Borehole geology,geomechanics and 3D reservoir modeling

FMI

Applications■ Structural geology

● Structural dip, even in frac-tured and conglomeraticformations

● Faults

■ Sedimentary features● Sedimentary dip● Paleocurrent direction● Sedimentary bodies and

their boundaries● Anisotropy, permeability

barriers and paths● Thin-bedded reservoirs

■ Rock texture● Qualitative vertical grain-

size profile● Carbonate texture● Secondary porosity● Fracture systems

■ Complement to whole core,sidewall core and formationtester programs● Depth matching and orien-

tation for whole cores● Reservoir description of

intervals not cored● Depth matching for sidewall

core samples and MDT*Modular FormationDynamics Tester probe settings

■ Geomechanical analysis● Drilling-induced features● Calibration for Mechanical

Earth Modeling● Mud weight selection

■ Geology and geophysicsworkflow● Deterministic reservoir

modeling● Distribution guidance

for stochastic modeling● Realistic petrophysical

parameters

OverviewThe FMI* Fullbore FormationMicroImager provides microresistivityformation images in water-base mud:

■ 80% borehole coverage in 8-in. hole size

■ 0.2-in. [5-mm] image resolution invertical and azimuthal directions

■ fully processed images and dip data in real time

■ top combinable with other wirelinetools such as the ARI* AzimuthalResistivity Imager and AIT* ArrayInduction Imager Tool.

The FMI tool generates an electricalimage of the borehole from 192 micro-resistivity measurements. Special focus-ing circuitry ensures that the measuringcurrents are forced into the formation,where they are modulated in amplitudewith the formation conductivities toproduce both low-frequency signalsrich with petrophysical and lithologicalinformation and a high-resolutioncomponent that provides the micro-resistivity data used for imaging anddip interpretation. The depth of inves-tigation is about 30 in., similar to thatof shallow lateral resistivity devices.The image is normalized throughcalibration with low-frequency, deeperresistivity measurements from the tool signal or from another resistivitymeasurement tool.

The spacing of the button electrodes,innovative pad and flap design, andhigh frequency of data transmitted bythe digital telemetry system result ina vertical and azimuthal resolution of0.2 in. This means that the dimensionsof any feature that is 0.2 in. or largercan be readily estimated from theimage. The size of features smallerthan 0.2 in. can be estimated by quanti-fying the current flow to the electrode.Fine-scale details such as 50-micronfractures filled with conductive fluidsare visible on FMI logs.

The physics of the FMI measurementmakes it a highly versatile geologicaland reservoir characterization toolthat produces complete and reliableanswer products. These real-timeanswers are used to characterize therock tectonics, identify and evaluatesedimentary features, measure the rock texture and complement the information obtained from coring programs. FMI data are also used ingeomechanical reservoir analysis toidentify drilling-induced features suchas breakouts. Stress field analysis ofFMI data provides critical informationfor controlling wellbore stability prob-lems through improved design of themud program.

Real-time FMI answers help in making quickexploration, production and drilling decisions.

FMI pad and flap assembly with horizontallyoffset rows of electrode buttons.

Benefits■ Cost and time savings: com-

plete interpretation with oneimage pass

■ Data obtained in difficultenvironments, including deviated and horizontal wells

■ Accurate and reliable interpretation of formationfeatures

■ Improved reservoir description

■ Accurate pay estimate

■ Wellsite answer products for on-site decision making

Features■ Borehole coverage of

80% in 8-in. borehole

■ High-resolution measurement(nominal 0.2 in.)

■ Image details to 50 microns

■ Pad tilting for application indeviated and horizontal wells

■ High-quality, real-time well-site image and Mean SquareDip (MSD) computation

■ Multiple logging modes: fullbore image, four padand dipmeter

■ Top combinable with mostservices

■ Excellent signal quality fromfocused electric currents

■ Electrical images less affectedby borehole rugosity andwashouts

Measurement physicsThe figure to the right illustrates theFMI measurement principle. An appliedvoltage causes an alternating current(AC) to flow from each electrode buttonin the lower pad section through theformation to the electrode on theupper cartridge housing.

As the current emerges from a buttonon the pad or flap, its path is initiallyfocused on a small volume of the for-mation directly facing the button. Thecurrent path expands rapidly to cover a large volume of formation betweenthe lower and upper electrodes. The current consists of two components:

■ high-resolution component, modu-lated by the resistivity variations in the formation directly facing the button

■ low-resolution component, modulatedby the resistivity of the zone betweenthe lower and upper electrodes.

Making the measurement in AC mini-mizes the effects of what are essentiallydirect currents created by the friction

of the pads against the borehole walland by variation of the spontaneouspotential (SP).

The microresistivity image of theborehole wall is created from the current measured by the array of but-tons. The high-resolution componentdominates the image because its valuevaries from button to button. The low-resolution component appears only as a gradually changing background.Microresistivity changes related tolithological and petrophysical variationsin the rock, which are conveyed mainlyby the high-resolution current compo-nent, are interpreted on the image interms of rock texture, stratigraphic and structural features, and fractures.

The FMI buttons produce only a profile of the conductivity—the responsecorrelates with the formation conductivity,rather than an actual conductivity/resis-tivity measurement. This is because thehigh- and low-resolution componentsare generally similar in magnitude andimpossible to separate. The buttonresponse is typically scaled to present

a resistivity curve that coincides withthe shallow laterolog over distancesthat are large in comparison with theFMI electrode spacing. Over compara-tively short distances, the scaled curveprovides a high-resolution, but approxi-mate, resistivity log.

The resolution of the FMI tool is the button size at 0.2 in. In theory, thismeans that an object larger than 0.2 in.appears as its true size. An objectsmaller than 0.2 in. may be visible,depending on the resistivity contrastwith the background rock. If it isvisible, it may appear to be as large as 0.2 in. In conformance with sam-pling theory, the FMI image is sampledat one-half the resolution (0.1 in.) vertically and horizontally so that the theoretical resolution is not com-promised. To achieve 0.1-in. samplinghorizontally for the button diameter of 0.2 in., the buttons are arranged on the pad and flap in two horizontallyoffset rows.

Lower electrodes

Current

Upper electrode

Mass insulated sub

FMI measurement current path.

Characterization of rock structureFMI images provide critical informationif the rock structure and sedimentaryfeatures are significant determinants |of formation productivity. The figurebelow shows paired carbonates andsandstones with the same porosity butcompletely different permeabilities andhence production capabilities.

Collapse Breccia (14% porosity, 6000 BOPD)

Vuggy Limestone (14% porosity, 0 BOPD)

Slumped Sandstone(18% porosity, 0 BOPD)

Turbidite Levee Deposit(18% porosity, 5000 BOPD)

1.5 ft

4 ft

1.5 ft

2 ft

FMI images differentiate the structures that result in vastly different production potential for formations with the same porosity.

Net pay determination in laminated sedimentsThe FMI tool is the preferred approachfor determining net pay in the lami-nated sediments typical of fluvial andturbidite depositional environments.The method for obtaining an accuratesand count is to compute an averageresistivity curve from the FMI meas-urements and apply a cutoff that

distinguishes sand from shale.Laminations as thin as 0.2 in. can beresolved. Sections with laminationsthinner than this intrinsic resolutionare analyzed by calibrating FMI litho-facies with the sand count from core.The resulting sand-shale curve can beanalyzed for the bed-thickness distri-bution and used to derive mappableparameters.

250

260

270

20 120

5 15

60-in. AIT Resistivity

(ohm-m)

20-in. AIT ResistivityDepth1:48(ft)

XX10

XX20

XX30

0.2 20

Shallow FMI Resistivity

(ohm-m)0.5 5.5

Sonic Porosity

(p.u.)0 60

Density Porosity

Neutron Porosity

(p.u.)0 60

Horizontal Scale: 1:9Orientation North

FMI Image

Resistive

0 120 240 360

Conductive

Shale5 15 (ohm-m)0.2 20 (p.u.)0 60

Gamma Ray

(gAPI)

Caliper X

Caliper Y

(in.)

(in.)

Sand

In highly laminated sediments, the FMI log clearly shows the net pay zones that can contribute to production.

Visualization of sedimentary featuresThe interpretation of image-derivedsedimentary dip data enables the visualization of sedimentary structuresthat define important reservoir geome-tries and petrophysical reservoirparameters. The figure to the rightshows eolian dunes separated byinterdune deposits.

Fracture permeability directionThe principal stress azimuths obtainedfrom borehole image analysis definethe maximum permeability direction in fractured reservoirs. The fractureset that is aligned with the maximumhorizontal stress contains the widestfracture apertures and dominates thedirection of fracture permeability.

The stylolite that appears as a dark,irregular feature in the center of theimage to the right acts as a verticalpermeability barrier.

720

730

740

750

760

770

780

790

Interdune

Interdune

Dune

FMI images and dip data clearly differentiate the dune and interdune deposits. The data wereacquired from an 8 1⁄2 -in. diameter borehole.

3 ft

Fracture aperture (mm)

Maximumhorizontalstress

180°Azimuth

90°

0°

Minimumhorizontalstress

Dip50° 90°0.1 0.010.010.0010.0001

FMI data analysis identifies the maximum permeability direction in a fractured reservoir, which isimportant information for designing a completion for optimal production.

1

13

27

36

37

A

B

AB: Layer 20% thicker in Well 2

Well 2Well 1

Start offolding

Truestratrgraphiclayerthickness

1 13 27 36 37

m

1.3 km(Scale 1:1)

Structural interpretationWell-to-well correlation is difficult in deviated wells with sections ofsteep and varying structural dip. The structural interpretation of seis-mic sections is greatly improved bythe use of high-quality bedding dips to compute accurate logs of the truestratigraphic thickness (two well logs

second from left in the figure below).The two correlated wells are plottedon the seismic section using struc-tural dip (magenta lines) and strati-graphic markers (cyan lines). Theramp anticline is well defined by thewell data compared with the seismicsection. The far-right plot in the figurecompares the thickness of strati-

graphic intervals 1 through 37 in bothwells. The thicknesses are the same in interval A, but from stratigraphicmarker 13 to marker 27, the layers inWell 2 show increasing thickness incomparison with the layers in Well 1,indicating a syntectonic sedimenta-tion differential.

1

13

27

36

37

A

B

A

A: Equal layer thickness in both wellsB: Layer 20% thicker in Well 2

Distortedzone

Well 2Well 1

Start offolding

Truestratrgraphiclayerthickness

1 13 27 36 37

1.5 km

1.3 km(Scale 1:1)

FMI images and dip data are an independent complement for structurally interpreting seismic sections. The ramp anticline is better defined by the dip data than the seismic surface data.

Three-dimensional reservoir modelingOnce the structural framework isestablished within the geology andgeophysics workflow, the reservoir is divided into lithologic units, whichare then populated with propertiessuch as porosity, permeability andwater saturation to support reservoirsimulation. The lithologic units arecreated deterministically or stochasti-cally. The latter approach is shown

in the figure below and employs the Fluvial Simulation (FLUVSIM)methodology.

Sand-shale logs can be derived fromwireline and logging-while-drilling(LWD) logs from vertical as well ashorizontal wells. Critical guidance forstochastic modeling of the sand-shaledistribution is provided by the geologi-cal information derived from FMI bore-hole images. Crossbedding translates

into channel directions, and FMIimages superbly define channelheights in amalgamated units. Othervariables, such as the channel widthand channel sinuosity, can be esti-mated using geological analogs basedon detailed sedimentological analysis of FMI image data in conjunction with other logs and core data.

NCrossbeddip-azimuthalhistogram

S

E

N

W E

3 ft

Sand

Shale

The three-dimensional display shows the distribution of sand (orange) and shale (blue) as modeled for a fluvial reservoir using FMI-derived sedimentological information in an 8 1⁄2-in. well.

Key input for Mechanical Earth ModelsA growing application for boreholeimages is providing input and verifica-tion data for the Mechanical EarthModel (MEM) used to optimize wellconstruction. Better understanding of borehole stability can save opera-tors millions of dollars by shortening

the learning curve during the initialfield development.

The example below shows the bore-hole breakout and hydraulic fracturesand their relationship to the horizontalstresses. An MEM can predict the typeand position of induced features onborehole images. Disagreement

between modeled and observed fea-tures on the borehole images is usedto tune the assumptions made in theMEM, such as changing the azimuthor relative magnitude of the principalstresses.

Minimumhorizontalstress

Borehole breakout

Induced fracture

3 ft

A Mechanical Earth Model can be used to predict the type and position of drilling-induced features in the borehole. Knowledge of the directions and relative magnitude of the principal stresses is needed to optimize a drilling program.

SMP-5822 ©Schlumberger

April 2002 *Mark of Schlumberger

www.connect.slb.com

FMI Specifications

Applications Structural geology, stratigraphy, reservoir analysis,heterogeneity, fine-scale features, real-time answers

Vertical resolution 0.2 in. with 50-micron features visibleAzimuthal resolution 0.2 in. with 50-micron features visibleMeasuring electrodes 192Pads and flaps 8Coverage 80% in 8-in. borehole (fullbore image mode)Max pressure 20,000 psiMax temperature 350°F [175°C]Borehole diameter

Min 57⁄8 in.Max 21 in.

Max hole deviation 90°Logging speed

Fullbore image mode 1,800 ft/hr with real-time processed imageFour-pad mode 3,600 ft/hr with real-time processed imageDipmeter mode 5,400 ft/hr with real-time dip processingInclinometer mode 10,000 ft/hr

Max mud resistivity 50 ohm-mFMI tool

Max diameter 5 in.Makeup length 24.4 ftMakeup length with flex joint 26.4 ftWeight in air 433.7 lbmCompressional strength (TLC* operations) 12,000 lbf (safety factor of 2)

Max pad pressure 44 lbfCombinability Top combinable with openhole wireline tools

Maximum tool diameter 5 in.

24.4 ft

Tool zero(0 ft)

Pad section (1.3 ft)

FMI tool string.