. Natural paths that fluids flow along deep inside the reservoir can be modified in the reservoir models based on the engineer's input parameters. This can inadvertently result in modifications of the flow capacity, investigated volume, and reserves estimate. The outcomes of these numerical experiments with analytical and numerical models will certainly challenge the existing understanding of the reservoir flow capacity, and the widely-accepted radius of investigation concept. Conventionally, these parameters are explained without any regard for the flow paths in reservoirs. This paper shows that the flow paths indeed have direct impacts on the transient-pressure behaviors, and on the key parameters as flow capacity, skin factor and volume of investigated region in heterogeneous reservoir systems. This study establishes a relationship between the parameter estimates and the flow paths in heterogeneous systems. Moreover, running into the pitfalls while building transient and simulation models for heterogeneous reservoir systems will be mitigated with the knowledge gained here.