Dynamic Depth Correction To Reduce Depth Uncertainty and Improve MWD/LWD Log Quality
- Dmitriy Dashevskiy (Baker Hughes INTEQ) | Thomas Dahl (Baker Hughes INTEQ) | Andrew G. Brooks (Baker Hughes INTEQ) | Derick Zurcher (Baker Hughes INTEQ) | Jeremy C. Lofts (Baker Hughes INTEQ) | Stephan Dankers (Baker Hughes INTEQ)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- March 2008
- Document Type
- Journal Paper
- 13 - 22
- 2008. Society of Petroleum Engineers
- 2.2.2 Perforating, 4.1.5 Processing Equipment, 5.6.1 Open hole/cased hole log analysis, 5.1.8 Seismic Modelling, 1.4.3 Torque and drag analysis, 1.12.2 Logging While Drilling, 4.3.4 Scale, 3.3.2 Borehole Imaging and Wellbore Seismic, 1.10 Drilling Equipment, 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc), 1.6 Drilling Operations, 1.12.1 Measurement While Drilling, 4.1.2 Separation and Treating, 1.11 Drilling Fluids and Materials, 5.1.1 Exploration, Development, Structural Geology, 5.1 Reservoir Characterisation, 1.6.1 Drilling Operation Management
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Accuracy of measurement while drilling/logging while drilling (MWD/LWD) depth measurements can be improved by considering the dynamic variation in drillstring length caused by pipe loading under changing drilling conditions. This paper details a new method that uses surface torque, hookload, and temperature measurements to determine force distribution in a drillstring and to compute apparent drillstring length. When available, torque, weight on bit (WOB), and temperature measured downhole are used to increase accuracy and robustness of the method.
Although logging depth is referred to as a measurement, in reality only the drilling block position is measured. Depth is inferred from it using drillstring length. In recent publications, physical phenomena affecting this were analyzed and quantified. Elastic pipe stretch and thermal expansion were found to be most significant. Techniques to compensate for these effects on the basis of empirical formulae have been proposed (Brooks et al. 2005), but they provide an averaged correction that has insufficient accuracy for many drilling and formation evaluation applications.
This paper presents experiments covering various wellbore profiles, temperature profiles, and drilling modes, which show that the depth fluctuation may be as much as 2.7 meters with a 7,000-meter (m) long drill string even when only the current rig operation mode changes. Among other factors considered in the paper, apparent depth fluctuation is the most significant contributor to commonly observed MWD/LWD log discrepancies when bed boundaries or other features are not logged at the same depth with each sensor. These errors lead to inaccurate petrophysical calculations, distortion of borehole images, and lost time caused by depth matching. Case studies illustrate the positive effect of dynamic depth correction on formation evaluation log quality.
The accuracy of a depth measurement is normally estimated in terms of its bias and uncertainty. A significant portion of the depth bias is caused by elastic stretch and thermal expansion (for example in a 7,000-m long vertical drillstring they can be 9 m and 6 m, respectively). The proposed method removes this bias and allows improved depth uncertainty.
Measured depth uncertainty (1 sigma) in the Industry Steering Committee on Wellbore Surveying Accuracy (ISCWSA) MWD model caused by drill string stretch is 2.2 X 10-7 m-1, multiplied by measured depth (MD) and by true vertical depth (TVD). Uncertainty in measured depth cannot be completely eliminated even by applying corresponding corrections because of the modeling and input data inaccuracies. Nevertherless, it is estimated that the proposed method significantly reduces this uncertainty (e.g., 50% and more, depending on the wellbore, available data, etc.).
Improved depth accuracy, in turn, reduces the uncertainty in computation of reservoir characterization parameters, such as net-to-gross and structural dip, especially when data from multiple wells are evaluated together.
Depth is one of the most important formation evaluation measurements, but one of the most difficult to define accurately. Previous publications (Wilson et al. 2004; Brooks et al. 2005; Pederson and Constable 2006) detail this problem as occurring to various degrees for both wireline- and drillpipe-based systems. With longer and deeper wells in deeper provinces around the world, and the use of drill pipe conveyance (MWD/LWD), this problem becomes more acute.
Awareness of the financial as well as technical implications for inaccurate depth is increasing. Depth accuracy is also vital for accurate calculation of structural dip from borehole images, picking perforation points, and correlation of geological units. MD is used directly in the calculation of TVD—the primary depth used for reservoir delineation. Wilson et al. (2004) explore one such case in which a 2-m TVD discrepancy in oil water contact (OWC) has a widespread implication for the field development plan, pressure support, and compartmentalization with a significant cost attached.
Errors in depth are difficult to detect using a single scalar measurement, however, comparison of multiple curves, which have similar character (Fig. 1) and array measurements, often readily exhibit these artifacts. It is common practice to use a cross-correlation and depth "rubber-banding?? technique to bring MWD/LWD measurements from different sources (even within the same bottomhole assembly [BHA]) together, using measurements from the sensor closest to the bit as a reference. This practice addresses the symptom but not the cause, with no reliable reference or rationale (e.g., the sensor closer to bit generally is not more accurate). Misalignment of curves serves as a good indicator of the formation evaluation (FE) measurements depth placement uncertainty. It should rather be used for validation of methods used to convert LWD/MWD data to depth logs.
This paper discusses the source of the marked relative variations between FE measurements and concentrates on improvement in the accuracy of drillstring-derived depth for MWD/LWD, which leads to smaller uncertainties around the calculation of final TVD.
The integrity of the depth measurement can be described by a variety of terminologies, generally depending on the application of depth under discussion. Wellbore placement, tying in formation evaluation data with seismic sections, and crosswell correlation all rely on accurate absolute depth (both measured and calculated TVD). Depth accuracy is the degree of conformity of the measured value to the true value.
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