Deep Drilling Practices in Mississippi
Authors: Kirk W.L. 
Published: December 2007

Three factors have been significant in the successful drilling of these deep, high-pressured sour gas wells: oil-base mads, the development of the C-90 grade of tubular goods, and the individual drilling contractor's willingness to upgrade equipment for sour gas service.

Introduction

In 1968, the Garrett No. 1 well, in the Thomasville field of Rankin County, was drilled and completed as Shell Oil Co.'s first deep, high-pressured, sour gas well in Mississippi. In view of the numerous problems that have been introduced by the sour gas (10 to 60 percent H2S and 5 to 60 per cent CO2) and by the high percent H2S and 5 to 60 per cent CO2) and by the high pressure in the formations (0.92 psi gradient), both pressure in the formations (0.92 psi gradient), both in the Garrett and subsequently drilled wells, a continuing effort has been made to find the best combination of drilling practices and to upgrade certain techniques. In addition, new materials and specifications have been developed for sour gas service equipment.

Geology

Before discussing the various aspects of drilling the Mississippi wells, it may be of interest to outline briefly the geology of the area, and to comment on the occurrence of the overpressures and H2S gas.

The area of interest (Fig. 1) may be defined by the Smackover belt. starting in the west with the Monroe Uplift and proceeding southeast through Wayne County, Miss., and into Alabama. The northern boundary of this system is formed by the updip limit of the Smackover and a graben system, and the southern boundary is the shelf edge.

Three factors are believed to be the primary contributors to the presence of the H2S gas and the abnormal pressures: the Jackson Dome, the Buckner formation, and the salt. The Jackson Dome, an igneous intrusion of Cretaceous time, formed the CO2 present in the Thomasville area (Rankin County) wells by exposing the carbonates to volcanic action.

Overpressured formations are found across the state. Apparently, the seal formed by the Buckner anhydrite in the western part of the belt prevents the dissipation of the gases that are formed by the Jackson Dome or that are generated in situ. Towards the east the Buckner grades into a salt and anhydrite. The geopressures are still present, but are often encountered above the seal and within the Buckner itself.

The underlying structure of this regional belt is the Louann salt. The rolling configuration of the salt forms the various structures of interest and may influence the formation of abnormal pressures. The salt domes and piercing (piercement) salt structures tend to break the impermeable seals by fractures or intrusions, and are generally associated with zones of normal pressure.

The Cretaceous remains fairly uniform throughout the region. However, there is a definite increase in induration towards the west in the Jurassic rocks, as evidenced by the slower drilling time below the 9-5/8-in. casing (see Fig. 2). Basically, the Cotton Valley in the eastern sector is a massive sand section with thin. friable shale beds. The shales thicken and become harder towards the west, resulting in slower penetration rates and higher well costs. penetration rates and higher well costs.

JPT

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