Until electrical micro imaging of borehole walls became possible more than a decade ago, the most reliable means of identifying and quantifying important details of subsurface geometry of formations and their geological characteristics was to core zones of interest and retrieve those cores to the surface for thorough hands-on examination.

Halliburton's electrical micro-imager (EMI™) was introduced in 1992. It was the first such imager that could scan more than 60% of the borehole and provide the operators with highly detailed, core-like images of the subsurface layers of the rock layers.

EMI, to a significant extent eliminated the need for costly, time-consuming coring operations in most downhole environments. The technology also far exceeded the capabilities of contemporary conventional open-hole logging tools and other imagers to identify and evaluate sub-centimeter-size features such as small fractures, vugs, bedding planes, depositional details, thin beds, and rock texture changes. This marked a major milestone in the successful application of wireline logs for reservoir description and had a direct bearing on expensive decisions such as choosing offset well locations, picking intervals for formation testing, perforations, and stimulation.

How the EMI tool works
EMI has a unique mechanical design incorporating six independent, articulating arms, each outfitted with 25 small electrodes on an electrically charged pad. Independent arm linkages and pad articulation facilitate maintaining optimum pad contact with a minimum of pad pressure, even in rugose, washed-out, or non-circular bore holes.

During logging, the six pads extend to make contact with the borehole wall at the desired downhole intervals. An electrical current flows from the pads into the surrounding rock then upward in the well bore to return at the top of the tool. Electrical micro-resistivity contrast in the rock layers generates the signal measured by the tool, which is then sampled 120 times per foot. Imaging software converts the raw resistivity signals into a cohesive, color-coded image of the borehole wall.

Conventional dipmeter information is embedded into the EMI image data, and a navigation package is included to provide accurate information on tool position and orientation within the borehole.

By virtue of the core-like Images it produces, the EMI facilitates detailed structural, stratigraphic, and sedimentological analyses, providing the opportunity to optimize offset well placement, completion tactics, and hydrocarbon-recovery efficiency. The technology is also used to accurately delineate thin beds, determine the orientation and connectivity of fracture systems, characterize secondary porosity, and improve estimates of net to gross pay.

Halliburton introduces the XRMI tool
Despite the effectiveness of EMI in most applications and its widespread acceptance among oil and gas producers, the technology has not been without limitations. This is especially true in an environment in which the formation resistivity is high and highly conductive drilling muds are used, creating extremely high ratios between true formation resistivity (Rt) and mud resistivity (Rm). Such conditions make it difficult for the EMI's electrical signal to penetrate surrounding rocks. If the Rt:Rm ratio is too high—100,000 or more, for example—the quality of the EMI images can be seriously degraded, to the point that important reservoir characteristics can be misinterpreted or overlooked altogether.

But now the logging and perforating business unit of Halliburton Energy Services has developed and has begun introducing a new electrical wireline borehole imaging tool—based upon the design of its popular EMI tool—for obtaining superior quality images of borehole micro-resistivity, even in high Rt:Rm environments. The so-called extended range micro-imager (XRMI™) tool was introduced in August 2003, and worldwide deployment of the device has begun.

XRMI specifications and features
The XRMI is a drop-in replacement for the EMI tool and requires little or no additional training to begin using the field. It uses the same data processing platform as EMI, and the new micro-resistivity imager's basic software package already is available at Halliburton computing centers, as well as in the possession of some operators.

The XRMI is effective in temperatures as great as 350 degrees Fahrenheit and in well bore pressures of up to 20,000 psi. The tool may be deployed via electric line or drill pipe-conveyed operations in well bores ranging in diameter from 6-1/4 inches to 21 inches, in wells drilled with either fresh- or salt-water drilling muds, and provides at least 67 percent coverage in well bores up to 8-1/2 inches in diameter.

Although the XRMI tool looks like—and to a significant degree acts like—its EMI predecessor, the operating range of the new tool greatly exceeds that of conventional electrical wireline borehole micro-resistivity imaging tools, serving across a wider scope of downhole conditions. This is achieved fundamentally with a new state-of-the-art digital signal acquisition architecture combined with a large increase in the power used to generate the excitation current injected by the pad electrodes into the formation.

An XRMI formation evaluation answer product generated by Halliburton's proprietary software "WXforecast". First image track shows the static equalized image and the second image track exhibits the texture-enhanced high resolution image produced by the application "texture-pro". Central dip-track shows the results of "Auto-Dip", an automated dip-picking application. The sharp change in the dip azimuths from NW to NE is interpreted to be due to "slump faulting". The base of the channel sand is interpreted as a scoured surface. This XRMI log is from a test well in the Fort Worth Basin, Texas.

The resulting signal-to-noise ratio of the raw micro-resistivity measurements is improved five-fold and the dynamic range is expanded by a factor of three. The resulting images offer superior resolution and fidelity in nominal downhole environments, as well as in formations with resistivity greater than 2000 ohm-m or in saline borehole fluids with resistivity less than 0.1 ohm-m.

Because it borrows the mandrel architecture from Halliburton's highly successful EMI tool, the pads mounted upon XRMI's six independent, articulated arms maintain contact in rugose, washed-out, elliptical, or highly deviated bore holes. The heart of the XRMI's improvement, however, lies in the fact that the XRMI employs ultra fast, latest-generation digital electronics very early in the signal processing chain. This significantly increases both quantity and quality of raw data samples coming into the tool, which intrinsically improves resolution of borehole features.

Another feature of the XRMI is that the radiated tool power (EMEX) is increased several times over that available from the EMI. Thus, for any given Rt:Rm, more current is available for injection Into the formation, and thus more signal is retrieved for image computation.

Finally, the XRMI offers improvement in operational reliability in that the electronic architecture is designed such that each arm-and-pad assembly is electronically independent. Thus, if one pad fails during open-hole logging operations, the other five pads will continue functioning normally, eliminating the need to trip the logging assembly out of the hole to change out a malfunctioning component.

XRMI in high Rt:Rm environments
Field tests of the XRMI tool show it provides higher resolution, higher fidelity micro-resistivity images than EMI technology in all types of subsurface conditions. Even under nominal Rt:Rm conditions, where the EMI tool works well, XRMI delivers across-the-board improvement, providing high-resolution digital images many times the quality of the analog images possible with EMI.
However, there is no alternative for XRMI technology for identifying and documenting the make-up of many extreme downhole environments, such as carbonate reservoirs drilled with saline mud, in which Rt:Rm ratios can reach as high as 1 million. Vugs and fractures in such formations frequently constitute the predominant pore types available for storing gas and liquids. Understanding the distribution of vugs and fractures in carbonate reservoirs can contribute significantly to the accuracy of fluid-flow modeling and estimates of secondary porosity, considerably enhancing the accuracy of reserve estimates and improving ultimate production.
 
High resolution XRMI images showing the micro-textural geological details in the fabric of a limestone section in a test well from Permian Basin, W. Texas. The Rt:Rm ratio exceeds 100,000 in this borehole.
 
 

Recently, XRMI logs of carbonate reservoirs from two wells in the Permian Basin were analyzed. Results clearly demonstrated the ability of the new imager to enable description and quantification of vugs and fractures in limestone and dolomite in bore holes characterized by high Rt:Rm ratios.

Reducing E&P risks>
Taken together, the core-like electrical micro-resistivity images generated by the XRMI tool can improve the effectiveness of hundreds of wellsite processes and decisions.

XRMI can help reduce exploration and development risks by taking the guesswork out of identifying and characterizing subsurface sedimentary sequences. The technology also can be used to reveal bedding dips that help rationalize the choice of next drilling location, or to help choose sidewall core zones, formation testing zones, and perforation intervals accurately by integrating XRMI images with other open hole logs.

By helping in the evaluation of subsurface structural and stratigraphic features and delineating bed and lamination orientations, XRMI can help improve net-to-gross pay estimations in laminated shaly sands and carbonates. The new borehole imaging technology also can rationalize well-stimulation and formation-testing decisions by characterizing the secondary porosity in reservoirs.

In addition, XRMI can help optimize drilling efficiency by evaluating and orienting borehole breakout, and optimize completion tactics and reservoir management by providing rock texture and electro-facies characterization.

The XRMI is now commercially available and Halliburton is engaged in deploying it worldwide.