Wireline logging

To derive a reservoir geological model various methods and techniques are employed mainly the analysis of core material, wireline logs, high resolution seismic and outcrop studies. These data gathering techniques are further discussed in Sections 5.3 and 2.2. 

A hole section which has been cored will subsequently be logged using wireline tools (see later in this section). A gamma ray (GR) measurement will be taken from the core itself, thus allowing calibration of wireline logs with core data. 

Figure 5.37 depicts the basic set up of a wireline logging operation. A sonde is lowered downhole after the drill string has been removed. The sonde is connected via an insulated and reinforced electrical cable to a winch unit at the surface. At a speed of about 600m per hour the cable Is spooled upward and the sonde continuously records formation properties like natural gamma ray radiation, formation resistivity or formation density. The measured data is sent through the cable and is recorded and processed in a sophisticated logging unita the surface. Offshore, this unit will be located in a cabin, while on land it is truck mounted. In either situation data can be transmitted in real time via satellite to company headquarters if required. 

One method of sampling reservoir fluids and taking formation pressures under reservoir conditions in open hole is by using a wireline formation tester. A number of wireline logging companies provide such a tool under the names such as RFT (repeat formation tester) and FMT (formation multi tester), so called because they can take a series of pressure samples in the same logging run. A newer version of the tool is called a modular dynamic tester or MDT (Schlumberger tool), shown in Figure 3.8. 

Perhaps the greatest stimulus for the development of such tools has been the proliferation of high angle wells in which deviation surveys are difficult and wireline logging services are impossible (without some sort of pipe conveyance system), and where MWD logging can minimise formation damage by reducing openhole exposure times. 

Kick Alert 1067. Horizontal Drilling, Geosteering 1070. Comparison of LWD Logs with Wireline Logs 1077. Comparison of MWD Data with Other Drilling Data 1078. 

The arrangement of Figure 4-218 is common to all directional tools based on the earth s magnetic field for orientation MWD tools or wireline logging tools. 

Total Gamma Rays. Total gamma ray logs have been run on electric wireline since 1940. The sondes are rather small in diameter (1.5 to 4 in. or 37 to 100 mm). 

The MWD total gamma ray tools cannot be calibrated in the standard pit, since they are too large. Their calibration in API units is difficult because it varies with the spectral content of the radiation. By spectral matching the MWD logs can be made to closely resemble the wireline logs. The logs which were recorded by the MWD companies in counts per second (cps) are now recorded in API units. 

Another difference between the wireline logs and the MWD logs is the logging speed. With a wireline, the sonde is pulled out at a speed of 500 to 2,000 ft/min (150 to 600 m/min). The time constant used to optimize the effect of the statistical variations of the radioactivity emission, varied from 2 to 6 s. Consequently, the log values are somewhat distorted and inaccurate. 

In MWD, the recording speed is the rate of penetration which rarely exceeds 120 to 150 ft/hr or 2 to 2.5 ft/min, two orders of magnitude less than the logging speed. Counters can be made shorter and time constant longer (up to 30 s or more). This results in a better accuracy and a better bed definition. Figure 4-269 shows an example of comparison between an MWD gamma ray log and the wireline log ran later. 

Figure 4-269. Example of good similarity displayed between the MWD gamma ray log and the wireline log.

Figure 4-272 shows an example of a MWD spectral GR log. On the left track, SGR is the total GR count, and CGR is this total count minus the uranium count. On the right side of Figure 4-272 the wireline spectral gamma ray in the same interval is displayed. The curves are similar but some differences occur in the amplitude of the three curves. 

Figure 4-272. Example of natural gamma ray spectral logs recorded while drilling and with a wireline.

The system is similar to the laterolog 3 used in wireline logging. A constant 1-k Hz AC voltage is maintained for all electrodes. The current flowing through the center electrode is measured. 

The drill collar acts as a series of elongated electrodes in a way similar to the laterolog 3 wireline sonde. The lower electrode, which is the drill bit, is used to get the forward resistivity curve. A lateral resistivity measurement is made between the two toroid receivers. An example of toroid logs is shown in Figure 4-279. 

A typical set of logs recorded while drilling is shown in Figure 4-280. The wireline caliper is shown in the gamma ray track. Displayed on this attachment are gamma ray, R curve, Pe curve, neutron and density curve. The delta-rho curve is the quality curve check for the density log. 

Figure 4-276. Comparison of the compensated dual-resistivity log resistivities run while drilling to the invaded and noninvaded resistivities calculated with wireline phasor induction data. The spurt loss is the ratio R /R (Courtesy Anadrill [tt3].)...

Figure 4-282 shows a typical comparison of wireline and MWD gamma ray and neutron logs in a borehole in excellent hole conditions. The MWD/LWD log matches the wireline log almost perfectly. 

Figure 4-284 shows a typical MWD density log compared to a wireline density log. The calipers are also shown. At 1620 ft, the wireline caliper detects a much larger caving because it was run several days later. 

The Gulf Coast logs shown in Figure 4-287 have been run in the same interval with MWD/LWD sondes and wireline sondes. 

Figure 4-287. Wireline and LWD logs showing the effect of invasion. (Courtesy Anadrill [113].)...

What is the invasion diameter at 8397 ft using the wireline logs What is the invasion diameter using the MWD/LWD logs ... 

According to chart in Figure 4-304, d, = 40 in. with the wireline logs. The d, cannot be calculated with the MWD/LWD logs since we have only two resistivity curves. 

Figure 4-289. Comparison of MWD ultrasonic caliper logs with the 4-arm wireline caliper logs run five days later. (Courtesy Anadrill [ 3].)...

Figure 4-292. Comparison of a wireline sonic iog with an LWD sonic log. (Courtesy SPWLA [116]. ...

Of particular interest in Figure 4-292 is the shaly sandstone in the 690-720-ft interval. In this zone, the LWD sonic measurements are consistently faster than the wireline measurements. Since the wireline logs were acquired 10 days after drilling, it is likely that shale swelling in the shaly sandstone has taken place. This phenomenon, known as formation alteration, causes the wireline sonic measurements to be slower. In this type of zone, LWD sonic yields a more correct At, which will better match surface seismic sections. 

Figure 4-294 shows a set of resistivity logs run in a sand-shale sequence of the Gulf Coast. We have one wireline dual induction log, one MWD resistivity log, a wiper-MWD resistivity log and one gamma ray log. 

Invasion causes curve separation for wiper and wireline logs. 

The log interpretation using logging while drilling logs is very similar to the interpretation made with wireline logs. One major difference is that the invasion is usually less important due to the short time elapsed between drilling and logging. 

Figure 4-301 shows sample neutron-density Pe log over the same interval. Figure 4-302 shows sample MWD resistivity (left) and wireline dual induction (right) for the same interval. 

Since the deep and shallow curve of the MWD log and the deep, shallow and guard (laterolog) of the wireline log shows a departure, the zone 2274-2356 ft is invaded, consequently permeable. Using the dual induction chart of Figure 4-304, we can plot the point at 2290 ft ... 

---