Conventional Measurement While Drilling (MWD) tools

A conventional Measurement While Drilling (MWD) system consists of a downhole probe, data transmission system and a surface equipment package. Directional data is measured by the downhole probe and sent by mud pulse telemetry or electromagnetic waves to surface. With most of the tools the different modes of operation can be changed by a pulse sequence.

Downhole probe

The downhole probe of a Measurement While Drilling (MWD) system conventionally comprises three solid state accelerometers to measure inclination and three solid state magnetometers to measure azimuth. The downhole probe is identical to that of solid state single and multi-shot tools and is placed in a non-magnetic collar.

Data transmission

Three primary means of transmitting data to the surface exist:

1.Mud pulse telemetry encodes the data in binary format and sends them to the surface by either positive or negative pressure pulses generated in the drilling fluid where they are detected by pressure transducers on the stand-pipe and decoded by a surface computer.

2.Continuous-wave telemetry, a form of positive pulse, employs a rotating device that generates a fixed frequency signal which sends binary information encoded in phase shifts on a pressure wave to surface through the mud column. Main advantage of the continuous wave telemetry system over the positive and negative system is the high pulse frequency reducing necessary survey time.

3.Electromagnetic transmission uses low frequency electromagnetic waves passing through the formation. These are received with an antenna placed in the ground adjacent to the rig site. The system has a limited depth range dependent on the resistivity of the formations. The lower the resistivity, the shallower is the useful depth range. At present this is typically between 1000 and 2000 metres. Contrary to the positive, negative and continuous wave telemetry systems, the electromagnetic telemetry system can be used if the well is shut-in, e.g. for under-balanced drilling .

Surface equipment

Typical surface components of a mud pulse Measurement While Drilling (MWD) system include pressure transducers for signal detection, electronic signal decoding equipment, and various analogue and digital readouts and plotters.

Quality assurance

Quality assurance of Measurement While Drilling (MWD) tools is identical to that of solid state single and multi-shot tools. In addition to this a function test should be performed prior to running the BHA to bottom.

Typical procedures:

1.Carry out surface function test. Check alignment of the Measurement While Drilling (MWD) tool with the bent sub, if applicable.

2.Carry out shallow test procedure.

3.The Measurement While Drilling (MWD) tool should be tested, whenever it is practical to do so, as close to the surface as possible. This is typically 1 to 2 stands of drillpipe below rotary. The procedure is as follows:

-attach kelly or top drive;

-take survey and wait for complete survey transmission. The criteria for a satisfactory survey are:

-inclination should be less than 1°;

-gravitational field should be within 0.003 g of the expected value;

-note that magnetic data taken inside the riser or casing are not valid;

-if the test is satisfactory, and mud pulses are decoded continue running in. If unsatisfactory, return tool to surface.

4.Carry out benchmark survey. Run in hole so that the Measurement While Drilling (MWD) sensor is at the benchmark station and carry out benchmark survey as follows:

5.The benchmark station is about 15 m (50 ft) below the previous casing shoe, but far enough from other wells to avoid possible magnetic interference to the Measurement While Drilling (MWD) system.

6.Carry out a check survey. This will be taken on bottom just prior to drilling and preferably as near to the last Measurement While Drilling (MWD) survey taken on the previous run as possible. It may be necessary to use the last but one Measurement While Drilling (MWD) survey from the previous run. This survey will confirm the accuracy of the previous run's survey data. When discrepancies of more than two degrees in azimuth and half a degree in inclination are observed in these check surveys, the office should be consulted to advise on the necessary action.

7.Run in hole and drill ahead taking surveys as required or orient toolface.

8.Any doubtful survey should be verified by taking another Measurement While Drilling (MWD) survey.

As a professional manufacturer of logging tools, our technical engineers have many years of experience in completion and logging, all the logging tools on sale can be customized according to customer requirements, of course, we can also provide you with on-site services to help you carry out on-site measurements. If you are interested in completion and logging equipment, please do not hesitate to get in touch with the Vigor team, we will provide you with the most professional technical support and product support at the first time.