Formation evaluation data used for geosteering is often limited by equipment availability to gamma ray measurements only. Gamma ray logging is effective but in many cases has

Figure 1. Sondex 4.75-in. compact propagation resistivity (CPR) tool is shown moving into place for makeup in the directional bottomhole assembly. (Image courtesy of Sondex)
produced mixed results. Gamma ray measurement is limited to a shallow depth of investigation and does not show significant response until the detector is close to an adjacent bed with significant radioactive contrast. Also, the gamma ray log is not sensitive to oil-water contacts or to adjacent beds without radioactive contrast.

When combined with the gamma ray log, resistivity measurements while drilling significantly enhance geosteering capability. Resistivity measurements offer deeper depth of investigation and respond to adjacent beds with resistivity contrast at a much greater distance.

In some cases, the resistivity tool will respond when it is still several feet away from an adjacent bed. Also, the resistivity tool responds to the oil-water contact in a thick producing bed. If the resistivity tool has multiple depths of investigation, the separation between deep and shallow measurements can be used to identify the approach to a nearby bed with contrasting resistivity.

Sondex recently teamed with Focus Directional Services Inc. to complete a trial run of its 4.75-in. compact propagation resistivity (CPR) tool in combination with its Geolink MWD System. The CPR tool uses two frequencies (400 KHx and 2 MHz) and three transmitter- receiver spacings to provide six resistivity measurements, which provide accurate formation evaluation data. The MWD system was used to acquire directional and gamma ray data, while the CPR was used to provide resistivity measurements. The company also used its surface software to gather real-time data and process memory data from the downhole tools.

Prior to running the tools, the borehole had been deviated to approximately 90° inclination with a maximum dogleg severity of 15°/98 ft (30 m). Casing was set to the end of the turn at 5,299 ft (1,615 m). The CPR and other tools were made up in the BHA, which included a mud motor with adjustable kickoff set to a 1.8° bend.

The well plan was for two horizontal (88 to 92° inclination) legs. Leg 1 reached 1,524 ft (470 m) from casing in a west-southwest direction. Leg 2 also reached 1,524 ft (465 m), in a southerly direction, after an openhole sidetrack at approximately 5,631 ft (1,634 m) measured depth. The target zone for both legs was a low permeability, high porosity dolomite — not more than a few meters in thickness — with expected true vertical depth from 4,937 to 4,970 ft (1,505 to 1,515 m). The low permeability, high porosity, and relatively thin payzone thickness are typical for plays that show the most benefit from horizontal drilling and geosteering.

The target resistivity as determined from nearby offset vertical logs was less than 1 ohm-m. The low-resistivity pay was underlying a lower porosity, higher resistivity zone above, including very high-resistivity evaporates. Below the pay was a lower porosity, higher resistivity limestone.

Together, the geologist and directional driller worked to geosteer the well using data from cuttings, drilling mechanics measurements and the gamma ray and resistivity logs. For the real-time log, 2 MHz deep and shallow uncompensated resistivity curves were presented along with the gamma ray measurement. The uncompensated resistivity measurements were selected to provide the quickest real-time updates, ensuring data density at the high expected rate of penetration (ROP) (up to 328 ft or 100 m/hr).

The borehole conditions were benign — mud resistivity was close to formation resistivity (Rt/Rm ~ 1) — and the borehole diameter was only slightly larger than the tool diameter. Under these conditions, the uncompensated measurements provided a high quality log.

When approaching a higher resistivity bed boundary at a high angle, the 2 MHz deep resistivity separates from the 2 MHz shallow resistivity. In Leg 1, the resistivity curves separate and the gamma ray log also increases at both 5,610 ft and 5,741 ft (1,710 m and 1,750 m) measured depth. Both gamma ray and resistivity detect a change in rock properties at this depth. However, in the zone from 6,069 ft to 6,266 ft (1,850 m to 1,910 m), the gamma ray log remains low while the 2 MHz deep resistivity steadily increases to a maximum between 6,266 ft and 6,299 ft (1,910 m and 1,920 m).

The separation between shallow and deep resistivity increases, indicating the approach to a higher resistivity bed nearby. Below 6,332 ft (1,930 m), the two resistivity curves converge — indicating that the higher resistivity bed is now farther away. Below 5,905 ft (1,800 m), the gamma ray measurement shows little character or indication of approaching bed boundaries, due either to a lack of radioactive contrast or range to the adjacent bed exceeding the depth of investigation of the measurement.

In Leg 2, resistivity measurements continue to show approaching bed boundaries while the gamma ray log remains relatively flat. At 5,987 ft (1,825 m), the 2 MHz deep resistivity spiked up sharply, signaling that the tool has crossed a bed boundary. A similar spike occurred at 6,758 ft (2,060 m) near the end of Leg 2, as the well approaches the higher resistivity beds above.

The memory logs confirm the data from the real-time logs. In all cases, the medium resistivity lies between the shallow and deep resistivity, indicating proper calibration and accuracy.
Despite slightly deeper measurements from the 400 KHz measurements, the 2 MHz data appears to be more sensitive to approaching bed boundaries, with greater separation between deep and shallow measurements. The general agreement between 400 KHz and 2 MHz confirms that no additional dielectric corrections are required for the 2 MHz measurement.

In conventional vertical or directional drilling applications where the borehole traverses multiple formations, the multidepth of investigation capabilities of the resistivity tool provide critical information to the log analyst.

When borehole pressure exceeds formation pressure in permeable formations, as when drilling overbalanced or drilling through depleted zones, mud filtrate invades the permeable formation. Mud filtrate alters the resistivity of the formation surrounding the borehole. If the resistivity of the mud filtrate is lower than the resistivity of the formation fluid (e.g., water-based mud displacing hydrocarbons), the altered or flushed zone resistivity will be lower than the true or virgin formation resistivity. If the resistivity of the mud filtrate is higher than the resistivity of the formation fluid (e.g., oil-based mud filtrate displacing salt-saturated formation water), flushed zone resistivity will be higher than true resistivity.

For decades log analysts have used wireline resistivity logs with three depths of investigation (shallow, medium, and deep) to detect the presence of invasion. The CPR also provides three depths of investigation at both operating frequencies. Using multiple depths of investigation, the log analyst can readily identify permeable zones that have been invaded during drilling. In real time, the shallow and deep readings are sufficient to signal the presence of invasion. After the tool is returned to surface, additional measurements stored in memory are available to confirm the real-time interpretation and perform more quantitative analysis. LWD data acquired within minutes or hours of drilling can also be compared to wireline data acquired days later after reaching total depth to determine the progress of invasion over time.

When the resistivity of the drilling mud is much less than the formation resistivity and the diameter of the borehole varies from the bit diameter, uncompensated resistivity logs can appear “spiky” or “noisy.” A spike in the log signals an abrupt change in the borehole diameter. These spikes can occur when the formation resistivity is high (e.g., 100 ohm/m) and the borehole fluid is salt-saturated water-based mud with low resistivity (e.g., 0.1 ohm/m). Under these conditions, the operator has the option of transmitting additional resistivity measurements in real time to provide a borehole-compensated resistivity log at the surface. The borehole-compensated log minimizes and in many cases eliminates the spikes in the uncompensated log. Data stored in tool memory always includes all measurements necessary to implement borehole compensation. Logs processed from memory data after the tool returns to the surface are always borehole compensated.