The decay of hydrostatic pressure due to gel strength development of a static cement results in a pressure underbalanced condition, which can initiate shallow water flow. By determining the static gel strength history verses time relationship of a cement slurry and calculating the annular pressure loss as a function of that static gel strength, the point in time when the pore pressure of the shallow water flow interval exceeds the annular hydrostatic pressure of the gelled cement and sea water can be predicted.
Shallow water flow (SWF) is the most feared of the geohazards associated with deepwater drilling. Normally encountered in the upper 2,500 ft of a deepwater well, SWF occurs when over-pressured, low-shear strength sands are drilled while the wellbore is in an underbalanced state. SWF is a slurry of water and sand, which if sufficiently pressured, flows up the wellbore to the sea floor. Once initiated, SWF's are capable of impacting well construction in any number of ways including:
The deposition of large amounts of sediment around the wellhead to a point at which re-entering the wellbore becomes problematic;
The abrasive force of the SWF eroding the sediment at the formation-cement sheath interface to produce a flow conduit back to the mudline, even at considerable depths below mudline;
The SWF removing a sufficient volume of formation to produce a near-wellbore subsidence which can shear the casing;
The SWF producing an annular washout which induces buckling of the casing due to lack of lateral support.
The tophole section of a deepwater well may contain several individual sand bodies each with its own flow potential. Above the individual sand body is usually a low-permeability, clay-rich section. The clay interval serves as an impermeable boundary, which prohibits the pressure within the sand body from equilibrating with its surroundings. A typical SWF sand is 10 to 50 ft in thickness, has a permeability in the range of 1 ? 10 Darcies and a porosity in excess of 40%. Shallow water flow has been experienced in the North Sea, Trinidad, Norwegian Sea, Caspian Sea, the Gulf of Mexico1 and West Africa.
It is important to note that SWF sands differ markedly in physical properties from the more familiar reservoir rocks. The sands are considered to be in transition between unconsolidated sediment and rock.2 The semi-solid nature of the sediments encountered in the tophole section is demonstrated during the drilling process: a rate of penetration (ROP) of 150 to 300 ft/hr is common. In fact, the biggest drilling challenge in the tophole section is not ROP, but solids/gumbo removal.
The metastable nature of the sands found in the tophole section is the key to understanding why the sands flow. Huffman and Castagna propose two possible mechanisms:3 First, is that the sands are already in an initial stage of failure, and the dynamic load imparted by drilling is sufficient to initiate flow.
