Abstract

Pressure transient tests of wells completed in multi-layer reservoirs have always been and continue to be a challenge for interpretation. Hence, characterizing layer properties from well tests, and determining and monitoring individual layer performance in commingled completions are complex and intensive tasks which could have significant impacts on well and reservoir management. Without accurate assessment of stimulation effectiveness and dynamic skin mechanisms, potential gains in long-term production may never be realized through appropriate action.

This paper discusses a hybrid approach for synergizing multi-layer pressure transient analysis with production logging analysis of flow and pressure profiles while accounting for carbonate matrix acidization physics. This approach uses two completely different but complementary tools, which are the existing multi-layer pressure transient analysis option in a pressure transient analysis package and a post-completion inflow performance analysis suite developed by the ExxonMobil Upstream Research Company to analyze carbonate acid stimulation effectiveness for RasGas wells. Based on field experience and acidized wormhole growth physics, RasGas and ExxonMobil jointly developed a new approach to multi-layer characterization using a workflow synergizing pressure transient analysis and inflow performance analysis to analyze post-completion well tests. A field example is described to illustrate the advantages and added value of enhanced understanding of strongly multi-layer producing reservoirs.

Background

RasGas is one of the major operators of the North Field, offshore Qatar. The North Field is the largest non-associated gas reservoir in the world. The subsurface formation of the North Field, Khuff, is a multi-layered carbonate formation. The Khuff reservoir is formed of four different and non-communicating reservoirs: K1, K2, K3 and K4.

As in most carbonate reservoirs, the Khuff lithology is a complex stratification of limestone (much of which is moldic) and dolostone in which the permeability varies by several orders of magnitude. The huge variability of the Khuff reservoir lithology, sometimes even within the same flow unit, is demonstrated by the distinctly variable MDT pressure profiles in a producing (dynamic) reservoir. Defining flow units is often a challenge for geologists and petrophysicists. The work presented in this paper presents a practical approach to quantifying reservoir flow behavior by generalized flow units.

Most RasGas wells are commingled producers across all four Khuff reservoirs. These wells are acid stimulated in multiple stages. Stimulations are designed so that some intervals (sub-layers) are treated more than others. This optimized stimulation design is driven by complex reservoir lithology, the large net pay of the reservoir to be treated, and operational requirements for safety and cost effectiveness.

The integrated pressure transient/post-completion analysis technique described in this paper was developed based on the company's long experience in dealing with the challenges of monitoring and understanding the well and reservoir performance of commingled producers. Integrated pressure transient/post-completion analysis establishes the baseline performance of a well for proactive assessment of the underlying causes of changes in well performance over the course of production.

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