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This paper presents our perspective of the shallow-water flow (SWF) problemin the Deepwater Gulf of Mexico (GOM). The nature of the problem, includingareal extent and overpressuring mechanisms, is discussed. Methods for sandprediction and shallow sediment and flow characterization are reviewed. Theseinclude seismic techniques, the use of geotechnical wells, regional trends, various MWD methods, and cements and settable spots. Finally, examples of flowincidents with pertinent drilling issues, including well failures andabandonment, are described. Total trouble costs due to shallow-water flow for all GOM operators probablyruns into the several hundred million dollars. Though the problem remains aconcern, advances in our knowledge and understanding make it a problem that ismanageable and not the "show stopper" once feared. Introduction SWF may occur while drilling shallow over-pressured formations at deepwatersites. It is a high profile problem in the GOM, though it does occur elsewhere(Ref. 1) and will likely be encountered in other deepwater regions (Fig. 1). Drilling shallow over-pressured sands may cause large and long lastinguncontrolled flows, well damage and foundation failure, formation compaction, damaged casing, and re-entry and control problems. Most spectacularly, eruptions from over-pressured sands may result in seafloor craters, mounds andcracks (Figs. 2 and 3). Eaton2, 3 has described the significant problems causedby SWF in the Ursa area. A recent inspection of 106 wells (Ref. 4) indicatedthat $175 MM has been spent on SWF and prevention and remediation on thosewells. Total industry costs due to SWF likely exceed several $100 MM. The problem is compounded by the difficulty in seismically imaging thesesands (Refs. 5 and 6). This stems from the relatively low sand/shale contrastin acoustic impedance. The impact of this problem on well site selection andwell design is significant, and makes drilling in SWF areas particularlychallenging.

A new method of obtaining improved zonal isolation using drilling fluid solidification technology has been developed. A water-base drilling fluid is converted into a cement using a hydraulic blast furnace slag. Hydraulic blast furnace slag is a unique material which has low impact on rheologid and fluid loss properties of drilling fluids, can be activated to set in drilling fluids which are difficult to convert to cements with other solidifrcation technologies, is a more uniform and consistent quality product than Portland well cements, and is available in large quantity from multiple sources.Because of its low impact on drilling fluid properties, blast furnace slag may be added to a drilling fluid at low concentrations during drilling operations. The Glter cake and drilling fluid in washed out sections thereby contain a hydraulic materiaL ARer reaching casing point, a mixture of drilling fluid containing chemical activators and higher concentrations of slag may be usedto cement the casing string. Chemical activators from this mixture cause the slag in the filter cake and any bypassed drilling fluid to set. The result is a more complete seal for the annulus. Fluid and hardened solid properties of blast furnace slag and drilling fluid mixturm used for cementing operations are comparable to properties of conventional Portland cement compositions. The design, testing and field application for this technology are similar to conventional cementing methods. Fluids with densities between about 1198 kg/m3 (10 lblgal) and 2397 kg/m3 (20 lblgal) may be prepared. The mixtures have been applied in web where temperatures range from about 4' C (40°F) to 315°C (600°F).This new solidification method provides the proper combination of fluid and solid properties, simplicity of design and application, improved zonal isolation, and broad applicability to bring drilling fluid solidification technology into widespread use.A fundamental weakness of the conventional cementing process is the uncertainty of establishing atrue, reliable seal at the borehole w a l l and cement interface. In many cases, alayer References and tables at end of paper.

In many maturing prospect around the world, operators are facing the challenge of having to drill through highly pressure-depleted formations in order to access lower-lying hydrocarbon-bearing zones. New technologies such as expandable casing are now becoming available to allow for extensions to conventional well designs in order to deal with depletion. However, before one can case off depleted formations, one first has to successfully drill them. This paper highlights key aspects in the planning and execution of the Ursa A-11 well, which was drilled through a 5500 psi depleted sand to a deeper horizon. Drilling complications included risks of excessive mud loss, internal blowout and differential sticking on the depleted sand. Moreover, fracturing of the depleted sand carried the risk of jeopardizing production at a nearby horizontal well. Key factors in the successful drilling of the Ursa A-11 well included special drilling fluid design, rock mechanics study, pro-active use of borehole strengthening technology, integration of supplier and operator expertise, and excellent communication between all parties involved. Introduction Producing a prospect's reservoirs "from the bottom up" may not always be feasible. Development economics often dictate that higher-reserves or better-quality reservoirs must be produced first before deeper-lying horizons can be accessed. In many maturing prospects operators are challenged to drill through zones that are severely depleted from past or ongoing production in order to unlock these deeper reservoirs. This situation applies to the deepwater prospect Ursa in the Gulf of Mexico (GOM). The main reservoir at Ursa is the Yellow sand, which is currently being depleted by three high-rate producing wells. Pore-pressures in the Yellow sand have typically dropped by 5000 - 6000 psi since production commenced in 1998. Production has not only reduced the pore-pressure but has also lowered the minimum horizontal stress in the Yellow sand (see Fig. 1). Such conditions greatly complicate accessing the Sub-Yellow reservoir, an untapped hydrocarbon-bearing zone at virgin pressure situated just below the Yellow sand. Significant challenges surfaced while planning the Ursa A-11 Sub-Yellow producer, for which the casing program is given in Fig. 2:The high GOM cost environment dictated the need for a high rate completion from a small Ursa template slot. Marginal economics on the Sub Yellow sand precluded any other development concepts (e.g. separate subsea well, use of a large Ursa slot etc.).Drilling risks included the possibility of an underground blowout from virgin-pressured sands above and below the Yellow sand (i.e. pore-pressures of adjacent sands are higher than the reduced fracture gradient / minimum horizontal stress in the depleted Yellow sand, see Fig. 1). Also, there was a high risk of differential sticking and associated loss of hole while drilling the Yellow sand at high overbalance (5500 psi).The optimum bottom-hole location for the Ursa A-11 well placed it in very close proximity (˜ 400 ft) to the high-rate Yellow horizontal producer Ursa A-6 (see Fig. 3 for a subsurface projection of the A-11 and A-6 wells). This introduced the significant risk of fracturing the A-11 well at the depth of the Yellow formation into the direction of the A-6 well. Propagation of drilling mud from A-11 to A-6 could result in impairment of the A-6 completion and thus compromise further production from A-6. To gain a proper perspective of the proximity of the A-11 and A-6 wells, it was estimated that hydrocarbons would be flowing by the A-11 well at an amazing rate of 2 ft/day due to ongoing production at the A-6 well. URSA A-11 Well Planning Significant effort went into the planning of the Ursa A-11 well to address the challenges associated with developing the Sub-Yellow sand. Planning was tackled by an integrated project team that included the Ursa prospect development team, drilling engineers and drilling fluids & cement team, R&D experts, and resources from various suppliers. Specific planning elements are discussed in detail below.

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