Previously, ExxonMobil had undertaken a multi-disciplinary approach to develop and integrate the required technologies for design, implementation, and evaluation of acid treatments in thick heterogeneous carbonate reservoirs.1 RasGas, in collaboration with ExxonMobil, has customized the technologies and integrated methodology for application in a major field in the Middle East with a high level of success. Acid placement and diversion are critical to achieving effective stimulation in heterogeneous carbonate reservoirs. While permeability is a major factor in the distribution of acid along a completion for many reservoirs, pre-stimulation skin damage, intermixed rock types with different acid-rock wormholing characteristics, distance between zones, and differential reservoir depletion also play important roles in the effective stimulation of the reservoirs. Important steps in the integrated methodology developed and implemented for matrix acidizing include:determine the stimulation requirements given the well/reservoir objectives,characterize the various rock types present in the formation,develop an integrated perforation/stimulation strategy,conduct appropriate laboratory tests with representative field core plugs,model the stimulation process with tools calibrated to the formation of interest,develop field procedures and implement the treatments as per design,evaluate stimulation effectiveness, andoptimize treatments based on post-stimulation performance and operational constraints. This paper features some of the technologies that have been developed and describes the integrated methodology used to effectively stimulate thick carbonate reservoirs in the Middle East. Introduction The technology of carbonate matrix stimulation has advanced significantly over the past 10 years through innovative laboratory testing, new fluid developments, and advanced computer models to simulate the process. However, the existing approaches are not sufficient to meet the challenges of optimized stimulation of wells in massive carbonate reservoirs. Typically, the intervals to be produced in these reservoirs are very thick and highly heterogeneous. The permeability can range from a few milliDarcies to several Darcies. ExxonMobil and RasGas have jointly developed an integrated methodology to optimize matrix stimulation of the Khuff reservoir, a large, complex carbonate reservoir in the North Field of Qatar. The integrated methodology is a continuous process which consists of five main elements: reservoir objectives, completion strategy, stimulation design, implementation, and evaluation. The process was introduced by ExxonMobil in an earlier paper, which focused on the general laboratory testing and process modeling.1 This paper discusses how the integrated carbonate stimulation methodology was customized and implemented by RasGas and ExxonMobil as a critical component of the North Field development, focusing on the stimulation strategy, design, and results obtained. The carbonate stimulation design methodology cycle, specifically as applied to the North Field, is shown in Figure 1. The reservoir objectives for the development of large fields often span multiple heterogeneous producing horizons containing many layers with varying rock properties. From a resource standpoint, the ultimate objective is to economically extract the maximum amount of hydrocarbon from the reservoir. In order to accomplish this, the optimum production flow profile for reservoir depletion is required.

Advancement in fluid diverter technology has enabled successful initial stimulation of long interval carbonates without mechanical isolation, in excess of 1000 ft. by using in-situ crosslinked polymer systems and viscoelastic surfactant systems. These fluids build viscosity upon reacting with the formation, enhancing diverter performance. However, heavily stimulated, i.e. wormholed, formations require a step-level change in diversion performance to effectively stimulation new intervals, or previously under-stimulated layers. This paper will present the qualification and optimization of degradable fibers for use in re-stimulation of carbonate formations.The key performance aspects desired and evaluated are: (1) ability to block existing wormholes near the wellbore, as opposed to needing to fill the entire wormhole structure, (2) permeability and diversion capability of resulting fiber bridge, and (3) confirmation of degradation and cleanup in a dry gas environment within a reasonable timeframe. New experimental methods were developed to address these requirements, including gas/fluid degradation testing, bridging tests with slots to represent wormholes and fractures, and bridging experiments with 3D synthetic models of a wormhole structure. To be able to run repeatable experiments, wormholed structures were printed with a 3D printer using high-resolution CT scans from an actual wormholed core plug.Testing results showed the ability of the fibers to bridge within the wormhole network and provided guidance in designing the fiber diverter stages for already-stimulated wells. Addition of degradable particulates was shown to further improve wormhole bridging and diversion performance. Modeling the wormhole diversion process within advanced stimulation software identified additional optimizations, such as increased stage size and increased fiber concentrations to achieve desired diversion. A related presentation will discuss the successful re-stimulation of three high-rate gas wells (B. Clancey 2015).

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An extensive big bore gas well drilling and completions program is in progress to develop the giant North Field, offshore Qatar. Benefits of the big-bore concept, compared to the prior 7-in. monobore design, include reduced development costs by requiring fewer wells, and deferred installation of compression by providing higher flowing wellhead pressures. This is the widest known application of a big bore design in one single field and involves a large number of wells. Many of these wells are drilled and completed to date and believed to be among the world's most prolific gas producers. These are also the first offshore wells to incorporate big bore well design for large-scale field development and feature industry advances in equipment design and manufacture. An extensive system of detailed design review, equipment performance testing, and quality programs has been implemented to meet challenging project requirements for well reliability. The wells feature a tapered tubing design known as the "Optimized Big Bore" (OBB). This paper discusses the challenges of planning and executing these OBB wells. It reviews the initial OBB well design, anticipated North Field drilling challenges, critical equipment specifications, and design revisions resulting from optimisation efforts over the past four years. Introduction Discovered in 1971, the North Field of Qatar is the world's largest non-associated gas field, extending over 6,000 km2, and contains approximately 900 TCF of abnormally pressured natural gas. Development has taken place in several stages, commencing with initial production by Qatar Petroleum in 1991 to supply the domestic gas market. This was followed in the mid-to-late 1990s by RasGas Company Limited (RasGas) and other projects to provide LNG to export markets. More recently, the growth in global LNG demand has sparked much more extensive development. RasGas is responsible for the development of a large area of the North Field, covering more than 500 Km2, to provide natural gas supply for both new LNG trains and the growing domestic gas market. The RasGas expansion plan requires a large number of wells to be drilled in the period 2002 to 2010 from several new wellhead platforms. Production is from the massive Khuff carbonate formation, which includes several productive intervals (K1, K2, K3, and K4), situated at approximately 10,000 ft TVD. This would differ from the earlier projects of the 1990's that developed only a part of the Khuff reservoir. In total, RasGas will operate more than 90 wells producing a total of 8 Bscfd. Well deliverability from the K1 to K4 reservoirs is very high due to both formation quality, near initial reservoir pressure, and high net pay thickness. Non-hydrocarbon gas composition varies between the four Khuff intervals within each well and also across the field. Well designs evolved from packer / tubing completions intended to produce at 60 MMscfd in the initial Qatar Petroleum project, to high-rate 7-in. monobore wells in the first LNG projects able to deliver in excess of 100 MMscfd.1 However, it was recognized by RasGas and its major shareholders Qatar Petroleum and the former Mobil Oil (now ExxonMobil) that higher capacity well designs, capable of producing in excess of 150 MMscfd, could be suitable for the planned RasGas expansion projects. Although individual wells would be more expensive to construct, there were two significant advantages that would result in a more cost effective project. These would be:

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