This paper shares RasGas Company Limited (RasGas) experience in developing and implementing a production optimisation technology for reservoir management, surveillance and production sustainability. RasGas has recently implemented the Production Optimisation, Management and Planning Tool (PROMPT) with the help of PETEX consultants. The tool was built by integrating the PETEX suite of software (PROSPER/GAP/RESOLVE/IFM/IVM). The entire production system was modeled from wellbore to slug-catcher and connected to a high frequency real-time database for real-time monitoring and system updates. The PROMPT system is model centric and relies on rigorous physics, while its strength relies on using multiple seamless automatic and manual workflows. Many key calculations take place automatically and continuously however, other central workflows rely on engineering judgment such as well models updates.The PROMPT system has successfully demonstrated its reliability in supporting RasGas efforts to achieve long term production deliverability and secure RasGas' contractual Liquefied Natural Gas (LNG) demand by meeting the LNG production targets and maximising recovery. This is attained by producing the field/wells per the optimum depletion strategy while honoring facility constraints, system availability (well/platforms/pipeline, planned and unplanned downtime, etc.) and operational limits.RasGas uses the PROMPT system to generate well production guidelines as per the optimum reservoir depletion strategy to meet short term production targets. The PROMPT platform is equipped with an optimizer "Excel Solver" where the desired depletion strategy is coded and implemented. This depletion strategy is translated to actuality by generating short-term production guidelines on a regular basis while honoring the production system constraints. PROMPT is effectively used for real time monitoring and compliance with production guidelines, such as monitoring deviations of daily production from predefined targets, and for making well rate adjustments during planned/unplanned shutdowns or increased demand. It gives the engineers the ability to test different well operating strategies in off-line simulation to fine-tune production guidelines to meet changing field conditions and enables effective data integration between RasGas engineers in the Sub-surface group with those in the Operations groups.

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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Monitoring individual layer performance (in commingled completions) and integrating the results into an overall understanding of field performance has always been a challenge. Overcoming this challenge allows for a better, layer-by-layer, understanding of the reservoir, and therefore will have a significant impact on well and reservoir management. This paper presents a success case for the integration of a novel approach, and specific applications are highlighted. The Khuff Formation of the North Field is a multi-layered carbonate reservoir formed from four main reservoirs; K1, K2, K3 and K4. As most of the RasGas wells are completed commingled through the main Khuff reservoirs, a hybrid approach for integrating multi-layer pressure transient analysis (PTA) with production logging tools (PLT) analysis of flow and pressure profiles was developed. The process also accounts for the physics of carbonate matrix acidisation. The outcome of this technique has helped RasGas better assess the stimulation effectiveness in commingled wells, and establish baseline performance for individual layers. This work was used to build improved inputs for reservoir simulation models, thus providing more accurate predictions of reservoir performance. Additional benefits of this technique have been: 1) identification of wells with impaired productivity (candidates wells for re-stimulation), 2) a better understanding of the stim jobs (areas for improvement for future jobs), and 3) an improved understanding of log kh vs test kh variation (to understand specific questions on well performance). This paper discusses the outcomes, applications, and the added value of this integrated methodology. The paper will also demonstrate examples where the technology was applied and the value that was added to the ongoing surveillance and future development activities.

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