TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractRecent improvements in magnetic survey quality control have revealed the presence of systematic azimuth anomalies, affecting the calculated trajectories of a significant number of wells. Although these anomalies display some features commonly associated with drillstring magnetic interference, they cannot be removed by traditional magnetic correction techniques. New magnetic survey quality control procedures have identified the problem as an attenuation of the transverse component of the earth's field, similar to that caused by magnetically permeable material surrounding the magnetometers. The drill collars housing the sensors were inspected but showed no magnetic susceptibility, which led to suspicion that the problems were caused by magnetic particles suspended in the drilling fluids. Laboratory testing has confirmed the magnetic properties of these fluids, and chemical analysis has verified the presence of iron in significant quantities. Field examples demonstrate the use of a multiple-survey quality control and correction method for identification and removal of these survey anomalies.

As modern drilling projects continue to include increased use of extended-reach wellbores directed to smaller targets, the need for an accurate assessment of uncertainty in bottom-hole location is becoming increasingly critical. Incorrect assessment of the probability of intersecting the target can lead to an equally incorrect assessment of the viability of the project. Most published methods for computing wellbore position uncertainty are based on the analysis of systematic errors in inclination, azimuth and measured depth. The underpinning of such analyses is that these various error terms are uncorrelated constants, but this assumption may not always be justified. The technique has therefore been generalized to make use of more fundamental input error terms. and to take account of the probabilistic nature of such terms, thereby calculating an ellipsoidal probability field for each point along the well. Examples are presented which illustrate the advantages of this method, including the ability to take into account the rotation of an electronic magnetic survey instrument such as an MWD tool. When one survey tool is followed in the hole by another, some error terms may behave in a systematic fashion across the tie-on point while others combine randomly. Since the computed uncertainty in bottom-hole location depends on the degree of correlation between one set of errors and the next, a flexible means is suggested for accommodating such tie-ons. Since a probability can be assigned to the hypothesis that a point in a planned or drilled wellbore occupies a given volume of space, this method can also be used to determine the probability that a particular point in the well might intersect an adjacent wellbore or an arbitrary target volume. This makes possible improved computation of the probabilities of wellbore collision or target penetration. Introduction The calculation of wellbore position uncertainty has been addressed by Walstrom et al and by Wolff and deWardt. It is commonly accepted today that such uncertainty is dominated by systematic errors, thus most popular models are based on the systematic analysis outlined by Wolff and deWardt. With extensive field usage of these models over the years, some limitations have become apparent. These include the deterministic nature of the systematic model as originally described, the definition of most instrument errors in terms of fixed inclination and azimuth uncertainties, failure to include random errors as well as systematic, and the inability to assign a probability to borehole position. A modified treatment of uncertainty which overcomes these limitations is described here. Although several of these improvements have been used for many years by a number of companies, until now there has been little documentation of them in the literature. Calculation of Wellbore Position Uncertainty Wellbore surveys are typically performed at a number of discrete survey stations along the course of the well by measuring components of the earth's gravity field and either the local magnetic field or a rotation vector. An instrument performance model is used to convert these raw measurements to inclination I, and azimuth A, which together with the along-hole depth L, make up a three-component measurement vector p. A wellbore trajectory model is then employed to convert the set of measurement vectors into a position vector r in a north, east, and vertical (N, E, V) coordinate system. An instrument performance model includes potential sources of error or uncertainty in the measurement. P. 411

Summary Recent improvements in magnetic-survey quality control have revealed the presence of systematic azimuth anomalies that affect the calculated trajectories of a significant number of wells. Although these anomalies display some features commonly associated with drillstring magnetic interference, they cannot be removed by traditional magnetic correction techniques. New magnetic-survey quality-control procedures have identified the problem as an attenuation of the transverse component of the Earth's field, similar to that caused by magnetically permeable material surrounding the magnetometers. The drill collars housing the sensors were inspected but showed no magnetic susceptibility, which led to the suspicion that the problems were caused by magnetic particles suspended in the drilling fluids. Laboratory testing has confirmed the magnetic properties of these fluids, and chemical analysis has verified the presence of iron in significant quantities. Field examples demonstrate the use of a multiple-survey quality-control and correction method for identification and removal of these survey anomalies. Introduction Boreholes are commonly surveyed by using the Earth's magnetic field as a north reference. Survey tools measure three orthogonal components of the local gravity and magnetic fields, and from these data, the tool's inclination, azimuth, and toolface orientation can be calculated. It is also possible to calculate the magnitudes of the gravity and magnetic field vectors and the angle between them, which is the complement of the local magnetic dip angle. These last three parameters may be used for survey quality-control purposes. Magnetic Interference. Unexpected measured values for the magnitude of the magnetic field vector and/or its dip angle provide an indication that the magnetic azimuth might be out of specification. A common cause of erroneous azimuth is magnetic interference. In the vicinity of the tool's magnetometers, the Earth's magnetic field can be distorted by magnetized material. Possible sources include the formation, casing of adjacent wells, drilling fluid, and steel drillstring components. The magnetometers are housed within a nonmagnetic drill collar, and additional nonmagnetic collars can be used to space the sensors longitudinally from the steel components of the drillstring. However, in most circumstances, the nonmagnetic spacing will be insufficient to isolate the magnetometers completely from the steel drillstring components. Magnetized formations and drilling fluids are considered rare, and adjacent wells are avoided, so the usual presumption is that magnetic interference originates in the magnetic drillstring components and is oriented along the axis of the drillstring. Accordingly, a number of correction techniques have been developed to remedy such problems.1–4 Single-Axis Corrections. The technique normally used to correct for magnetic interference assumes that only the axial (z-axis) magnetometer measurement is corrupted by drillstring interference. The local Earth field components are determined independently, such as from a geomagnetic model or chart or from a magnetic site survey. It is then reasonable to assume that the most likely value of z-magnetometer interference is that which results in the minimum vector distance between the post-correction total field components and the reference field. Magnetic corrections of this type1 have been made for the last 2 decades. Because the correction picks the point at which the data best fit the reference total field and dip angle, the residual errors in total field and dip are generally expected to be smaller than for well-spaced, uncorrected data.5 Multiple-Survey Corrections. More recently, multiple-survey magnetic corrections have been developed by several companies.3,4 These techniques process data simultaneously from a number of surveys, assuming that all the surveys are affected by identical disturbances. They can then determine effective bias and scale-factor error terms on all three axes, which minimize the overall variance between the corrected measurements and the expected reference field. Anomalous Surveys. It has recently been found that a significant proportion of magnetic surveys fail quality-control limits on total magnetic field or dip angle criteria and fail to achieve predicted azimuth agreement with other survey systems. This problem might have existed widely for some time, but it was only recently identified through the application of more rigorous survey and survey quality-control methods than are normally applied. These included application of accurate magnetic field data acquired from local measurements rather than a global model, application of more valid quality-control parameters, updating measurement-while-drilling (MWD) surveys with high-accuracy gyro or inertial navigation surveys, and the supervision of survey quality-control by dedicated survey management personnel. It was noted that the measured total magnetic-field strength was often lower than predicted. If the cause of the anomalies had been axial drillstring interference, then single-axis magnetic interference corrections should have resolved the problem; however, in several cases, the application of such corrections did not improve the surveys. In some examples, the correction adjusted the azimuth in the wrong direction. A possible cause of all these symptoms is incorrect reference values for the magnetic field (i.e., the global model predictions for the well were incorrect). However, several examples occurred in which the magnetic field values had been obtained from local measurements. The application of a multiple-survey correction to these data frequently indicated negative scale-factor errors of similar magnitude on the x and y transverse axes. Collar Susceptibility. Negative x and y scale-factor errors indicate shielding or attenuation of the magnetic field seen by the magnetometers, which might be expected if the sensors are housed within magnetic material. Tests were, therefore, performed on a number of sonde housings and nonmagnetic drill collars, but in all cases, the magnetic susceptibility was found to be within specification. Drilling-Fluid Testing. The possibility remained that the magnetic shielding was caused not by the drill collar material, but by the fluid annulus surrounding the magnetometers. This hypothesis was confirmed by laboratory tests in which attenuation of the measured magnetic field could be directly observed when a three-axis probe was immersed in certain drilling-fluid samples returned from the field.

Time-dependent current fluctuations in the Earth's ionosphere cause inaccuracies in wellbore directional surveying. These inaccuracies increase at higher latitudes, and although monitoring and correction are possible, they become less valid as the distance between the monitoring site and the rigsite increases, which is a particular problem for offshore drillsites. The characteristics of the ionosphere currents indicate that the most favorable location for monitoring stations is on the same geomagnetic latitude as the drillsite. Such an arrangement has been used to monitor and correct directional surveys at the Haltenbanken area of the Norwegian Sea over a period of approximately 2 years. Haltenbanken is approximately 200 km west of the Norwegian coast at latitude 65 N, where magnetic-storm activity can have a significant effect on directional surveying. A monitoring station was set up on the coast at the same geomagnetic latitude as Haltenbanken. To test the idea that magnetic disturbances are similar along constant magnetic latitude, an additional monitoring station was established 200 km east of the main station. The data broadly confirmed the hypothesis, although isolated events were observed when this was not the case. The challenges of surveying at offshore sites north of 62 N latitude are probably greater than the oil and gas industry is accustomed to-but such challenges will become more significant if the Arctic Ocean is opened to drilling operations. The technique described in this paper may contribute to safer and more-productive offshore operations at high latitudes.

Depth is a critical measurement in the economic development of a hydrocarbon asset.Almost all downhole activities, from making petrophysical measurements to setting packers, are performed remotely from surface.The common reference for all such activities is depth.A vertical depth error of less than one meter can have a financial impact counted in millions of dollars.However, despite the Industry's heavy reliance on depth, its accuracy is poorly specified. This paper describes a set of error terms which allows proper quantification of along-hole depth uncertainty for commonly used measurement systems.Additionally, the terms include correlation coefficients that allow quantification of the relative uncertainty between two competing measurements. Although the physical measurement that is made at the rig site is normally along-hole depth, it is vertical depth that defines the relationship between sub-surface features.The quantification of along-hole measured depth uncertainty is therefore only a partial solution; it is also necessary to estimate vertical uncertainty. The directional survey of the wellbore defines vertical depth for any along-hole depth, and directional surveys are routinely accompanied by an estimate of positional uncertainty.A method is described for combining the directional survey's estimate of the wellpath's vertical position uncertainty with the along-hole depth uncertainty associated with another downhole operation, resulting in a valid vertical uncertainty for that operation. Adoption of the techniques described in this paper will result in valid estimates of depth uncertainty, which it is hoped will encourage better depth management practices, and result in more productive wells. Introduction There are frequent calls from the end users of formation evaluation (FE) logs for improved depth accuracy.[1]Zones of interest within the wellbore identified from FE logs (e.g. zones targeted for production, injection, etc.) are subsequently exploited using tools and procedures that are also applied at specified depths.It is therefore desirable that improvements made to the measurement and management of FE depths are applied to all other depth measurements. It has been proposed that rational improvement in depth measurement accuracy is not possible until current performance is better understood and properly quantified, and that the directional survey tool error models, commonly used in the Industry to predict wellbore position uncertainty, offer a useful starting point for modelling the performance of depth measurement systems,[2]Survey tool error models quantify accuracy largely in terms of uncertainty or probability.Their outputs are position bias and position uncertainty, but these values are derived from estimates of the biases and uncertainties associated with the measured values of along-hole depth, inclination and azimuth.Along-hole depth is more commonly referred to as measured depth (MD). Several directional survey tool error models are described in the literature.[3–8]These models include MD terms, which can be extracted, revised and added to, to produce a dedicated MD error model.The most recent papers on the subject[7,8] were written under the auspices of the Industry Steering Committee for Wellbore Survey Accuracy (ISCWSA).The models described in these papers are now being widely adopted within the Industry, and are likely to become de facto standards.In 2004, the ISCWSA was assimilated into the SPE as its Wellbore Positioning Technical Section. The new Technical Section saw the development of a comprehensive depth error model as a natural extension of the earlier error modelling work of the ISCWSA, and as something that might benefit the wider wellbore construction community.This paper is a first step in meeting the Section's objective of providing a standard depth error model.