The term Measurement While Drilling (MWD) includes any tool that can be used while drilling, including wireline steering tools. In practice it means 'wire-less' measuring systems, e.g. systems using mud pulse or electromagnetic telemetry.
Measurement While Drilling (MWD) surveys can be used both as orientation surveys with steerable BHAs, or to replace magnetic multi-shot surveys while rotary drilling. Measurement While Drilling (MWD) surveys are of comparable accuracy as wireline solid state magnetic surveys and can be used as definitive surveys.
The simplest Measurement While Drilling (MWD) system is the Teledrift offered by Scientific Drilling International, which is a mechanical inclination-only mud pulse tool. Most basic Measurement While Drilling (MWD) systems take surveys of inclination and azimuth at discrete survey stations. More sophisticated systems have the capability to have extra sensors, for drillstring mechanics (e.g. downhole weight on bit, downhole torque, downhole shock, downhole pressure and temperature) or petrophysical evaluation (e.g. gamma ray, resistivity, density and porosity). Most of the systems transmit the data to surface using pressure pulses in the drilling mud inside the drillpipe.
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.
Running procedures
The different tools have different procedures for initiating the survey (e.g. by stopping the pumps). Some tools can have the mode (e.g. survey or steering) changed by communicating with the tool from surface, usually by cycling the mud pumps.
Quality control
Quality control of Measurement While Drilling (MWD) surveys is mainly governed by correction programme.
Uncertainties of Measurement While Drilling (MWD) tools
Uncertainties of Measurement While Drilling (MWD) tools comprises tool uncertainties, geomagnetic uncertainties, correction uncertainties. System uncertainties are BHA deflection, drillpipe tally uncertainties and toolface dependent misalignment. Toolface dependent misalignment is corrected for by correction programme by use of a rotational shot.
Geo-steering
This new technique has been primarily developed for horizontal wells and thin reservoirs. The well is drilled and steered using data from petrophysical logging information sensed by Formation Evaluation tools such as Gamma Ray, Resistivity and Sonic. The success of geo-steering depends on having an accurate reservoir model and log information.
A typical sequence for planning and drilling a well using geo-steering is as follows:
1.Review the available reservoir models and petrophysical data to determine a suitable target for the well and the tolerances.
2.Plan the well path.
3.Decide at which point geo-steering will be started. Make a note of geological markers that can be used to check the progress of the well and see if there are any unexpected geological changes.
4.If necessary, plan a non-horizontal pilot hole through the reservoir so that it can be logged and markered within the reservoir defined. The pilot hole can then be plugged back.
5.Drill the well to the point for starting geo-steering, using directional surveys, but monitoring geological markers to see if the reservoir position and well plan need revising.
6.Change to the geo-steering BHA and steer the well through the reservoir using the geology rather than according to the geometric well plan.
7.If required, a further pilot hole and plug back may be needed to check the reservoir characteristics.
Most Measurement While Drilling (MWD) companies have systems and sensors that can be used for geo-steering. The essential elements are a steerable BHA, with logging sensors as close to the bit as possible. These often include inclination measurements, but not magnetic azimuth because it is not possible to have a sufficient length of NMDC. However, accurate azimuth data is not always required as geosteering is mostly used where vertical depth hence inclination is critical. The systems integrate directional drilling, drilling mechanics and petrophysical data measured down hole, with relevant surface measurements.