Dynamic Nodal Analysis in Well Testing Interpretation

Abstract

In well testing, the subject of study is composed by the reservoir, wellbore and piping.

The traditional well testing interpretation approach focuses in the transient behavior of the pressure inside the reservoir. Flow rates are considered input information; generally assumed instantaneously constant during each flow period.

On the other side, the traditional nodal analysis concentrates in the pressure drops inside the wellbore and pipelines reducing the behavior of the reservoir to a pseudo-stationary state.

These simplifications can be applied for a limited period of time in high potential wells.

However, especially in low potential wells, the productivity index varies continuously, flow rates can not be approximated by constant values; the effects of liquid loading, slugging, and other regimes of multiphase pipe flow affect decisively the conditions in the reservoir. Understanding system's operation implies the analysis of every component and the relations between them.

Introduction
The models can be concatenated in series so that the output of one model becomes the input of the next.

The purpose of this paper is to show that integrating the tools each method provides, solving the models as a system in a dynamic and simultaneous way, significant advantages are obtained:

• Reproduce well conditions not detectable, or wrongly interpreted using separate analysis techniques.
  
• Improve the quality of models, increasing prediction and forecasting capabilities by eliminating the need of restricting assumptions.
  
• Be able to utilize every relevant measurement taken at different points and times.
  
• Validate acquired data, possibly detecting zones in which the simulation is not compatible with the measured or calculated information.
  
• Maximize confidence in obtained results, by using multiple criteria and convergent measurements.
  
• Enhance the value of each taken measurement as a control point of the simulation.

System Description
A typical well testing procedure consists in measuring transient pressures and flow rates simultaneously while inducing changes to the flow conditions.

As can be seen in Figure 1 the hydraulic system is composed of the reservoir, connected to the wellbore that conveys the fluid to surface, a flow control device such as a Choke or restriction, a surface pipe line to derive the flow to a separator or other metering device where rates are measured.

Pressures are taken at several points: at bottom, using a pressure and temperature gauge, at the well head, after the choke or start of surface pipe line and at the separator. Flow rates are commonly measured after separation.