Successful Cementing Procedures Developed for Mobile Bay
Authors: Stange A.F. 
Published: October 2008

Many factors combine to make drilling and completing a well in Mobile Bay, offshore Alabama a difficult engineering task. Of particular concern to operators and service companies alike are hostile environmental conditions: high temperatures (375 to 450°F), extreme depths (21,000 to 25000 ft), massive translayeral salt movements, critical casing designs, critical mud and cement programs, and the possibility of encountering CO2 and H2S. Wells drilled to the Norphlet formation have proven to be particularly challenging with respect to cementing operations. An examination of cementing considerations for a typical Mobile Bay completion, with particular emphasis on the development of cement designs for long, deep liner conditions, provides insight into the current state-of-art in cementing technology, from design to job execution. Computer-aided cement job design and recently developed cementing materials have been successfully employed to achieve an effective cementing program for the difficult conditions found in the Mobile Bay Field.

INTRODUCTION

The offshore drilling environment of Mobile Bay presents some of the most challenging drilling and completion problems in the entire world. Deep production (below 20,000 ft), salt formations, high temperatures, and the presence of H2S and CO2 combine to push well completion technology to its limits. Well cementing is especially critical to help provide maximum protection from these hostile downhole conditions. Designing cement slurries that provide this protection has been an on-going concern for the industry. The current state-of-the-art in cementing deep, hot wells relies upon recent developments in cementing materials combined with proven, effective cementing practices to provide optimum cementing efficiency.

Major features of a typical cementing program for Mobile Bay, Norphlet formation completions include salt saturated cement: slurries with recently developed additives for high temperature salt cement compositions, and spacer fluid systems for enhanced mud removal and cement bonding. Salt saturated cement slurries present special challenges in cement slurry design, especially when they must be designed for extremely high temperatures. Many standard cement additives are less effective in the presence of high salt concentrations. Recently, however, a cementing material has been developed which provides fluid loss control, dispersion, and retardation properties at high temperatures. An examination of the conditions found in Norphlet completions and the design of cement slurries for those conditions illustrate application of recently developed cementing technology for hostile well conditions.

THE NORPHLET FORMATION

The Norphlet section consists primarily of sandstone, occupying an apparent transition between the Jurassic and Paleozoic systems. The Norphlet conformably lies in the Upper Coachuilan Series from 'South Alabama to the offshore region. It is generally 50 to 1000 ft thick, is dark grey to black in color with fine grained sand, has good apparent porosity (approximately 177.), an average permeability of 4.69 md, Young's Modulus values range from 0.88 to 2.4 × 106 psi, and Poission's Ratio ranges from 0.18 to 0.40. 1