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Formation evaluation and drilling mechanics information in real time are critical for effective geosteering (the act of directing a drill bit along a defined path while drilling a well). The path is chosen to maximise both the geological information acquired in a well and the surface area of any oil or gas-bearing reservoirs that are in direct contact with the wellbore, so increasing the ultimate production rates.
Monitoring While Drilling (MWD) was first developed to measure direction in deviated boreholes in order to avoid time-consuming directional surveys. The objective of MWD tools is to monitor inclination and azimuth transmitted, via a coded series of pressure pulses within the mud column, to a pressure sensor (transducer) at the surface that picks up the signals. These are decoded in the Mud Logging Unit. Access to real time information such as this can reduce the cost of most wells by improving casing point selection, minimising non-productive time and improving drilling efficiency.
Soon after the development of MWD, logging-while drilling (LWD) was developed so that a driller, with advice from the Well-site Geologist, could geosteer the bit, always knowing in terms of the geology exactly where it is. At first a formation evaluation sensor – the gamma ray sensor - was combined with the directional MWD tool to collect details on the gamma ray response (defining shale intervals) of the formations being drilled. In this way the well's gamma ray curve could be plotted.Modern LWD systems can collect a range of complex electrical and other signals, which give information on the rock layers being penetrated during the drilling process information that was formerly only available through conventional logging after the bit was withdrawn. Along with the gamma ray curve, parameters include resistivity, neutron and formation density, giving valuable information on both the formation rock types and the fluids contained within them. LWD also has the advantage that it measures a clean formation relatively undisturbed by drilling mud.
As the number of down-hole LWD sensors increase the data send rate becomes critical.To obtain wireline quality information it is necessary to obtain at least one data point every 15cm but conventional LWD telemetry systems are limited to a data rate of 0.5 to 1.5 bits per second. If three logs are being run together about 80 bits of information usually need to be transmitted. Thus, at typical ROPs, insufficient data can be transmitted to meet the 15cm target (a point every 50cm is perhaps achievable). Newer telemetry systems are now improving data rates to around 6 to 12 bits per second and, with data compression, much better LWD quality will result.
Low-density gas-charged mud and deep waters also pose problems. Mud-pulse systems cannot transmit signals when the gas volume at the wellhead exceeds around 17%. In deep waters the low temperature mud in the riser is more viscous, weakening the signal as it travels to the surface and, in extreme cases, completely attenuating it. Furthermore, with new dual gradient mud systems (discussed in Chapter 9) returning mud is no longer pumped up the riser, so the heating effect of the mud flowing down the drill pipe is lost. It is thus necessary to significantly increase the magnitude of the pressure pulse generated by the LWD tools.Alternatively an electromagnetic (EM) system transmitting through the formation to a seabed-receiving antenna can be used for MWD. If formation resistivity is too low then signal attenuation will make surface detection difficult so an extended range system including a downhole wireline-deployed transmitting antenna is required for depths below around 500 to 1,000m.
LWD tools are being continuously and rapidly upgraded. New tools are being developed such as seismic logs that can look ahead and allow correlation of the well with seismic data during drilling. LWD can provide a formation evaluation service comparable to that of wireline logging and it is now standard on most deviated development wells and many straight holes saving costs in the process.However, there is some resistance to completely replacing conventional wireline logging runs in appraisal and exploration wells partly because conventional logs are generally of better quality and more reliable and partly due to a general dislike of change.Nevertheless, Exxon-Mobil's joint HooverlDiana field development for example, located in the Alaminos Canyon area of the deep water Gulf of Mexico, was developed entirely with LWD tools run whilst drilling a total of 11 straight, highly deviated and horizontal wells.
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