TECHNOLOGY Logging while tripping cuts time to run gamma ray

A new logging tool, sent down the drillstring before a pipe trip out of the hole, logs the well as the pipe is pulled from the hole, cutting the total time required for open hole logging on some wells.

This logging while tripping (LWT) technology allows an operator to run a gamma ray and neutron log more quickly than with standard wire line equipment.

The drill pipe conveys the logging tools up the well as the pipe is tripped out, and the logging data are stored in the tool's memory. A "transparent" logging collar, which meets API 7 and RP7G requirements, holds the tools in the drillstring without interfering with the logging measurements.

Tool designer LWT Instruments Inc. says this logging format can reduce the risk of losing logging tools downhole, a problem associated with conventional wire line methods. Furthermore, total logging time can be cut by about 50% by eliminating multiple trips and mud conditioning sequences involved in a logging cycle, according to LWT Instruments.

To prepare for conventional open hole logging after drilling a section, an operator typically circulates and conditions the mud for several hours before pulling the pipe out of the hole. The logging unit is rigged up, and the wire line logs are then run. If casing is to be run, the operator usually makes a trip back in the hole to circulate and recondition the mud. The drill pipe is pulled out, and then the casing is run.

With LWT, the biggest advantage comes in the elimination of the time needed to trip the pipe back in the hole, recondition the mud, and then trip back out. The other large time savings comes from the lack of rigging up and rigging down wire line logging equipment at the surface. Table 1 [29000 bytes] compares the time needed to run a conventional wire line log to the time needed to run an LWT log. In the table, the actual logging time for LWT is the trip out time, which is only slightly longer than tripping the pipe normally.

Another advantage of logging while tripping, though not as easily quantified, is a reduction in the possibility of well bore collapse. The longer an open hole section remains uncased, the greater is the chance for well bore problems from swelling or sloughing formations.

Currently, the only LWT tools available are gamma ray and neutron porosity. Other logging while tripping tools under development by LWT Instruments include inductive resistivity, spectral gamma ray, density, dip meter, microresistivity, compensated sonic, and caliper. The company is also investigating a spontaneous potential tool.

Applications

The LWT technology has been developed during the past 6 years and has undergone field trials in Canada in the past 18 months with Amoco Canada Petroleum Ltd., Chevron Canada Resources Ltd., Renaissance Energy Ltd., and Talisman Energy Inc. According to some of the operators who have field tested the tools, LWT technology has several specific applications:

Depth correlation for open hole drill stem tests
Use for minimum logging requirements, with a waiver from the Energy Resources Conservation Board in Calgary, for the intermediate hole and the main hole (if determined dry by other means)
Picking of formation tops and identifying faults
Logging wells which cannot be logged with conventional methods because of unstable hole conditions or the inability to get logs to bottom.

The LWT tools can be run in inclined or horizontal wells for the same cost as in vertical wells, according to LWT instruments. Once the tool is run inside or pumped down the drillstring, the log is made as the pipe is pulled, irrespective of hole angle.

The logging while tripping tools are small, which allows them to be run in production logging applications as well as in drilling. Also, multiple tools can be run in the same trip. The tools are not articulated yet, however. Given their length and size, they cannot go through any curve which exceeds 33/100 ft.

The logging tool (111/16-in. OD) is lowered into the drillstring on a survey line. If the drillstring is inclined, a pumping sub is connected at the surface, and the tool can then be pumped down to the landing sub. Once the logging tool reaches the landing sub, the sinker bars above the tool will compress a release mechanism and allow the survey line and sinker bars to be retrieved. Fig. 1 [22000 bytes] shows the basic tools used.

When the logging tool is located in the landing sub, it is still possible to circulate through the drillstring. The restriction, however, increases the circulating pressure up to 35 psi.

Once the tool lands in the sub, the drillstring can be tripped out of the hole. The tool will begin recording logging information at a predetermined time, which is programmed into the tool at the surface beforehand. The tool can also be programmed to log as the drillstring is run in the hole.

The tool memory can store up to 18 hr of 16-bit data, when a reading is taken every 0.25 sec. If further logging is necessary before the pipe is completely pulled from the well, the tool can be retrieved with an overshot sub and survey line. The memory can be dumped at surface so the tool can be rerun to continue logging.

Ordinarily, the pipe is pulled at about 30 fpm for optimum logging. This speed is slow enough to allow logging, yet fast enough so that the tool's memory or battery life do not become limiting factors.

The batteries can last 100 hr downhole. The tool can withstand a temperature of 375 F., but the batteries have a temperature limitation of 350 F.

Once the tool is pulled from the landing sub at the surface, the data are downloaded via a serial port into a personal computer at the rig site. The log data are then merged with a depth/time log which is acquired through a simple acquisition system rigged up prior to the trip; this process takes about 2 hr and can be done days ahead. The merged files then produce a log to American Petroleum Institute standards.

The depth acquisition system on the surface consists of a set of targets on the crown's fast sheave and a pair of inductive sensors. The sensors, in combination with software, determine the direction of the sheave's rotation and apply incremental values of drillstring position change to the depth/time log.

Used in conjunction with this system is a pressure transducer attached to the dead line. The pressure transducer, after calibration, detects whether the block moves up or down with the drillstring or if the drillstring is in the slips. On some wells in which the tools are used to log as the pipe is run in the hole, the pressure transducers are overridden. The operator then enters the control data. This step is necessary on some wells because of the effect of tight hole on dead line weight.

Data resolution

To match wire line quality log data, LWT Instruments uses different scintillation crystals (than those used in wire line tools) in the gamma ray tool. The gamma ray tool has better resolution than some measurement while drilling (MWD) formats, according to LWT Instruments.

To reduce signal attenuation caused by drill collars, a special drill collar was developed with internal and external relief. A new patent-pending drill collar eliminates signal attenuation for acoustic, nuclear, and inductive tools.

Pipe tripping speeds are controlled, usually to 25 fpm, because of the effect on data resolution. Faster pipe tripping reduces the amount of data recorded per foot. Similarly, when the drillstring is stopped to set back a stand or lay down pipe, the tool records a large amount of data as the pipe stands still momentarily. Software analysis either smoothes or dumps the extra data.

Talisman Energy Inc., with partners Imperial Oil Resources and Sceptre Resources, ran a comparison test of an LWT log and a standard wire line gamma ray log in its C-55-I/93-I-14-ST well (Fig. 2) [10000 bytes]. The two curves in the log correlate well, indicating little difference between the LWT gamma ray and the standard wire line gamma ray. As a result, Talisman and several other operators in Western Canada run the LWT logs and do not repeat the logged section with wire line later.