BEST PRACTICES IN TESTING AND ANALYZING MULTILAYER RESERVOIRS

Layered formations are the norm rather than exception among oil and gas reservoirs. Knowing the individual layer properties is important for making development strategies. It plays a significant role in secondary recovery. This paper presents best practices in the design, execution, and analysis of multilayer pressure transient tests in the "super‑giant" Tengiz oil field in western Kazakhstan. Our experiences prove that the use of multilayer testing and analysis techniques result in a lower level of uncertainty in layered reservoir characterization than can be obtained by the simpler commingled pressure transient test alone. The formations in the platform area of Tengiz consist of multiple zones with different reservoir pressures and cross flow often occurs during shut‑in periods. In some cases, the impact from different layer pressure and skin distribution could result in 100% difference in individual layer permeability estimations. To reduce the uncertainty, a Selective Inflow Performance (SIP) production logging technique is used to measure the bottom‑hole pressures and flow contributions of individual layers during a commingled pressure transient test. Pulsed Neutron Capture logs also provide information about stimulation effectiveness (skin) in each zone. Then a robust step by step analysis method developed to utilize all these data and information is applied to obtain individual layer properties. The best practices of using this methodology and the associated uncertainty are discussed in this paper. The challenge in characterization of layered reservoirs lies in the large number of unknown parameters. The approach described in this paper resolves enough of these unknowns in a systematic fashion that a more direct solution is obtained with resultant lower uncertainty. By sharing our testing experiences in Tengiz we hope to provide means and ideas for petroleum engineers to face the challenge and reduce the uncertainty in the development and management of multilayered reservoirs.