te deepwaterdrilling problem of shallow-water flows(SWF) has proven to be particularly challenging. It isneither easily identified nor avoidedand has the potentialto cause total well failure when not handled properly.During the past 10 years of drilling in the deepwater Gulfof Mexico,offshoreWestAfricaand other basins, SWFhasbeen one of the most expensive hazards for deepwateroperators, causing significant cost over-runs and re-spudding of numerous wells. Shallow-water flow eventsalso have been the cause of template failures involvingmultiple wells, such as the Ursa template failure in theGulf of Mexico,and may be the single most costly and

DEEPWATER. JULY 2003

dangerous hazard in the deepwater exploration and pro-duction business.

The nature of SWFShallow-water flows have been observed in water depths fromI,500ft to 7,OOOft(457m to 2,135m) and up to 4,OOOft,(1,220m) below the seafloor in areas where rapid sedi-mentation has caused sands to retain abnormally highporosities at or near the critical porosity for the material.Critical porosity is that at which a rock begins acting like aload-bearing solid. Shallow-water flow sands exist at loweffectivestresses and are close to incipient failure. This resultsin a situation where even the agitation of the drillbit can causethe formation to begin collapsing into the well during

Drilling-Shallow Water Flows

Sel.1 .2

Pressure (kpsl)~ .S -6 .9-S.7.3

LEGEND

100 It waler depth

4000 It waler depthOwrburdm

(100 fi \\'D)

E1fectM Stress

(100 Ii \\'D)

Effective Slmt$(4000 ft \\'Di

Overourdm(-toaG ft WO)

Figure 1: A comparison of shallow water and deepwater pressureenvironments shows the differences between the effective stress/

overburden and pore pressure for water depths of 100h (30.Sm)and 4,OOOft(1,220m).

penetration or soon after. Some SWFevents have been delayedby several hours to several days. Because it "excavates" theformation, SWF has resulted in failure of the shallow portionof wells long after the section had been cased.

To understand SWF, it is useful to understand the basisof the old truism that "compaction begins at the seafloor."This empirical observation is based on the physicalphenomenon that compaction, which includes con-solidation, reduction of porosity and strengthening of thesediments, is because of the grain-to-grain forces in thesediments. This is the effective stress or the differencebeh'l'een the overburden and the pore fluid pressures.

In cases of rapid sedimentation, pore fluids may notescape, as the load increases. The fluids bear some of theweight of the overlying solids and become overpressuredbeyond the normal hydrostatic pressure of the overlyingwaters. The effective stress on the sediments is abnormallylow and the porosity is preserved.

Consider a comparison of stress conditions in a shallow-water case vs. a deepwater case (Figure 1). The effectivestress on the formation at the mudline is zero, and it increaseswith a gradient of about 0.535 psi/ft if hydrostatic com-munication is maintained with the water column. Inshallow water, the effective stress becomes non-zero at 100ft(30.5m) below sea level where the total overburden stress

from the water co'umn is relatively smalL At a water depthof 4,OOOft,however, the overburden stress is largerbecause of the higher water column. However, as long as thepore fluids are in pressure communication, the porepressure will be increased an equal amount. Theeffective stress remains unchanged between shallowand deep water at the same depth below the mud line. As aresult, the consolidation of the sands is the same for equivalentdepths below the mudline. Thus, not only are the sediments inthe deepwater case less compacted, but they also are atrelativelyhigher pore pressures for the same depth below sealevel compared with their shallow water equivalent.

In addition to the low compaction state for deepwatersediments, the severity of SWF may be exacerbated by thepresence of structural hyper-pressuring, also known as thecentroid effect. This concept suggests a sand body positionedon a structure or slope will develop a pressure gradient that is

e hydrostatic, even though the gradient in the surroundingsediments is non-hydrostatic. For the shallow burial conditionsin which SWF events occur, the amount of structural hyper-pressuring required to cause a seal failure is not great.

SolutionsThe industry has chosen to address SWF in two pri-mary ways:

... pre-drill detection and avoidance, and

... detectionand mitigationwhiledrilling.These two efforts have occurred in parallel, often with no

interaction between the efforts. The pre-drill predictioneffort has been focused on seismic prediction and logging-while-drilling/pressure-while-drilling (PWD) analysis, whilethe mitigation effort has been addressed by new drillingtechniques, new mud circulation approaches (especiallydual-gradient drilling) and new mud chemical treatments toprevent hole collapse.

Pre-drill prediction and avoidanceThe most common approach to SWF prediction is tied toanalog wells and analysis of seismic reflection character ondata processed with standard techniques. Well locationshave been chosen to avoid or minimize exposure tosuspected SWF zones. Avoidance relies on "pattern recog-nition" and has not always been successful. Of course,seismic time-to-depth conversion is important for pre-dicting the depths of these zones.

Another key pre-drill tool is seismic velocity analysis forpressure prediction. Departures of these velocities from"normal" trends have proven useful for quantifying the degreeof overpressure and allowing accurate casing program design.The bases for these techniques date to the late 1960s and havebeen recently applied with better accuracy and understanding.

The geophysical community has been making newadvances in the pre-drill detection of SWF. Research at the

JULY 2003. DEEPWATER

Z

3

4

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6

7

8

9

10

Universityof Oklahomaspon-soredbya consortium ofcom-panies led by ConocoPhillipsand jointly fundedby the U.S.MineralsManagementServicedemonstrated the usefulnessof seismic data for detectingthe low shear wavevelocitiesand abnormally high VpNsratios that are diagnostic ofSWF sands. The basic con-cepts and feasibilityof thesemethods have been demon-strated and extended withfurther analysis showing thedistinct signatures of SWFsands in a deepwater area withknown SWF events (Figures2 and 3).

Detectionand mitigationwhile drillingThe overpressures ofSWFsand sections can be detected duringdrilling in two ways. First, the most basic form of detection isobservation of the wellhead with remotely operated vehiclevideo while drilling of the tophole section of the well. Thissection usually is drilledwith seawater instead of drilling mudsand without mechanical pressure control, so SWF events arereadily observed as dramatic flows of sandy slurries from thewell. The more precise method of detection is through the useof PWD technology. This has proven helpful in real-timedetection and quantification of the overpressures so weightedfluids may be introduced rapidly for flow control.

Mitigation of SWF is accomplished in two ways. Someoperators chose to "topset" the suspected SWF zone andthen set another string of casing immediately below thezone after drilling through the zone with weighted mud tocounterbalance the overpressured pore fluids. This methodprotects the wellbore from the influx of pore fluids andunconsolidated sands while the SWF section is drilled, thenprotects the weak formation from the heavier weightedmuds needed to drill the deeper portions of the well. This

Drilling -Shallow Water Flows

~~, ... 'o.!_~_!'L~'~~_~~!'.!'U2' ~..~'~._~._"'2..~..111 ... 7" ". 7.. "" 7911813621"2 " ... ... ... .29 .., ... 972

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Figure 2: When estimating Vp/Vs ratios for shallow-waler Row detection, abnormally high Vp/Vs ratiosor blue and purple anomalies between 600 ms and 700 ms indicate possible sand boc/ies responsible forshallow-water Row.

approach has proven effective but is expensive because ofthe requirement for the extra casing string.

An alternative is to use dual-gradient techniques. Thesimplest of these is riserless drilling using weighted drillingfluids. In the technique known as "pump and dump," theweighted drilling fluids are not returned to the surface butare allowed to vent from the wellbore at the seafloor. Thedual-gradient is because the normal pressure build-upthrough the water column and the higher gradient ofpressure increase because of the weighted drilling fluidsfrom the seafloor downward. This technique reduces thepressure of the drilling fluids on the formation and avoidsfracturing and fluid loss into the SWFzones while being ableto counterbalance overpressured pore fluids.

This technique has proven effective,but it has some draw-backs. Other systems are under development to mimicriserless drilling by providing mud lift from the sea floor."Closed" dual-gradient systems are being developed to returnthe drilling fluids to the surface, which has several advantages:

... expensive muds can be recovered;a wider variety of muds can be used; andcuttings can be returned and examined on the rig.

/n..the technique known as (pump and dump,"tkg weighted drilling fluids are not returned to the surface but are allowed

tq,pe,gt.frorn the wellbore at the seafloor.

DEEPWATER. JULY 2003

About the authors: Alan R. Huffman

(huffman@fusiongeo.com) is presidentand chief executive officer of FusionPetroleum Technologies Inc. in The Wood-lands, Texas. Robert J. Bruce is a fusion

resenlOir associate at Fusion Petroleum Technologies Inc.John P. Castagna is a professor of geophysics at theUniversityof Oklahoma.

Drilling -Shallow Water Flows

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common data point locations on the line

Figure3: In analyzing the deviation in the estimated Vp/Vs ratio from background Vp/Vsfrom a seismic inversion, purple and red zones have abnormally high Vp/Vs, possibly

indicating sand bodies responsible for shallow water flows.

ConclusionWhile SWFsands have been a major ha7,ardto deepwaterdrilling and development, experience has taught much

about the nature of the phenomenonand how to cope with it. New andemerging techniques are availabJe forpredicting the existence of the sands andthe degree of overpressure in them aswell as for drilling them safely. TheskilJs and knowledge of several diversedisciplines are needed in this effort.With communication and a commonunderstanding of the problem by thegeoscience and engineering disciplines,progress has been made in mitigatingthe hazard and drilling efficientdeepwater wells. ~

103 W. Boyd St.Norman, OK 73069405-364-8663 (phone)405-321-7571 (fax) o~s IF u

www.fusiongeo.com

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