Decoding Drift Curves: Unraveling Well Tie Calibration and Interpreting Sonic vs Checkshot Calibration in Geophysical Earth Science

Construction of a Drift Curve in Well Tie Calibration

In the field of geophysics and earth sciences, one of the fundamental tasks is to establish a reliable correlation between well log data and seismic data. This correlation, often referred to as well tie calibration, is critical for accurate interpretation of subsurface geologic features and for making informed decisions in the exploration and production of hydrocarbon reservoirs. An important aspect of the well tie calibration process is the construction and interpretation of a drift curve.

A drift curve represents the relationship between the vertical depth measured in a well and the corresponding two-way travel time (TWT) derived from seismic data. It serves as a critical link between the time domain of seismic data and the depth domain of well log data. The construction of a drift curve involves several steps. First, well log data such as gamma ray, resistivity and sonic logs are used to establish a depth-time correlation within the well. This correlation is achieved by identifying distinctive markers or reflectors in both the well log and seismic data, such as prominent stratigraphic boundaries or key geological features.
Once the depth-time correlation is established, the next step is to compare the TWT derived from the seismic data with the calculated TWT based on the well log data. Any discrepancy between the two measurements is referred to as “drift”. The drift can be positive or negative, indicating a time lag or lead between the seismic data and the well log data. The drift curve is constructed by plotting the drift values against the corresponding depth values along the well trajectory.

Interpretation of the drift curve is essential for understanding the relationship between the well log data and the seismic data and for identifying potential sources of error or uncertainty in the calibration process. A consistent and smooth drift curve suggests a good correlation between the two data sets, indicating an accurate depth estimate from the seismic data. Conversely, irregularities or abrupt changes in the drift curve may indicate problems such as data misalignment, incorrect depth selection, or uncertainties in the seismic velocity model.

Sonic Calibration vs. Checkshot Calibration

When constructing a drift curve for well tie calibration, two commonly used methods for establishing the depth-time correlation are sonic calibration and checkshot calibration. Both methods aim to determine the velocity of seismic waves traveling through the subsurface, which is critical to accurately converting seismic data from the time domain to the depth domain. However, there are some key differences between sonic calibration and checkshot calibration.

In sonic calibration, sonic logs from the well are used to determine the interval velocities of the subsurface formations. These interval velocities are then compared to the corresponding seismic velocities derived from the seismic data. The goal is to find a consistent relationship between the two sets of velocities that can be used to establish the depth-time correlation. Sonic calibration is a well established and widely used method in the industry. It is based on the assumption that the interval velocities derived from the well logs are representative of the subsurface formations in the vicinity of the wellbore.
On the other hand, checkshot calibration involves performing special seismic surveys, known as checkshot surveys, in which precisely timed seismic shots are recorded at different depths in the well. The recorded travel times of these shots are then compared with the calculated travel times based on the seismic velocities derived from the well log data. Checkshot calibration provides a more direct and accurate measure of subsurface seismic velocities by eliminating potential uncertainties associated with assuming representative interval velocities. However, checkshot calibration requires additional time and resources to perform the dedicated checkshot surveys.
The choice between sonic calibration and checkshot calibration depends on several factors, including the availability of well log data, the level of accuracy required, and budget and time constraints. Sonic calibration is often preferred when well log data is readily available and accuracy requirements are less stringent. Checkshot calibration, on the other hand, is preferred when a higher level of accuracy is required or when sonic log data is unreliable or insufficient. In some cases, a combination of the two methods may be used to take advantage of their respective strengths and mitigate their limitations.

FAQs

Construction of a drift curve in the well tie calibration and how to interpret it?

In well tie calibration, a drift curve is constructed to correct for the time-depth discrepancies between different data sources, such as well logs and seismic data. The drift curve represents the vertical shift or drift in the well log data compared to the seismic data. To construct a drift curve, the well log data, such as sonic, density, or neutron logs, are correlated with the seismic data at specific depth intervals called tie points. The tie points are typically chosen where the seismic reflectors are well-defined and easily identifiable in both the well logs and seismic data.

The process of constructing a drift curve involves:

1. Identifying the tie points in the well logs and seismic data.
2. Aligning the well log data and seismic data at the tie points.
3. Calculating the depth differences or drifts between the well logs and seismic data at each tie point.
4. Plotting the drifts as a function of depth to create the drift curve.

The interpretation of the drift curve involves analyzing the trend and magnitude of the drifts. A steeply increasing or erratic drift curve suggests poor data quality or incorrect tie point selection. A smooth and gradually changing drift curve indicates a good match between the well logs and seismic data. The drift curve can be used to apply depth corrections to the seismic data, ensuring accurate positioning of seismic reflectors in subsurface interpretation and mapping.

Sonic calibration vs checkshot calibration

Sonic calibration and checkshot calibration are two methods used to establish the relationship between travel time and depth in a well. They are often employed in well tie calibration to correlate well log data with seismic data. Here’s a comparison between the two methods:

Sonic calibration:

In sonic calibration, a sonic log is used to measure the interval transit time (ITT) of compressional waves traveling through the formation. The ITT is then converted to an interval velocity, which represents the velocity of sound waves in the subsurface. The interval velocity is plotted against depth, creating a velocity-depth profile. This profile is compared to the seismic data to identify tie points for well tie calibration. Sonic calibration provides a continuous velocity-depth relationship, allowing for more accurate depth conversion of seismic data.

Checkshot calibration:

Checkshot calibration involves the use of controlled seismic sources, such as explosive charges or vibrators, to generate seismic waves at specific depths in the well. These seismic waves are recorded by geophones and the travel times are measured. By comparing the measured travel times with the predicted travel times from the seismic data, the depth-depth relationship between seismic data and the well log data is established. Checkshot calibration provides discrete depth control at specific depths, but it may not capture the continuous variation in velocity between the measured points.

In summary, sonic calibration provides a continuous velocity-depth relationship, while checkshot calibration offers discrete depth control. The choice between the two methods depends on the data availability, the desired level of accuracy, and the specific objectives of the well tie calibration process.