FORMATION MICRO IMAGER TOOL

INTRODUCTION

The FMI is the latest generation electrical imaging device and belongs to the family of imaging services provided by most of the service companies.

The FMI (Fullbore Formation MicroImager) provides micro-resistivity formation images in water-base mud.

FMI uses scanning electrodes arranged in 24 electrode per pad/flap arrays (of four pads and four flaps) are used to provide a high spatial sampling of formation micro-resistivity in both the vertical and azimuthal directions on the borehole surface.

These two-dimensional micro-resistivity data are then mapped to gray scale or color to produce “core-like” borehole wall image that allows fine scale geological features to be described with a very good vertical resolution.

HISTORY

In the late 1980’s Schlumberger introduced the concept of borehole electrical images by processing variations of the shallow micro-resistivity of wellbore walls recorded by a tool Called the Formation Micro-Scanner (FMS).

This tool was developed by Schlumberger in 1991 as an improvement on the FMS4 (4 Pad-tool) developed earlier in 1987.

In the early stages this tool only measured closely spaced arrays of focused shallow resistivity readings that are related to changes  in rock composition and texture, structure, and fluid content.

With improved technology, micro-resistivity of deeper wells can now be determined.

WHAT DOES FMI MEASURES?

Image logs are resistivity or acoustic devices that measure certain physical properties of the rock at or near the well that can be displayed as images of the wellbore, which can then be interpreted on a computer.

Typically rock properties are controlled by factors such as variations in composition, diagenesis, grain size, grain orientation, pore fluid variations.

 Image logs can provide detailed picture of the wellbore that represent the geological and petro-physical properties of the section being logged.

TOOL SPECIFICATION

This tool is a semi-active focusing device , so the response cannot be output directly as resistivity but is relatively proportional to the conductivity of the formation.

A 16 Khz voltage is applied across each button with a return to the cartridge and the current at the button is measured at the same frequency.

The voltage is increased automatically against resistive formations and lowered against conductive formation to ensure activity on the individual micro-conductivity curves.

Electrodes: There are in total 192 electrodes distributed on four pads and four flaps.

Resolution: The high resolution component of response gives button resolution ~ button size (5mm).

The tool has a very large dynamic range - from less than 0.1ohm-m to more than 10,000ohm-m.

The maximum logging speed is 1600 ft/hr (500m/hr), but outside zones of interest, it can be run at 3200 ft/hr (1000 m/hr).

WORKING

The measurement principle of the micro-resistivity imaging devices is straightforward.

Formation Micro Imager (FMI), records an array of micro-resistivity measurements from 192 sensors on eight pads mounted on four orthogonally placed caliper arms. 

The spacing and position of the pads provides 80% coverage of an eight-inch diameter hole and a resolution of 5 mm.

The FMI yields a continuous, high-resolution electrical image of a borehole (color-coded for resistivity values), and therefore complements whole cores cut in the  same well.

An applied voltage causes an alternating current to flow from each button electrode into the formation and then to be received at a return electrode on the upper part of the tool.

The microelectrodes respond to current density, which is related to localized formation resistivity.

The tool, therefore, has a high-resolution capability in measuring variations from button to button.

The resistivity of the interval between the button-electrode array and the return electrode gives rise to a low-resolution capability in the form of a background signal.

The tool does not provide an absolute measurement of formation resistivity but rather a record of changes in resistivity. 

4 Arms - 8 Pads 192 Electrodes

FMI TOOL

INTERPRETATION

Structural:Fault,unconformities,sequence boundary

Stratigraphic:Depositional environment, orientation

Reservoir:Thin beds, Permeability trends, Fractures/Vugs,Borehole geometry

ADVANTAGES

Determine net pay

The FMI (formation micro Imager) gives micro-resistivity formation images in water-base mud. This is the preferred approach for determining net pay in laminated sediments of fluvial and turbidite depositional environments.

Visualize sedimentary features to understand structure

Sedimentary features define important reservoir geometries and petrophysical reservoir parameters. The interpretation of image-derived sedimentary data helps you understand sedimentary structures.

Interpret seismic sections

Well-to-well correlation is difficult in deviated wells with sections of steep and varying structural dip. Greatly improve your structural interpretation of seismic sections with high-quality bedding dips to compute accurate logs of true stratigraphic thickness.

Get more data

Geological information from FMI borehole images helps with stochastic modeling of the sand-shale distribution. FMI images define channel heights superbly in amalgamated units. Other variables, such as the channel width and channel sinuosity, can be estimated using geological analogs, based on detailed sedimentological analysis of FMI image data.

Improve well construction plans

Borehole images improve mechanical earth models, which in turn helps to optimize well plans. Better understanding of borehole stability can save millions of dollars during field development.

BENEFITS

Obtain accurate pay estimates

Interpret formations accurately

Improve reservoir descriptions

Make decisions on site

Get data in difficult environments, including deviated and horizontal wells

Save time and money with complete interpretations in one image pass

APPLICATIONS

Structural geology

● Structural dips, even in fractured and conglomeratic formations

● Detection and determination of faults

Sedimentary features

● Determination of sedimentary dips

● Definition and characterization of sedimentary bodies and their boundaries

● Recognition of permeability barriers, and permeability paths

● Recognition and evaluation of thinly bedded reservoirs

Rock texture

● Qualitative vertical grain-size profile

● Determination of carbonate texture

● Detection and evaluation of secondary porosity

● Detection and evaluation of fracture systems

Geo-mechanics

● Identification and analysis of drilling-induced features

● Mud weight selection

Reservoir characterization workflow

● Deterministic modeling of reservoir bodies

● Guidance for the distribution of reservoir bodies

● Realistic population of reservoir bodies with petrophysical parameters

DISADVANTAGE

FMI Tool cannot be used with oil based mud

CONCLUSION

The lighter the shade the more resistive the formation.

FMI measurements respond primarily to fluid properties.

Formation micro-imaging tools have proved superior to the ultrasonic tele viewers in the identification of sedimentary characteristics and some structural features such as natural fractures in sedimentary rocks.

They are especially useful for net-sand definition in thinly laminated fluvial and turbidite depositional environments.

REFERENCE

Petrowiki

Wikipedia

AAPG Wiki

http://www.slb.com/resources/publications/industry_articles/oilfield_review/1990/or1990jan05_microscanner.aspx

http://petphy.blogspot.in/2011/12/formation-micro-imager-logs-fmi.html

THANK YOU…