Best Practices in Testing and Analyzing Multilayer Reservoirs
Authors: Denney Dennis 
Published: February 2015

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper SPE 132596, ’Best Practices in Testing and Analyzing Multilayer Reservoirs,’ by Yan Pan, SPE, Chevron Energy Technology Company, and Michael Sullivan and David Belanger, SPE, Tengizchevroil, prepared for the 2010 SPE Western Regional Meeting, Anaheim, California, 27-29 May. The paper has not been peer reviewed.

Layered formations are the norm rather than the exception among oil and gas reservoirs. Knowing individual-layer properties is important for development strategies, especially for secondary recovery. A set of best practices was established in the design, execution, and analysis of multilayer pressure-transient tests in the Tengiz oil field in western Kazakhstan. Use of multilayer testing and analysis techniques provided a lower level of uncertainty in layered-reservoir characterization than can be obtained with a simpler commingled-pressure-transient test alone.   

Introduction

Formations in the platform area of Tengiz consist of multiple zones with different reservoir pressures. Therefore, crossflow often occurs during shut-in periods. In some cases, the effect from different layer pressures and skin distributions could result in 100% difference in individual-layer permeability estimations. To reduce uncertainty, the selective-inflow-performance (SIP) production-logging technique is used to measure the bottomhole pressures and flow contributions of individual layers during a commingled-pressure-transient test. Pulsed-neutron-capture (PNC) logs are used to gain information about stimulation effectiveness (skin) in each zone. Then, a robust step-by-step analysis method that uses all data and information is applied to obtain individual-layer properties. The best practices for use of this methodology and the associated uncertainty are detailed in the full-length paper.

The challenge in characterizing layered reservoirs is the large number of unknown parameters. The approach described in the paper resolves enough of these unknowns in a systematic fashion for a more direct solution to be obtained, with a resulting lower uncertainty.

Tengiz Field

The central platform of the Tengiz carbonate reservoir is dominated by matrix porosity. The surrounding rim and flank regions exhibit evidence of fracture networks. Three major zones contribute to oil production. The upper reservoir units have been produced for a longer period of time and, therefore, are more depleted.

All Tengiz wells are essentially vertical and are producing single-phase oil with flowing bottomhole pressure above the bubblepoint, which provides good conditions for production-log profiling. Because most wells produce no water, the connate water has low salinity, and irreducible water saturation is approximately 10%, elevated chlorine-concentration levels remained in the near-wellbore region of the formation after acid stimulation, which increased the chance for a PNC log to detect acid effects.

Sour-gas injection for miscible flooding began in late 2007 in the central-platform area. Understanding individual-zone formation properties was important in the success of the hydrocarbon-miscible flood.