Characterization of a Carbonate Reservoir Using Pressure-Transient Tests and Production Logs: Tengiz Field, Kazakstan
Authors: Chambers K.T. , Hallager W.S. , Kabir C.S. , Garber R.A. 
Published: December 2007

The combination of pressure-transient and production log analyses has proven valuable in characterizing reservoir flow behavior in the giant Tengiz field. Among the important findings is the absence of clear dual-porosity flow. This observation contradicts an earlier interpretation that the reservoir contains a well-connected, natural fracture network. Fracturing and other secondary porosity mechanisms play a role in enhancing matrix permeability, but their impact is insufficient to cause dual-porosity flow behavior to develop.

Flow profiles measured with production logs consistently show several thin (10 to 30 ft) zones dominating well deliverability over the thick (up to 1040 ft) perforation intervals at Tengiz. A comparison of production log (PL) results and core descriptions reveals a good correlation between high deliverability zones and probable exposure surfaces in the carbonate reservoir.

Contrary to earlier postulations, results obtained from pressure-transient and PL data at Tengiz do not support rate-sensitive productivity indices (PI's). Inclusion of rate variations in reconciling buildup and drawdown test results addressed this issue.

We developed wellbore hydraulic models and calibrated them with PL data for extending PI results to wells that do not have measured values. A simplified equation-of-state (EoS) fluid description was an important component of the models because the available black-oil fluid correlations do not provide reliable results for the 47 API volatile Tengiz oil. Clear trends in reservoir quality emerge from the PI results.

Introduction

A plethora of publications exists on transient testing. However, only a few papers address the issue of combining multidisciplinary data to understanding the reservoir flow behavior: Refs. 1–4 are worthy of note. We used a synergistic approach by combining geology, petrophysics transient tests, production logs, and wellbore flow modeling to characterize the reservoir flow behavior in the Tengiz field. Understanding this flow behavior is crucial to formulating guidelines for managing the field.

Permeability estimation from pressure-transient data is sensitive to the effective reservoir thickness contributing to flow. Unfortunately, difficulties associated with the calibration of old openhole logs, sparse core coverage, and a major diagenetic overprint of solid bitumen combine to limit the identification of effective reservoir at Tengiz based on openhole log data alone. Consequently, production logs have been used to identify effective reservoir in terms of its flow potential. A limitation of production logs is that they only measure fluid entering the wellbore and are not necessarily indicative of flow in the reservoir away from the well.

Pressure data from buildup and drawdown tests, on the other hand, provide insights into flow behavior both near the well and farther into the reservoir. The combination of pressure-transient analysis using simultaneous downhole pressure and flow rate data along with measured production profiles provides an opportunity to reconcile near-wellbore and in-situ flow behavior.

Expansion of reservoir fluids along with formation compaction provide the current drive mechanism at Tengiz because the reservoir is undersaturated by over 8,000 psia. As the field is produced, reservoir stresses will increase in response to pressure decreases. Increased stresses can significantly reduce permeability if natural fractures provide the primary flow capacity in the reservoir. Wells producing at high drawdowns provide an opportunity to investigate the pressure-sensitivity of fractures within the near-wellbore region.

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