A New Method for Improving LWD Logging Depth

Abstract

This paper will describe a new method for improving Logging While Drilling (LWD) depth accuracy. Case studies that describe this technique will also be presented. It is generally accepted that using the Drillers depth measurement for LWD applications has been the most practical solution to a complex depth problem. The various sources of depth errors have also been described and quantified in the industry. Two of the main contributors to drillpipe based depth error are mechanical stretch and thermal expansion. Of these, the mechanical stretch is governed largely by the well profile, linear weight of the pipe, and the frictional forces that can be calculated using industry standard torque and drag calculations. The coefficient of linear thermal expansion of the drillstring components and the distributed temperature of the drillstring assembly at the time of measurement govern the change in length due to temperature. To compensate for these effects requires a series of algorithms that identify the mechanical condition of the drillstring at the time of measurement based on the operational drilling mode. Then, a standard torque and drag model is used to calculate the mechanical stretch, and a thermal expansion algorithm subsequently applies the temperature component of the depth correction. The results of these computations are a corrected logging depth, and an improved time to depth conversion file that can be used to recalculate the logging data. The results from case study data strongly support that; uncorrected standard LWD depth accuracy today is often at least as good as that provided by comparable wireline logs; and that LWD depth can be significantly improved using this technique. This new method for improving logging depth will lead to enhanced single well evaluation and the improved well-to-well correlation of reservoir features, and hence the value of the reservoir model. Drillers Depth LWD measurements are referenced to Drillers depth, and this is generally based on a listing of hand measurements made on each length of pipe lowered into the well. To provide a continuous depth for the log data, the movement of the block is tracked at surface, using a geolograph or drawworks encoder1 (figure 1). For each foot that the block travels up or down, it is assumed that the bit travels and equal distance out of or into the hole. Bit movement is only updated when the pipe is apparently "out of slips". In and out of slips is determined by software that monitors a hook-load sensor attached to the drilling deadline. As drillpipe is moved up and down in the well, the LWD depth tracking inevitably starts to deviate from Drillers depth due to errors in determining in and out of slips, depth sensor calibration errors and errors in the pipe tally itself. As a result, in practice the LWD depth is periodically adjusted to match driller's depth. Measuring the bit depth (and hence the LWD true sensor depth) at surface also neglects the changes in pipe length due to hole geometry, temperature and mechanical stretch. Surface depth measurements assume that the drill pipe is a rigid body and that any movement at surface is immediately translated into the equal movement of the bit down-hole, which may be several miles away. So even if it were possible to make a perfect depth measurement at surface, bit depth would still be in error.

Improving LWD Image and Formation Evaluation by Utilizing Dynamically Corrected Drilling-Derived LWD Depth and Continuous Inclination and Azimuth Measurements

“…Pipe tally errors can be neglected, if standard surface depth tracking policies are carefully observed as shown by Chia et al (2006). Downhole clock drift can also be neglected provided it is not more than 3 seconds.…”

“…This procedure does not require model calibration and can provide good match with wireline logs (Chia et al 2006). Modern industry employs rotary steerable systems (RSS) drilling, more and more frequently which significantly reduces dynamic changes in the drillstring length due to pipe compliance while drilling.…”

Improving LWD Image and Formation Evaluation by Utilizing Dynamically Corrected Drilling-Derived LWD Depth and Continuous Inclination and Azimuth Measurements

“…Following these recommendations Chia et al (2006) proposed a methodology for correcting MD errors which concentrates on taking into account the drillpipe mechanical stretch and thermal expansion. They have demonstrated that performing such depth correction corrections significantly improves LWD depth accuracy and reduces depth mismatch between LWD and wireline logs.…”

“…They also provided numerous case studies showing the reduction of the depth mismatch, for example, from 30 ft to 3 ft for a well of 15,000 ft length in MD. Chia et al (2006) have identified and proposed a treatment for the two components of the depth correction: static, which represents the bulk depth shift slowly growing with depth; and dynamic, which is caused by variations of the drilling mechanics parameters with time. The impact of the dynamic correction on LWD log and image quality has been described in detail by Dashevskiy et al (2006), who proposed similar methodology for LWD depth correction.…”

“…These are average sliding friction factor, effective block weight, average pipe wear and mud weight. The methodology combines drillstring weight, WOB, and friction effects in one computation following the technique of Chia et al (2006). This technique estimates both static and dynamic drillstring stretch and thermal expansion, provided that the model parameters are calibrated to the actual surface drillstring weight and given the actual drilling mode.…”