A Review of Three Years' Work in Europe and Africa with an Instrumented Motor
Authors: Bruce S. , Bezant P. , Pinnock S. 
Published: December 2007

Since its introduction in late 1992, the GeoSteering Tool has been used by 13 different operators on nearly 50 wells in Europe and Africa alone. This paper reviews the overall use of the tool and discusses several examples in detail (from both oil and water based mud jobs) to highlight different applications.

At BP Wytch Farm, a new technique has been pioneered which is critical to their extended reach wells where sliding is a problem. The combination of GeoSteering Tool near-bit inclination data and a variable gauge stabiliser positioned on top of the motor has enabled the wells to be drilled almost entirely in rotary mode. Use of the technique at Wytch Farm is discussed, as is its potential application in highly over balanced drilling where the risk of stuck pipe is high.

The importance of minimising the distance from the bit face to the sensors was highlighted on a well drilled by KerrMcGee: a single decision taken on the basis of the GeoSteering Tool bit resistivity meant 14% more productive footage than could have been achieved with behind-the-motor sensors.

At least two sidetracks were avoided on a well in a Texaco development in the UK North Sea: the wellsite geologist used near-bit azimuthal gamma ray to identify whether shale was being encountered from below or above and by steering correctly was able to bring the well back into the reservoir sand.

Throughout Europe and Africa, the GeoSteering Tool and the experience gained in using it are combining to allow wells to be attempted that could not previously have been contemplated.

The Industry's First Fully Instrumented Motor

In late 1992, the industry's first fully instrumented, steerable positive displacement motor, the GeoSteering Tool, was introduced into service in Europe.

The primary components of this tool are a steerable motor, an instrumented housing / sub and a fast wireless telemetry system that passes data from the GeoSteering Tool to the MWD system above (fig 1).

The Steerable Motor. The steerable motor part of the GeoSteering Tool is based on the standard (noninstrumented) 6 3/4" PowerPak motor: in fact, the power section, surface adjustable bent housing and bearing section are identical to those of the PowerPak. However, the GeoSteering Tool bearing housing, unlike that of the PowerPak, is machined with a fixed bend - either 3/4 or 1 1/4 - such that, when the GeoSteering Tool is made up, the fixed and adjustable bends are in approximately the same plane (less than 10 mismatch for adjustable bend settings in the range 0 - 1). This overall design makes it possible for the same hardware to be used for long-radius drilling or medium-radius buildup sections - see table 1 for typical build rates obtainable.

The Instrumented Housing / Sub. A four foot long instrumented sub is built into the motor between the surface adjustable bend and the bearing section. This sub contains the electronics package for the sensors, some of the sensors themselves and the batteries which power the tool. The remainder of the sensors are located on the motor housing.

Inclination and toolface data are provided from three inclinometers which are mounted orthogonally in the sub and which measure tool inclination with an accuracy of 0.2 (at 2 sigma). The sensors are sampled not just when pumps are cycled but continually as drilling proceeds: typically, inclination data is updated every 60 seconds (since, although a faster update rate can be achieved, this is not usually required). The measure point is 8.6 ft behind the GeoSteering Tool bit box.

The GeoSteering Tool contains a gamma-ray sensor 7.9 ft behind the bit box, the sodium iodide crystal / photo multiplier tube package being located in the instrumented sub. As the motor drive shaft goes through the centre of this sub - the entire electronics / sensor package is annular - gamma-rays have to travel through the drive shaft as well as the collar to reach the crystal from one side of the tool but only through the collar to reach it from the other side. This gives the measurement azimuthal sensitivity.

In water based mud, the GeoSteering Tool provides two laterolog resistivity measurements.

One, the bit resistivity, has a measure point typically only 1.25 ft behind the bit box and a depth of investigation of roughly 18 inches. The transmitting electrode for this measurement is the collar of the GeoSteering Tool below a ring antenna plus the bit itself. Because of the length of this electrode, the measurement has a vertical resolution of the order of 4.5 ft, the precise number depending - as does the distance to the measure point - on the length of the bit. The range for the measurement is 0.2 to 1000 ohm-m.

The second resistivity measurement, the arc resistivity, has a measure point 3.5 ft behind the bit box. The transmitter is an arc in the collar of the GeoSteering Tool which is electrically insulated from the rest of the collar but held at the same potential as the rest of the collar. The vertical resolution of this measurement is typically 4 inches and the depth of investigation about 12 inches.

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