THE LOGGING PROGRAM
A logging suite should be selected on the basis of
· type of well--wildcat or development
· hole conditions--depth, deviation from vertical, hole size, mud type, and temperature
· formation fluid content--(fresh or salt) connate water
· formation type--clastic or carbonate
· economics--rig time, logging dollars, and so forth
Each tool is designed with a specific set of conditions in mind. Outside these limitations, the tool will fail to provide the intended measurement, and its use is discouraged.
Depth, Pressure, And Temperature Considerations
The majority of logging tools are rated at 20,000 psi and 350° F. These parameters are adequate for logging most holes. For higher temperatures, special tools may be available on a limited basis from the logging service companies.
Hole Size And Deviation
Six inches is the standard minimum hole size for correct and safe operation of normal logging tools. Slim-line, small-diameter tools are available for smaller-diameter holes on a limited basis.
Maximum hole diameter is difficult to define. Most pad contact tools (compensated formation density logs, micro-focused logs, dipmeter, and the like) have spring-loaded, hydraulically operated arms that push the relevant sensor pad against the borehole wall. The arms open to about 20 inches, although this limit varies from tool to tool. If holes are deviated, good pad contact might still be obtained, since the tool will tend to"lean" on the low side of the hole. However, optimum performance cannot be guaranteed. Running a pad contact tool in a hole of greater than 20 inches in diameter is risky because the pad may not be able to make contact with the wall of the wellbore. Similarly, tools that need to be run eccentered--for example, compensated neutron tools--are less accurate in enlarged holes.
Resistivity devices, such as induction and laterolog, suffer in a progressive fashion as the borehole gets bigger. Theoretically, there is no fixed limit to the hole size. In practice, however, there is a limit because borehole corrections to the raw data become so large that nothing useful can be determined from the logs.
Logging large-diameter surface holes may thus cause a problem. In this respect, the logging program may dictate the drilling of a medium-sized borehole, which will subsequently be underreamed to the desired gauge after logging.
In today’s offshore environment, the deviated hole is the norm rather than the exception. The greater the angle of deviation from vertical, the greater the difficulties of physically getting a logging tool to the bottom of the hole. In general, hole deviation greater than 40° causes problems for wireline logging, and deviations in excess of 52° will require more drastic measures, such as logging on the end of drill pipe. A number of techniques have been tried to get logging tools safely to bottom. Among them are
· keeping the openhole section as short as possible
· removing centralizers and standoff pads
· using a "hole finder," a rubber snout on the bottom of the logging tool string
· using logging tools especially adapted to be run to the bottom of the hole on drillpipe
In difficult situations, the hole may have to be logged through open-ended drillpipe with a slim logging tool physically pumped down by mud circulation. Using this technique, holes with deviations as high as 65° have been logged.
Most logging programs call for a minimum of an SP-Resistivity log, along with a measurement of porosity. However, where hydrocarbon reservoirs are more difficult to evaluate, a variety of porosity devices may be required to provide more accurate porosity data and lithology information. In addition, the reservoir engineer, the completion engineer, and the geophysicist may need additional information for evaluation and completion of the well. With the addition of computers to aid in formation evaluation, such comprehensive logging programs offer greater utilization of the measurements recorded.
Mud resistivity, formation water resistivity, hole conditions, and formation types dictate the type of devices needed. The extent of the logging program is also a function of the information obtained in previous wells.
A cross-reference list of tool nomenclature of the various service companies is presented in the Reference Section under Service Company Terminology.
Influence Of The Mud Program
The mud type influences the choice of logging tool, especially the choice of resistivity tool.
Air-drilled holes, which have no conductive fluid in them, must be logged with an induction device. Likewise, holes drilled with oil can only be logged with an induction log. Where conductive fluids are in the borehole for logging operations, the choice between induction and laterolog devices is controlled by the salinities of the mud and the formation water.
· Fresh muds and salty formation waters favor the induction log.
· Salty muds favor the laterolog.
All samples should be protected from excessive fluid losses so that porosity and saturation can be adequately determined. Bit cuttings can be sufficient to interpret lithology and determine proper constants for log evaluation formulas. Thus, the mud program should be designed for both the drilling and the logging operations.
It is possible for a logging program to succeed or fail strictly because of the design of the mud program. For example, filtrate from a high-water-loss mud can invade a formation so deeply as to mask the measurement of true resistivity, reduce the amplitude of the spontaneous potential curve, obscure the detection of the residual hydrocarbons, and result in water recovery on a drillstem test from zones that would otherwise produce oil. Invasion of oil from oil-base or oil-emulsion muds can increase the resistivity (Rxo) and decrease the water saturation (Sxo) of the invaded zone. This effect would erroneously indicate the presence of oil in water-bearing formations, or reduce formation porosity values calculated from microresistivity devices.
The practice of "mudding up" just before reaching the objective zone affects interpretation when mud filtrate invades the formation beyond the radius of investigation of the resistivity device. Friable formations, as well, drilled with natural high-water-loss muds are usually badly washed out and can prevent the logging tools from going down the hole because they hang up on ledges and/or bridges. Borehole contact devices cannot obtain effective contact with the side of the borehole in highly rugose holes and will give erroneous measurements. Normally, the extent of washouts through shale is in proportion to the water loss of the muds (i.e., the higher the water loss, the larger the washouts). Since many development and semi-wildcat wells are drilled with natural high-water-loss muds through the shallower formations, reliable analysis of logs through these intervals is most difficult. The decision to drill with natural high-water-loss muds through shallow formations is normally based on the erroneous assumption that the shallow formations are of no interest. However, the logs through the shallow formations are invariably consulted later to find zones for recompletion, to determine prospects for new hydrocarbon-bearing zones in the area, to locate and evaluate high-pressure zones, and for general correlation work.
Deciding When To Log
Most logs should be run just prior to running the casing string. Once casing is set, logging choices are severely limited.
It is recommended that openhole wireline logs be run (1) if hole conditions suggest that a section of hole could be "lost" (caving, washouts, etc., which would contraindicate the running of a logging tool), (2) if cuttings indicate that an unexpected formation has been encountered, and/or (3) if one is otherwise "lost" structurally.
However, one’s enthusiasm for running logs must be tempered somewhat by the economic and practical realities of service company price lists and fee structures. Each time a logging truck is called, a setup charge is assessed to cover costs of mobilization. In addition, a depth charge is assessed per foot of hole from surface to total depth. Finally, a survey charge is assessed over the actual interval logged. The full cost of a logging operation is thus, more than anything else, a function of the depth of the well. To log a l00-ft section at 10,000 ft is an expensive proposition, while a 4000-ft survey at 5000 ft total depth is probably less expensive.