For the deepwater blocks in the Gulf of Mexico, the seafloor relief is rugged, the water bottom fluid and the seabed are unstable. Within 1500 m under the mud surface, the probability of the existence of shallow gas and gas hydrates is high. These unstable factors pose a threat to the surface layer drilling to a large extent [3].

Shallow gas: the undisturbed structure of the layer under the mud surface may be destroyed and the effective formation pressure could be reduced. In the surface layer operation, if the shallow gas is encountered during the drilling, the wellhead would collapse or blow out and other high potential risks may be caused.

Hydrate: in the surface layer operation, due to the change of gradient of pressure between the seafloor sediment and upper stratum, the formation cavitation pressure and bursting pressure space ratio is small, the well leakage, collapse, well kick or even jamming risks greatly rise.

With the continuous increase of water depth, the formation fracture pressure gradient decreases. Therefore, the window between the formation pore pressure gradient and formation fracture pressure gradient is very narrow [4]. According to the actual drilling example of a deepwater block in the Gulf of Mexico, obvious changes may take place in several hundred meters with regard to the formation pressure coefficient. It raises a tougher requirement for the mud system. Once the treatment is improper, the well leakage or even blowout may easily occur. At present, the common technology in the Gulf of Mexico is the lowering of the multi-layer casing. However, while the risk is avoided, the number of bottom hole assemblies required for drilling is increased; resulting in a great increase in drilling costs.

In the area of the deepwater block of the Gulf of Mexico, there are tremendous salt layers (Figure 2). According to the exploration of deepwater blocks in the Gulf of Mexico which belongs to the USA, some salt layers could be up to 6100 m thick [1]. The deepwater blocks of the Gulf of Mexico in the Mexico region have not been developed on a large scale. When PEMEX carries out the exploration, it has discovered the salt layer. Many reservoirs are under the salt layer. During the drilling, the drilling of the salt layer is inevitable, which poses a great challenge to the drilling operations.

Since the salt body density is fixed (the formation apparent density gradually increases as the pressure coefficient increases), the salt body density is lower than the formation apparent density. As the salt body moves upwards, the fracturing rubble zone at the salt bottom or on the salt body side is caused. The cavitation pressure, fracture gradient and fissure of the formation cannot be quantitatively forecast, so, high well risks control is necessary.

When the salt body is encountered, due to the wriggle of the salt body [1], the salt body reversely intrudes into the borehole. Under the high temperature and high-pressure conditions, the reverse intrusion into the borehole is often faster, leading to the jamming risk.

The change of temperature gradient of deepwater operations is great. Generally, the seafloor temperature is 3˚C - 5˚C, and it could be lower than 0˚C in some areas. Before the arrival at the formation, the drilling mud or well-cementing fluid will undergo a process of obvious temperature decrease. Consequently, adverse effects may be left on the drilling fluid, such as a great increment of the viscosity and jelling. Besides, in a low-temperature environment, gas hydrate may easily occur, and icing or even pipeline blocking may easily happen too [5]. Since the change of formation pressure coefficient is obvious, tougher requirements are raised for the mud. Under different formation pressure coefficients, the pressure inside and outside the borehole should be balanced, to guarantee the stability of the well wall. The low bursting pressure gradient and possible abnormal high-pressure formation need tougher requirements for the well cementing.

The operator of the XXX 1 well is PEMEX, drilled using the Deepwater Drilling Platform BICENTENARIO. XXX 1 well is in the north of the Gulf of Mexico (Figure 3) and it is 178 km away from Tamaulipas. It is adjacent to the border with the USA in the north, connected with Kamay and Pegaso in the east, and the Perdido Fold Belt in the west. It is parallel to Northern Latitude 24˚30' in the south and it is close to Silvertip, Tobago, Trident and Great White oilfields of the USA. The water depth of the area is 2535 m, the design total depth of the well is 6224 m, the final drilled well depth is 6119 m, and the drilling time was 143 days. The seafloor temperature is 5˚C, and the maximum bottom hole temperature is 146˚C.

Refer to Table 1 for the analysis of formation lithology of XXX 1 Well.

In order to effectively avoid the geological risks and effectively balance the formation pressure coefficient, the well depth structure and casing procedure (Table 2) of the well have been optimized, and the structure of 7 different casings has been designed. Within 1500 m under the mud surface, 4 different casings are used. In particular, at 3050 m - 3450 m, the pressure change is considerable, requiring a mud density between a range of 1.05 - 1.70 g/cm³, being too obvious the pressure increase, and it is especially important to increase the number of casings of the well structure. At the same time, the multi-layer casing can favorably avoid the risk of intrusion of the salt gypsum bed into the borehole (for the deepwater blocks of the Gulf of Mexico, the salt gypsum bed is normally present in the oilfields near to the border between the USA and Mexico in most cases). The advantages of large-time drilling drill bits are as follows: when the complexities are encountered and the drilling of the borehole of such stage is finished in advance, 1 - 2 conventional casings could be added to the well structure, and the geological risk is effectively isolated.

Even if no salt gypsum bed is encountered in this well, this area is close to the border between the USA and Mexico. Through the optimization of bottom hole assembly, RSS, reaming while drilling technology, drill bit selection and the like, the salt gypsum bed event is prevented in this well.

The water depth of the Well is 2535 m, and the seafloor temperature is 3˚C. In order to reduce the effects on the drilling due to the temperature decrease, and prevent the obvious change of rheological property of drilling fluid, the manifold is enveloped by the insulating layer, so as to reduce the jelling of drilling fluid, constrain the gas hydrate generation and avoid the manifold blocking [7]. In terms of casing well cementing, considering the seafloor low-temperature effect and the effect of low formation fracture pressure gradient, low-density cement is used, the setting time of cement is reduced, the internal and external pressure is balanced, and the effective well cementing is achieved.