ABSTRACT

Water flowage from shallow sands during deep water drilling perations is a relatively common phenomenon; because Drilling operations are vulnerable at shallow depths below the Sea floor before casing is set and blowout presenters are Installed. The shallow water flow phenomenon is caused by The presence of fluids under greater than hydrostatic pressure Within highly permeable loose sands at shallow w depths Beneath the sea floor. Massive sand formations are regularlyDeposited during sea level low stands and can be identified By the seismic stratigraphic method. When applied to The interpretation of high resolution geophysical survey Records sequence stratigraphyperrnits identification and mapping Of potential problem-causing sands. Once the depth And thickness of sand formations are established an appropriate Casing and drilling program can be set up to reduce the risk of encountering shallow water flow problems during Drilling operations.

INTRODUCTION

The occurrence of shallow water flows during deep water Drilling operations is a relatively common phenomenon for which various aspects have been addressed by Gonzales The shallow water flow phenomenon occurs when fluids under Greater than hydrostatic pressures are present within ighly permeable sands, with very little consolidated overburden; Such that, when drilling operations encounter these Sands, without securely placed casing, fluids will flow out of The formation and up the borehole and drill-string.

While at present there exists no methodology for the direct Detection of such formations, the shallow water flow phenomena Occurs principally within loose - highly permeable - sands. And such sands are primarily constituents of the low stand Systems tracts stratigraphic components; components which Can be interpreted from high resolution seismic reflection data Using the seismic sequence stratigraphic methods originally Presented by Vail and colleagues.

While the sequence stratigraphic methodology is relatively Straight forward, it should be noted that it only identifies potential Sand-prone layers, which may or may not produce Shallow water flows, and does not represent an absolute indication Of a potential problem. Hence, the methodology suggestedHerein provides an indication of potential shallow water flow Zones for which recommendations can be made to at Least mitigate the problem through appropriate drilling techniques and casing programs.

ORIGIN OF THE PROBLEM

Serious drilling problems have been encountered by a number of deep water operators in the Gulf of Mexico1,2&3. The Problem stems from the presence of geologically young unconsolidated Sands which provide an excellent conduit forHigh pressured interstitial fluids when pierced by the drill-bit. Furthermore the shallow nature of these formations is problematic In that casing is not always installed when the sands Are perforated, and mud weights necessary tic overcome Formation pressures cannot be used as they may fracture the sands and cause a rapid loss of drilling fluids.

A partial solution to the problem may be obtained by mapping The shallow seismic sequence stratigraphy with emphasis On the depth, thickness, and types of sand-bearing formations Or "systems tracts" according to the model of Vail And Wornardt showed in Figure 1. In addressing the shallow water flow problem, it is paramount to understand the origin.

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