This study examines the effects of drilling, completion, and production operations and their associated cyclic stresses on a cement sheath. The operations performed after cement placement can damage cement sheath integrity and bond with the casing or formation resulting in loss of zonal isolation and sustained casing pressure often requiring remediation and reducing productivity. This paper describes evaluation of cement sheath failure resulting from cyclic stresses experienced while drilling, fracturing, and producing shale gas wells and showcases optimization of cement systems used for Marcellus shale play intermediate casing strings. This study correlates the durability of each cement system with mechanical properties of the cements to determine each system's ability to resist failure under cyclic stresses. Two different cement compositions were used to cement intermediate casings on a number of wells. Both had similar performance properties and were designed to prevent gas migration following cementing. Wells cemented with one of the blends experienced a higher incidence of seal failure as evidenced by sustained casing pressure. In laboratory stress endurance testing, each cement system failed at a specific quantified magnitude of cumulative energy input. The results show a higher maximum endurance for the more durable cement, which also exhibits the higher field success rate. The fatigue endurance limit gives an approximation of the stress magnitude that a cement composition can withstand. Correlating laboratory endurance measurements and mechanical properties of the cement systems to field performance further quantifies the mechanical properties needed to optimize zonal isolation. U.S. shale gas production is a major component in the future of U. S. energy supply. As such, there is focus on the drilling and production of U.S. shale plays. This study takes measured look at annular seal failure and lays the ground work to calibrate it to actual field results allowing operators and service companies to select more durable cement systems. Results from this study can ultimately decrease time and funds spent repairing compromised cement seals. Furthermore, improved seal performance equates to improved well performance with lessened environmental risk and impact. IntroductionThe study reported here is a derivative of a long--term investigation aimed to improve zonal isolation for horizontal wells drilled in the Marcellus shale with the Research Partnership to Secure Energy for America (RPSEA). The impetus for this long--term study is to optimize drilling and completion practices to reduce cost, improve zonal isolation, and improve well success rate. Due to the complex environment encountered in drilling and completing horizontal shale wells, it can be difficult to determine all of the factors necessary to optimize zonal isolation. One of the initial actions of the long--term study was assessment of well performance in the Marcellus play. Interestingly, the well success rate was much lower in the intermediate strin...

A drilling-and-casing method (Odex 115 system) 4 utilizing air as a drilling fluid was used successfully to drill through various rock types within the unsaturated zone at Yucca Mountain, Nevada. This paper describes this method and the equipment used to rapidly penetrate bouldery alluvial-colluvial deposits, poorly consolidated bedded and nonwelded tuff, and fractured, densely welded tuff to depths of about 130 meters. A comparison of water-content and water-potential data from drill cuttings with similar measurements on rock cores indicates that drill cuttings were only slightly disturbed for several of the rock types penetrated.Coring, sampling, and handling methods were devised to obtain minimally disturbed drive core from bouldery alluvial-colluvial deposits. Bulk-density values obtained from bulk samples dug from nearby trenches were compared to bulk-density values obtained from drive core to determine the effects of drive coring on the porosity of the core.Rotary coring methods utilizing a triple-tube core barrel and air as the drilling fluid were used to obtain core from welded and nonwelded tuff. Results indicate that the disturbance of the water content of the core was minimal. ••Water-content distributions in alluvium-colluvium were determined before drilling occurred by drive-core methods. After drilling, watercontent distributions were determined by nuclear-logging methods. A comparison of the water-content distributions made before and after '"rilling indicates that Odex 115 drilling minimally disturbs the water t intent of the formation rock.

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