Central Graben Extreme Offshore High-Pressure/High-Temperature Cementing Case Study

Abstract

This paper presents the cementing case histories of six wells in two offshore high-pressure, high-temperature (HPHT) fields in the Central Graben area of the North Sea. Well depths averaged 6000 m true vertical depth (TVD), temperatures were greater than 200 o C, the fracture/pore-pressure margin was small, and the bottomhole pressure exceeded 1,100 bar. The well deviation varied from near vertical to a maximum of 35°. These downhole conditions are the most extreme yet experienced in a field development in the North Sea.Because of the extreme HPHT conditions, all aspects of cementing operations had to be carefully evaluated. In addition, new and enhanced equipment, processes, and materials had to be developed. Even though the potential for gas influx of the cement was high, the cement slurry formulation and placement techniques for resisting gas migration were successfully designed through the use of materials with a low environmental impact. Because the formations were weak and unconsolidated, a special cement blend was used at the top of the well. The mixing equipment was modified to withstand the high-density fluids required for the liner section.For job success, both a well-defined top-of-cement (TOC) and good casing support had to be obtained without exceeding the allowable equivalent circulating density (ECD) limit. Therefore, effective mud displacement, correct slurry placement, and precise rheology measurement were required.No remedial HPHT cementing operations were necessary during these operations, which was an important objective. Remedial operations could have posed extreme risks and high associated costs. This paper describes how the cementing process was optimized for HPHT field development, thereby minimizing risk and costs. Considerable emphasis is placed on the production liner cement job, because it was the most critical string on the well. Improvements to equipment, slurry testing, placement, and bond logging procedures are also described.

Challenges and Remedy for Cementing of HPHT Wells in Nigerian Operation

“…In HPHT wells, the slurry becomes sensitive to high temperature so that the thickening time of the slurry is highly reduced, making the cement set faster than in average temperature wells. North et al (2000) in a paper on a cement case study on wells in the North Sea outlined that there are two temperatures of importance in the well and they are: Bottomhole Circulating Temperature; this is the temperature the slurry encounters as it is being pumped into the well and it is the one that affects thickening time. Ravi and Sutton (1990) mentioned that the plastic viscosity (PV) and yield viscosity (YV) decrease with an increase in temperature.…”