Well-Intervention Challenges To Service Wells That Can Be Drilled

Abstract

Abstract Drilling rigs are continually setting new records in measured depth versus true vertical depth ratios, horizontal section lengths, and delivering sophisticated versatile yet often operationally complex completions. Experience suggests that common well intervention technologies including slick line, electric line, e-line tractors, Coiled Tubing (CT) tractors, pump-down services, and CT and CT enabling technologies may have difficulty fulfilling all the possible intervention needs of the wells that can and are being drilled and completed. This paper will present an overview of available well intervention technology, outline limitations of each technology, and use numerous case histories from challenging interventions performed in Alaska to support the discussions and conclusions. Introduction Advances in drilling technology have enabled operators to drill complex wells ranging from long step-out wells to multi-lateral completions with horizontal laterals that may extend over 7,000 ft. Examples of the fields with wells considered as step changing include Wytch Farm (UK) from 1978 through the 1990's, Sakhalin Island (Russia) and Alpine Field (Alaska) development in the 1990's through today, and the current West Sak Field (Alaska) development with long multi-lateral completions. These advancements in drilling technology has allowed operators to reach distant offshore targets from onshore facilities such as used in the Wytch Farm and Sakhalin Island and to drain large areas from small environmentally conscious pads such as in Alpine, West Sak and other Alaska North Slope fields (Fig. 1). The engineering and new technology used in drilling and completing these complex wells is impressive. Numerous papers are written detailing the tools and techniques developed to drill these wells. However, when the rig moves off, the production department is left with wells that cannot be serviced with ‘conventional’ service tools and techniques. Shortcomings in servicing these unconventional wells have driven numerous intervention technology advancements including pump-down equipment, electric line tractors, extended reach tools and techniques for CT applications, CT tractors, and use of hydraulic workover units for accessing the wellbores. Still, even with the ability to access these complex wells, performing routine jobs such as setting plugs, shifting sleeves, production logging, profile modifications, and fill cleanouts can become orders of magnitude more difficult and costly to perform than similar services in conventional wells. Overview of Well Intervention Conveyance Methods The following section will consider the intervention tools and techniques available to service these wells. The overview will provide a general review of limitations to each type of service, but should not be considered the absolute limit to each of the services. Among other parameters, well conditions, well bore fluids, geometry variations, and potential logistic problems preclude determining such limitations. Slickline Depth limitations for slickline working in straight holes vary dependent on numerous parameters including the material it is made of, fluid in the hole, weight of the tools, and the work needing to be performed. Modeling slickline operations in a vertical well suggests that the achievable depth limit is around 33,500 ft to 35,400 ft using a light 100 lb tool string and limiting hanging weight to 50% of breaking strength. There was no provision for performing work at this depth. Modeling included slickline sizes ranging from 0.092 in. to 0.125 in. diameter with the larger diameter achieving only slightly greater depths than the smaller diameter lines. This makes sense when considering the base material is the same and that increases in the yield load with increasing diameter are nearly offset by corresponding increases in the weight of the material. Achievable depths based on modeling using the aforementioned parameters and a 0.25 friction factor suggest that slickline could reach approximately 50,000 ft in a well of constant 60 degree inclination. Factors such as tubing material properties, amount of wellbore debris present, and wellbore tortuosity will dramatically reduce these depths. Experience in highly deviated wells also indicates that the friction factor will most likely be higher than 0.25.