Wellbore designs have developed from being vertical wells to modern complex 3-D designs. In order to extend the lifetime of hydrocarbon reservoirs, operators have high focus on accurate wellbore positioning.

Directional drilling is the science of directing a wellbore along a predetermined trajectory to intersect a designated sub-surface target.

The directional measurements that are provided by Measurement While Drilling (MWD) tools are vital in the drilling of any well, as they are used to determine the position of the wellbore. The ability to navigate safely around other wells drilled from a highly crowded platform, and to then place the well in such a way as to maximize production are made possible only by highly accurate and reliable directional measurements.

A great selection of survey tools and techniques are available in order to achieve the directional design objectives. The most common technique for lateral directional measurement is by the use of magnetometers.

The selection of survey tool(s) and techniques will need to consider the wellbore design objectives, tool limitations and also cost. A main objective is to select a survey tool that is fit for purpose.

The wellbores for petroleum production is often positioned and surveyed with directional tools equipped with magnetic sensors. Thus the geomagnetic field becomes a key input to estimation of the bearing along the wellbores. Global models describing the major trends of the geomagnetic field; local models of the crustal anomalies and monitoring of the fluctuations caused by solar activity are all important for the quality of magnetic survey data.

This presentation shows how the uncertainties of the geomagnetic components propagate into the estimation of the directions and the positions of the wellbore. The impact of some correction and estimation techniques are discussed.

The effect of natural geomagnetic disturbances on offshore directional drilling has been part of the activity of Tromsoe Geophysical Observatory since 1993. During the 15 years elapsed we have set up a chain of instruments along most of the Norwegian coast providing data for offshore drilling operation at the continental shelf. Today we deliever in near real time graphics tailored to user requirements, numerical values of the field's deviation from its normal value and alerts by e-mail and mobile phone SMS when a magnetic disturbance is developing.

Only three of our 13 magnetometers installations are traditional observatories with corresponding absolute calibration and stability. The others are variometers with only occasional calibration, but good short time stability. Considering the relatively low demands for accuray in directional drilling such simplifed installations are sufficent.

In the auroral and subauroral zone the geomagnetic latitude of the reference magnetometer should match that of the wellbore site within roughly one degree, corresponding aproximately to 100 km north-south. If this requirement is fulfilled the geomagnetic information is useful at a distance of approximately 400 km in the east-west direction.

Modern drilling technology gives the ability to drill wells along complex paths reaching over 10 km horizontally and 5 km vertically to targets with linear dimensions as small as 100m. There is a requirement to monitor the well bore position as drilling proceeds to ensure that the well conforms to the planned path until the target is reached. Both gyroscopic and magnetic down-hole survey tools are commonly used to survey the well path as drilling progresses. The tools must be able to achieve sufficient accuracy to ensure that the well path remains within an acceptable ellipse of uncertainty and so the various sources of error must be well characterised.

Survey tools employing magnetic sensors rely on reference values of the geomagnetic field vector at the location and time of each survey measurement. Ideally, the reference values would be supplied by an on-site absolute vector magnetometer. This is not usually feasible, particularly for offshore locations, and so methods are needed to estimate the values indirectly. It is important to quantify the errors in the reference magnetic field as these contribute to well position uncertainty.

The geomagnetic field is the vector sum of magnetic fields from several sources and a reference field estimate should include the effects of each the sources that would create an error of significance for drilling if it were ignored. (The desired accuracies in the direction and strength of the geomagnetic field are around 0.1° and 50nT, respectively.)

The major component of the geomagnetic field is the main field, generated in the Earth's core which may be represented by a global geomagnetic field model. Such models are nowadays based largely on data from magnetic survey satellites. Locally, magnetic fields are created by rocks containing magnetic minerals. The magnitude of this crustal field is generally less than 1% of the main field, but it can cause changes in field direction much larger than 0.1° and the effect is a static bias. Aeromagnetic and marine magnetic survey data can be used to quantify the crustal field perturbations. There are (relatively) regular daily magnetic field changes due to ionospheric currents, and space weather events energise various current systems in the near-Earth space environment that may create irregular magnetic field disturbances causing short-term changes of several degrees in the direction of the magnetic field at higher latitudes. Ground-based magnetometers at observatories and in variometer chains monitor these external magnetic fields and provide relevant data. (The time-varying external magnetic fields induce electric fields in the solid Earth and the oceans giving rise to electric currents which are then a secondary source of magnetic fields.)

To construct a reference field value for a drilling location, an appropriate combination of representations of the core, crust and external fields must be assembled. The importance of space weather effects is assessed on a case-by-case basis to decide whether there is a need to include estimates of external field variations. The ability to represent the external field variations at a drilling site depends on the number and proximity of reference observatories or variometer stations where the external field component can be accurately isolated. The correlation between the magnetic field variations at a remote reference station and at the drilling site is a function of geomagnetic latitude. During magnetic storms the correlation is likely to be smaller than at magnetically quiet times. The accuracy achievable is, therefore, dependent on geographic location and on magnetic activity levels.

The British Geological Survey (BGS), working with Halliburton (Sperry Drilling Services) and other industry users, has developed services to meet industry needs. The requirements on quality, continuity and real time availability of data from reference magnetometers to be useful for external field estimation, and operational aspects of the BGS services, will be described. The value of geomagnetic activity forecasts will be discussed.

During an aeromagnetic survey one records (typically) a time series of the total intensity of the geomagnetic field and identifies the difference between the recorded value and the value of the part of the internal geomagnetic field which is generated in the Earth's core as the geomagnetic anomaly. The geomagnetic anomaly is assumed to stem from sources in the solid Earth's crust and possibly from the uppermost solid part of the Earth's mantle and is therefore also called magnetostatic anomaly. The recorded magnetic field also contains contributions from external sources, i.e., from ionospheric and magnetospheric electric currents and secondary currents induced in the conducting ground and - if the survey is made close to or above the sea - in the highly conducting sea water. The magnitude of the ground magnetic field of the external currents is often comparable to that of the magnetostatic anomaly, and the task is to separate the effect of the external sources from the magnetostatic sources.

The principal difference between the external field and the geomagnetic anomaly field is that the sources of the former vary in space and time while the latter vary only in space. It is therefore custom to use remote reference recordings from a fixed base station to clean the survey measurements by removing the undesired effect of external sources Depending on the relative locations of the base station magnetometer and the survey area such a cleaning can be more or less reliable, and survey managers have developed procedures to decide when cleaning is justified and when not. In the latter case a re-flight becomes necessary which incurs additional operation costs. A reliable forecast of geomagnetic activity adapted to the specific needs of aeromagnetic survey managers is therefore a challenge to the geomagnetism research community.

We give in this presentation an outline of the problem and report on some experiences of survey managers and discuss attempts to devise a useful geomagnetic activity forecast for aeromagnetic surveys.

Nonlinearly evolving dynamical systems, such as space plasmas, generate complex fluctuations in their output signals that reflect the underlying dynamics. The non-extensive Tsallis entropy has been proposed as a measure to investigate the complexity of system dynamics. We employ this method for analyzing Dst time series. The results show that Tsallis entropy can effectively detect the dissimilarity of complexity between the pre-storm activity and intense magnetic storms (Dst < -150 nT), which is convenient for space weather applications.

5 minute poster presentation

K-derived planetary geomagnetic indices provide characterization of the overall energy status of the magnetosphere. They are in particular used in modeling the response of the magnetosphere/thermosphere/ionosphere system to the solar wind/magnetosphere coupling.

K-indices are proxies of the energy related to the geomagnetic activity. They are directly related to the range during 3-hour intervals of the irregular variations in the horizontal components of the magnetic field. There are accordingly 8 K-derived planetary indices per day, corresponding to UT intervals 00-03, 03-06 up to 21-24.

The 3-hour resolution is a strong limitation for precise modeling of the magnetosphere/thermosphere/ionosphere system. We therefore introduce new indices based on another proxy of the magnetic energy, namely the root mean square (rms) of the irregular variations in the magnetic horizontal components. Using such proxy does not put constraints on the length of the time interval over which the indices are derived.

At each observatory, the irregular variations are processed using the FMI algorithm. From the computed K indices, rms indices are calculated. Planetary magnetic indices are then derived following algorithms similar to those applied for am and aa planetary geomagnetic indices. rms-derived planetary geomagnetic indices based on different time intervals are presented, and their statistical relation with K-derived planetary indices is discussed. Such indices are computed over time intervals significantly shorter than 3 hours (typically few tens of minutes). They should provide a better description of the magnetic activity contribution to the Earth' thermosphere behavior.