MWD Survey Accuracy Improvements Using Multistation Analysis

Abstract

Multistation analysis is a technique that provides drillstring magnetic interference compensation to magnetic surveys. This technique is now in common use in the industry, and this case study data verifies the capabilities of the multistation technique to improve measurement-while-drilling (MWD) azimuth measurement quality and accuracy. This study quantifies a significant improvement to standard MWD surveyed position uncertainty using actual survey data from drilling assemblies used in more than one hundred and twenty runs in over thirty-five different wells worldwide. The use of multistation analysis, and the subsequent reduction in wellbore position uncertainty demonstrated can significantly reduce the overall surveying and drilling costs for the well, removing the need for correction runs and allowing for the penetration of smaller targets than previously possible with standard MWD surveying. This correction technique can be applied on a real-time basis at the point of surveying. Forecasting of the magnetization effects of the bottom hole assembly (BHA) using the same software, will also allow the driller to place the well more accurately. In some cases the use of multistation analysis with standard MWD surveys will enable the removal of one or more gyrocompass surveys from the survey program. This can reduce overall surveying time and cost, and the exposure of the BHA to additional stuck-in-hole risks that occur when running wireline surveying tools through drillpipe in extended-reach wells. The main analysis method is based on a direct comparison of MWD survey data before and after multistation correction, against independent overlapping gyro survey data. The results achieved by this study are translated directly into improvements in current industry standard error models, and this paper proposes a standard error model for multistation analysis corrected MWD survey data. Introduction Modern directional drilling operations depend on magnetic surveying instruments to provide confirmation that the positional objectives of the well have been met. These MWD instruments are deployed in the bottom hole [drilling] assembly (BHA) in order to provide real-time surveys to the driller, and minimize the rig time taken for surveying overall. Magnetic survey tools are robust, relatively easy to calibrate, maintain and run, and their performance is well known and modeled to a common standard in the industry1,2. The main difficulty in making accurate magnetic survey measurements today is largely driven by uncertainty resulting from environmental error sources. Amongst these, one of the largest is interference caused by drillstring magnetism. Drillstring magnetic interference is almost always present because of the desire to place the surveying instrument as close to the bit as possible, and to minimize the cost and impact of placing large quantities of nonmagnetic material in the BHA to magnetically isolate the sensor package. Measured Magnetic Field Components When the BHA is relatively well magnetically spaced, about 95% of the measurement obtained by the magnetometers comes from the Earth's main field. The remaining portion of the magnetometer sensor measurement comes from a combination of local crustal effects (or magnetic features trapped in the solidified Earth's crust), the disturbance field (or the effect of upper atmosphere magnetic effects in the magnetosphere, largely caused by solar activity), the presence of any cultural artifacts (or manmade external magnetic interference, such as from nearby drilling rigs, or other adjacent wells) and drillstring magnetic interference. Drillstring magnetic interference is characterized in two forms; remanent magnetism, which is where components of the drilling assembly are actively magnetized; and permeable effects, which is where the drillstring components have the capacity to be magnetized, and the magnetic field lines are ‘bent’ as they pass through the permeable material. All standard steel drillstring components such as drill collars, drillpipe, stabilizers, roller reamers, crossovers etc, can be magnetized. Even those components that are not initially magnetized, can become magnetized by the effects of the inspection process, laying in the yard or on the pipe deck next to other magnetized components, making up and breaking connections and also from the hammering that occurs during the drilling process itself.

Ditch Magnet Performance

“…Multistation analysis provides drillstring magnetic interference compensation to magnetic surveys and improves the azimuth readings of the MWD directional tool. This method will not be elaborated any further and information can be found in the work done by Chia and Lima (2004) [14].…”

Improving models of the Earth’s magnetic field for directional drilling applications

“…Lowdon and Chia, 2003;Chia and Lima, 2004), ultimately MWD relies upon 47 measuring the direction of the well-bore relative to the direction of the local geomagnetic field 48 (Russell et al, 1995). Hence accuracy is inherently limited by knowledge of the direction of the 49 Earth's magnetic field at the MWD survey position.…”