Data Correction

Depth Shifting. Log analysis programs that use curves in combination (Water saturation, etc.) require bed boundaries which are the same for all curves. If the reservoir is 20 feet thick and the curves are off depth by four feet then the various curve calculations will be incorrect over 40% of the reservoir. With our proprietary auto-shift program, we shift the log curves so that all the boundaries for any one curve are within one foot of the same bed boundaries for any other curve. EDS' geologists then review each foot of the curve and fine tune the shifting as needed. In practice, most curves are shifted so that bed boundaries are within 1/2 a foot. 

Curve Normalization. The objective of normalization is to adjust certain curves in each well so that they register similar values in the similar rock types. Normalization eliminates the effects of different borehole conditions (mud weight, hole size, etc.), different logging companies and/or poor well site calibrations. Neutron logs in counts/second are rescaled to rock porosity units. Well log normalization is a necessary step for the accurate mapping of oil and gas fields. 

Straightening the SP Curve. Mechanical shifts and other SP anomalies can make the SP difficult to work with, especially on the highly compressed display of a seismic work station. Without straightening the SP, using the computer to accurately calculate the net sand is problematic. 

Volume of Shale Curve. In many Gulf Coast oil fields drilled in the 1940's through 1960's, few gamma ray curves are available. A Volume of Shale (Vsh) from the SP curve can be defined using a clean sand baseline as well as the shale baseline, adjusting for SP suppression in the pay sands as needed. If a few gamma ray curves are available, a non-linear transform can be used to make the SP-based Vsh compatible with that from the gamma ray.  In Alaska I have an SP-derived VSH curve for Cook Inlet and a GR-derived VSH curve for the North Slope.

Curve Editing. Raw log data from the blue-line logs or field tapes may contain meaningless data, such as resistivity or porosity log data below the first reading or some kinds of data recorded in the casing from open hole logging tools. It may also contain misleading data, such as noise spikes and cycle skips on the sonic, porosity log data in washed-out zones and high frequency data on the SP curve. 

Construction of Composite Curves. Field tapes or diskettes usually have a separate file for each log run and each rerun. They also may contain many curves in addition to the 7 to 11 curves generally used for log analysis. EDS constructs composite curves, depth shifting the various runs to be compatible if requested by the client. 

In Areas with Deviated Holes and/or Steep Dips, I can provide True Vertical Depth (TVD) curves, True Stratigraphic Thickness (TST) curves or True Vertical Thickness (TVT) curves. Or I can resample the curves to TVD, TST, or TVT thickness. 

* Note: When a vital curve is missing from a well, in many cases the best solution is to derive a pseudo curve from the curves that are available using a nearby well in which most curves are available. Using normalized data, any "transform" equation can be executed in batch mode to quickly generate the required pseudo-curve in all wells or in selected wells. 

 

| Top |