Heavy Weight Cement System for Corrosive Environment: Preventing Strength Retrogression and H2S Corrosion

Well cementing in high temperature and hydrogen sulfide (H2S) corrosive environment presents challenges in preventing cement compressive strength retrogression and selecting weighting agents inert to H2S. This paper presents the development of a cementing system for high pressure high temperature (HPHT) well with bottom hole static temperature in excess of 165°C, a drilling fluid density of 2.19 SG and a high concentration of H2S. A major operator in the Caspian Sea region accepted the cement design and successfully used it on the production liner section of the HPHT well. Cementing of the production liner was complex due to the requirement for a high-density cement system, narrow margin between the pore pressure and frac gradient, HPHT conditions and 18% H2S concentration in the formation fluid. Comprehensive laboratory testing was performed to evaluate the properties of the cement system including measurements of thickening time and compressive strength evaluation using a UCA and destructive method using ultra-HPHT curing chamber for cube sample curing. The presence of H2S limited the use of conventional weighting agents such as hematite and hausmannite, and the high temperature environment dictated the need for quartz silica. These factors required a nonstandard approach to cement blend formulation and flowability assessment. During cement system optimization, the target slurry density was achieved using barite which has a lower density compared to other common weighting agents and significantly reduces cement content in the blend but also is inert to H2S corrosion. A further challenge encountered during cement system optimization was strength retrogression that could not be prevented by the conventional approach of adding 30-40% quartz silica by weight of cement into the system. To overcome strength retrogression, much higher concentrations of silica were required. As a result, the low cement content led to insufficient compressive strength development at liner hanger depth. A solution was found by adding a Vinylamide/Vinylsulfonated polymer (VA/VS) polymer in a certain proportion to the slurry design. Thus, at elevated temperatures, it was observed that the VA/VS polymer tended not to delay compressive strength development while still extending the slurry thickening time. The developed heavy weight cement system was successfully implemented to isolate the 7-in liner on HPHT well. All the stages of job planning, design and execution, along with the slurry optimization process are presented.

Manufacturing of LSAW Pipes Using Hot Rolled Sheets for Sour-Service Application

The exploration and transportation of fluids from corrosive fields or reserves demand corrosion-resistant pipelines. The pipelines for sour-service application demand higher resistance to H2S corrosion because of higher sulphur content observed in the geographical locations. The pipelines laid in the corrosive and marshy lands demand high wall thickness to allow for wall thinning due to corrosion. The linepipes manufactured using TMCP plates are widely accepted for sour-service applications. The use of cut-to-length sheets from hot rolled coils for the manufacturing of LSAW pipes is economical but having limited acceptance by the pipeline operators. Welspun took an initiative to develop the linepipes for an onshore sour-service application using API 5L X60MS grade cut-to-length sheets from TMCP hot rolled coils procured from an approved steel mill. The LSAW pipes of 24” OD × 14.27 mm WT were manufactured by the JCO-E press at Welspun Pipes Mill in Anjar-Gujarat-India. The experience of 24”OD × 14.30 mm WT, API 5L X60MO grade pipes manufacturing using TMCP plates procured from another approved steel mill, was used to decide factors such as alloy design, cleanliness, strength, DWTT, hardness, HIC, SSCC, CVN for the selection of cut-to-length sheets. This paper deals with sheet-to-pipe behavioural changes in properties before and after cold mechanical expansion. Wherever possible, the comparison was also made with plate-pipe behavioural changes in properties.