Testing and Interpretation in Layered Reservoirs
Authors: Ehlig-Economides Christine A. 
Published: December 2007

Introduction

This paper summarizes recently developed techniques for testing layered reservoirs that field tests have shown to be successful. The effects of layering on. conventional pressure-transient tests have been studied by a number of investigators, starting with a model for the commingled reservoir system by Lefkovits et al. in 1961. An extensive list of publications on this subject is given in Ref. 2.

In a multilayered reservoir, each layer may have distinct values for its thickness, porosity permeability, and formation skin factor. In gas -wells, each layer may be subject to an additional skin because of non-Darcy flow near the wellbore. Layers may communicate vertically in the reservoir by formation crossflow. For interpretive purposes, the reservoir system is organized into zones, which are groups of communicating layers. Between adjacent zones there are impermeable barriers. Interpretation procedures assume that each zone is either infinite-acting or confined by either a constant-pressure or no-flow outer boundary. Finally, single-phase flow is assumed in the formation.

Each change in the surface rate causes downhole transients, which depend on the system properties. Under commingled flow, the bottomhole flowing pressure is affected by the total system. The fluid flow rate to or from each layer, however, is a function of the properties of the layer and the adjacent communicating layers. Often, there may be flow in the wellbore from one layer into another. Determination of the layer properties by these techniques is not prevented by the presence of wellbore crossflow and/or wellbore storage.

Layered Reservoir Test Data Acquisition

The layered reservoir test (LRT) relies on a combination of measurements, including production log surveys, and pressure and flow-rate transients acquired with the sensors maintained in a stationary position. The simplest LRT technique is called selective inflow performance (SIP). To perform the SIP test, it is necessary to achieve stabilized flow in the well for several surface injection or production rates. Under stabilized flow conditions, the wellbore pressures and flow rates are surveyed, and flow for each perforated interval is computed as the difference between the flow rates measured above and below the interval.

To determine permeabilities and skin factors for each layer, it is necessary to perform a multilayer transient (MLT) test." A typical MLT test sequence is shown in Fig. 1. Following an initial pressure and flow-rate survey, the flowmeter sensor is positioned above one of the perforated intervals. The surface flow rate is changed, and the tool is left in the same position until nearly the end of the flow period. Then a pressure and flow-rate survey is taken, and the tool is relocated above another perforated interval for the next transient measurement. As shown in Fig. 1, this procedure is repeated for each interval to be tested.

If the well reaches stabilized steady- or pseudosteady-state flow before the end of each flow period, then the pressure and flow-rate surveys can be used for SIP analysis. If the system has reached stabilized infinite-acting radial flow at the time of each vertical survey, then the SIP analysis is modified slightly by use of the transient SIP interpretation procedure.

Because flow-rate measurements are required for the analysis, it is important to conduct the test at flow rates high enough to be measurable in the wellbore.

It is useful, although not required, to run a conventional pressure buildup test at the end of the LRT sequence.. Hence, the simulated test sequence in Fig. 1 is terminated with a pressure buildup.

JPT

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