Effective stimulation of thick, layered carbonate reservoirs is required to reach production targets for present and future demand. It is common that the completed intervals intersect several layered reservoirs, commingle multiple zones, and extend hundreds to more than a thousand feet in length. Even with the best petrophysical measurement and interpretation available in the industry, the key formation parameters considered in acid stimulation are still clouded by uncertainty. Several matrix acidizing techniques have been applied to stimulate these types of reservoirs based on the knowledge of petrophysical data. This paper strives to outline some of the key pitfalls that can occur due to the uncertainty in this data. Using reservoir simulations, the long term adverse impact of these pitfalls on both production and overall recovery can be shown. A new acid diverter, based on degradable fiber and visco-elastic surfactant technologies, has been applied, along with a placement model, to optimize treatment design and to maximize diversion in heterogeneous carbonate reservoirs. The system is robust, diverting from high permeability streaks, fissures and natural fractures with very little diverting effect from the low permeability zones, and therefore generates a more uniform stimulation than conventional fluid systems. This approach has been used to optimize various stimulation campaigns on carbonate fields throughout the Middle East. Maximizing treatment fluid coverage across the entire intervals can be achieved across thick, even naturally fractured, carbonate intervals using degradable fiber technology within a visco-elastic surfactant fluid. The diversion technique does not overly rely on certainty in petrophysical data, such as permeability and porosity, and can be successful in a variety of potential scenarios.
Recent trends in unconventional oil and gas developments have seen longer horizontal wells drilled to achieve greater reservoir contact while minimizing cost and surface impact. Challenges for completing longer laterals include achieving effective fracture stimulation and performing clean out of the well bore after stimulation is complete. A case study was performed in Shell Groundbirch, an unconventional gas development in British Colombia, Canada, focusing on stimulation and extended reach cleanouts. Five long lateral wells +3600m lateral length with measured depth to true vertical depth ratio of 2.5, were drilled and completed; the resulting wells are 60% longer than the standard development well. Based on operational efficiency, coiled tubing (CT) was determined to be the preferred method for performing the millouts and well cleanup. 73.0 mm CT with an aggressive taper can reach the required set-down depths with enough weight on bit available to mill-out all completion plugs. Operational plans and milling tools were developed to maximize the probability of success. This paper outlines the technical details which contribute to an important case study that can help define and push the limits of extended reach CT interventions in industry.
Installing coiled tubing velocity strings is a common deliquification strategy used in gas wells in which the flowrate has dropped below the critical rate required to remove produced liquids from the well. Initially in Shell’s Groundbirch unconventional gas development, a standard carbon steel coiled tubing grade was used for these applications. Due to elevated levels of chlorides, carbon dioxide and in some cases H2S, a long-term inhibition schedule was then needed to minimize corrosion. Recently, however, a special CRA material has been employed, that Shell had previously helped develop for subsea applications (Li et al, 2002), which has eliminated the need for costly long term inhibition. This paper outlines the selection process and implementation of this material change. Standard 13Cr alloys are well known in the industry but could not be used in the Groundbirch fields due to the high chlorides levels of the produced water as well as H2S levels sometimes above NACE thresholds. On this basis, higher chromium coiled tubing materials were evaluated based on their ability to resist corrosion, sulphide and chloride stress cracking, as well as their mechanical properties. A fit for purpose material testing program was used, combining standard NACE test protocols with unconventional specimen shapes and crevice corrosion assessments in simulated well environments. Factors such as the direction of primary loading stresses, mechanical deformation during installation, as well as coiled tubing manufacturing characteristics such as the longitudinal seam, bias welds, and orbital (butt) welds were also considered. From this fit-for-purpose testing, not only was the preferred corrosion-resistant material, Nitronic 19D duplex-steel, selected for the coiled tubing strings, but an operating envelope has been developed for it, as well as manufacturing specifications have been defined for the strings for optimum performance. Installation guidelines to reduce the mechanical damage inflicted on the strings have also been created. Although the campaign startup was delayed due to manufacturing issues, once the campaign did start, the installations have been efficient, with future campaigns planned. By selecting a single, robust material grade for CT Velocity String material for a variety of potential conditions (high salinity, CO2, H2S), and also developing some guidelines for their use, installations can be done efficiently in Groundbirch in large campaigns, with long term cost savings from eliminating the need for continuous inhibition. Furthermore, the long term integrity of the strings should benefit, ultimately leading to longer completion life, less production deferment, and less intervention issues due to corroded and failed CT strings.
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