Schuh, Frank J., Member SPE-AIME, ARCO Oil and Gas Co.
The conductor and surface casing setting depths required in offshore areas are dependent primarily on the minimum formation fracture gradients below the casing shoes. The conductor string must be set at a depth that allows circulation or the drilling fluid required to drill the surface hole. The surface casing must be set deep enough to permit circulating the maximum required drilling fluid density and to contain any kick fluids encountered in the intermediate hole. Since formation fracture gradients decrease as the depth of the water increases, the required lengths of conductor and surface casing also increase with the depth of water. This paper combines the effect of water depth on fracture gradient with kick control requirements to define the conductor and surface casing setting depths as a function of water depth for typical Gulf of Mexico sediments.
Formation fracture pressures have been shown to be equal to the formation pore pressure plus a fraction of the difference between the overburden pressure and the pore pressure. Empirical studies have shown that this stress ratio fraction can be defined as a function of the formation depth offshore and the subsea floor depth offshore. It is therefore reasonable to expect to be able to predict deepwater offshore fracture gradients with reasonable accuracy.
Fig. 1 compares the fracture pressure predictions for typical Gulf Coast formation properties under 500- and 2,000-ft water depths. This figure shows that the fracture pressure reaches the equivalent of a 10 lb/gal gradient from the surface in 500 ft of water at casing penetration depth of 650 ft. In 2,000 ft of water, 1,450 ft casing penetration is required to achieve a 10-lb/gal gradient in this same formation.
Fig. 2 is a plot of offshore fracture gradient vs water depth for depths from 0 to 6,000 ft. These were calculated using the following assumptions.
The overburden gradient and stress ratio is equal to published values for Gulf Coast sediments (Fig. 3).
The formation pore pressures are normal and begin with a seawater gradient at the sea floor and gradually increase to a 9 lb/gal gradient at 3000 ft of penetration.
The flowline elevation is 35 ft above the sea level.
Note that the penetration depth required to reach a given fracture gradient increases significantly with water depth. For example, a 13-lb/gal fracture gradient only requires 2,500 ft of casing penetration at zero water depth, but requires 8,000 ft of penetration in 5,000 ft of water. penetration in 5,000 ft of water.
Conductor and surface casing setting depth requirements have evolved from the industry's efforts to provide adequate well control, to protect freshwater zones and to drill at minimum cost. In onshore areas, the primary role of the surface casing string is to protect freshwater zones and that of the conductor is to raise the mud returns to the elevation of the mud pits. If the surface casing depth does not provide adequate well control, intermediate casing strings are set below the surface string.
In the offshore areas this evolution provided significantly greater setting depths for the surface and conductor casing strings. Conductor requirements increased because greater setting depths were required offshore to allow circulation of returns to the mud pits. Setting depths of 500 ft to 1,000 ft of penetration are typical in water depths of 500 ft to 1,000 ft. As can be seen in Fig. 2, these depths approximate the penetration depths required to achieve a 10-lb/gal fracture gradient. Since the primary role of the conductor string is to permit circulating mud returns to the surface, this data indicate that our present drilling practices require a 10-lb/gal fracture