15a.1 Discussion: Why Bother

Depth is the most important sub-surface parameter measured. Depth defines the along hole position of all the sub-surface parameters measured, and hence defines the well construction activity, the geologies drilled and the horizons produced. Without accurate, consistent and credible depth measurement the entire database related to subsurface is compromised and the uncertainty associated with any subsurface measurement cannot be reasonably ascertained.

Along hole depth is the key component of TVD, which is the main descriptor in well bore positioning and reservoir geometrical descriptions.

However, there is a great deal of discussion and lack of clarity on how depth is measured. There is a correspondingly significant spread of measurement results that bring into doubt the validity of depth as a
measured parameter. The scale of the depth measurement discrepancy has been described by Forsyth et.al.1

Even operators do not have a common platform for expectations in depth accuracy or requirements for the associated uncertainty.

Definitions of terms

The word “depth” is described as commonly understood to refer to the distance from an acknowledged reference point, usually assumed to be at surface (typ. MSL, GL, ORT, etc.), along a described path (e.g. along hole Measured Depth, MD, or True Vertical Depth TVD, from surface) using described units (ft or m). The use of these three descriptors is critical in understanding what “depth” is being referred to.

MD is the basis for TVD. TVD is not usually measured directly and relies on deviation surveys, defining azimuth and inclination along hole to derive the actual vertical depth of any point.

MD is described by any of several quite different types of measurement. These include “Indicated Depth”, “Raw Depth”, “Calibrated Depth” and “Corrected Depth”. Ultimately, the actual along hole depth is referred to as “True Along Hole Depth”, and is provided then with an uncertainty estimate. (See WDDrev4.0 p. 152 or Forsyth et.al. for a full definition of commonly referred to depth terms).

Defining accuracy expectations
Accuracy expectations vary according to the application of the data. Seismic depth accuracy is limited by the resolution of seismic signals that is defined by the frequency of the acoustic signal. This is typically 10’s of meters at 1,000 M, so can be assumed to be around 5:1,000 at best. For most well construction, accuracies in the order of 1:1,000 are typically sufficient. Most wireline companies quote 5:10,000 to 2:10,000, but few actually achieve this. Forsyth et.al. demonstrated that there is very little evidence to suggest that these accuracies are routinely achieved. For compaction studies where movements of formation boundaries and markers are measured, accuracies of at least 1:10,000 are required.

Table 1, overleaf, illustrates a number of different applications where the accuracy requirements can be seen to be different. As increasing accuracy requires increasing calibration, verification and correction requirements, it is clear that increasing accuracy can only be attained at increased operating cost.

Driller’s Depth and Wireline Depth

Driller’s Depth, attained from the measurement of drill pipe in the well bore, is typically uncorrected, and can at best be described as being “indicated Depth”. The reason for this is that there is not a credible calibration of measurement verification process associated with drill pipe measurement, and typically corrections are not routinely or consistently applied across the industry. The movement of the drill pipe is also irregular, with depth measurements being made under differing pipe loads and pipe stress conditions (torque, pressure support, temperature, rotational and sliding friction, etc.).

Wireline cable can be calibrated in length, and the length measurement made can be verified. The measurement made can be subject to systematic environmental corrections that accounts for temperature, stretch and other influences. The measurement is made only during pull out of hole (POOH) when ascending to surface with an increasing tension regime distributed along the cable form the tool string to surface3. This means that the cable tensional regime can be modelled, and hence corrected. The ability of wireline depth to provide a calibrated, verified and corrected measurement is a major differentiator to Drillers’ Depth.

Drillers’ Depth and wireline first primary

Driller’s Depth is made continuously during the drilling process, and the core and MWD/LWD data is accumulated and presented according to the first measured depth principle. As has been pointed out above, Drillers’ Depth measurement usually lacks calibration and verification, and is not corrected. Also, as the drilling process is dynamic, significant changes in indicated depth occur during the drilling process as WOB is increased and decreased, as stand-pipe pressures are varied, and mud flows and mud weights vary an as the pipe as rotated and slid along the hole. There are a multitude of other factors, such as BHA assembly and drill pipe string composition that also affect the actual bit and LWD/MWD sensor positions.

Wireline Depths are usually measured more consistently, but current practices are varied between logging companies – see Table 2. These variances include whether or not calibrated cable length is made available and what (if any) verification processes are deployed. There are also fundamental differences in the application of correction, with few companies having a comprehensive, credible and published correction determination and application process.

By convention, the wireline “First Primary” is when the along-hole depth is defined by wireline for the first time. It is self-evident that this measurement should be scrutinized for the applied calibration, the measurement results verification and judicial application of the corrections. By convention, all subsequent wireline depths are synchronized to the First Primary to assure that the depths of wireline logs results correspond. For this reason, it is critical that the processes used to define First Primary log depths are closely scrutinized. This includes detailed QC of the utilized measurement methodologies (including calibration, verification and correction). It is also important that the First Primary log is accompanied with an uncertainty statement that then defines the accuracy of the depth measurement provided.

Review of practices
Different companies log depth in different ways, and they result in different measurements. Table 2 gives a summary of 4-different wireline companies surveyed in the same location and illustrates that very different methodologies are used in depth measurement as well as totally different correction mechanisms.