Blocking - A New Technique for Well Log Interpretation
Authors: Kerzner Mark G. , Frost Elton 
Published: December 2007

Almost all well log measurements at a given depth are influenced by the properties or formations up and down the wellbore. Blocking is a way to eliminate these influences and restore the original signal, while at the same time performing bed thickness corrections. In addition to obtaining more accurate log values for estimating formation properties, blocking opens the way to a zone-by-zone analysis on the computer.

Introduction

A very basic fact about well logs is that they all require interpretation. One reason is that the log reading at a given depth does not represent the exact value at this point but is affected by formation characteristics up and down the well. The usual way to deal with this problem is to divide the log into zones corresponding to geological formations of relatively constant character and work with average values in every zone, applying all possible corrections. The number of intervals that can be picked possible corrections. The number of intervals that can be picked is limited by the amount of calculations that have to be performed. The values thus obtained are averaged. Computerized well log data processing allows the log analyst to perform all of the calculations at every depth where the measurements were made, thus providing much more data. The point-by-point approach usually taken has its weakness, however, point-by-point approach usually taken has its weakness, however, in that the total zones are not considered. Taking the Spontaneous Potential (SP) curve as an example, we see that it measures correct Potential (SP) curve as an example, we see that it measures correct values only in thick beds, and far enough from bed boundaries. Calculations based on the SP curve close to bed boundaries will be incorrect. If the bed is thin enough, we may not be getting correct calculations anywhere in this bed. Computed well logs, therefore, do not give the answers that are absolutely correct, and have to be interpreted. The error in computed logs becomes pronounced as we get closer to bed boundaries. Some of the calculations are completely wasted. The purpose of this work is to show how to do correct bed by-bed analysis, either manually or with the help of a computer. Since manual processing is already visually taking bed boundaries into account, one can use the algorithm below to obtain better corrections for the log values in cases of complex stratification. For computer-oriented well log processing, this work helps achieve the following goals:

* Differentiate between meaningful and random features on the logs by use of specially designed filters. This latter technique is used here to help apply blocking to different logs. It also has other applications, as described below.

The whole idea that log values should be constant in each bed, or sedimentary facies, is not new. In fact, it follows from the definition of facies. In the case of an SP curve, the relationship between a block-form static SP and observed SP is a standard topic. These publications also give correction charts for the most simple case of one thin bed surrounded by two identical thick beds - a "sandwich". This work extends the previous results for cases of stratification of any complexity. Another idea related to blocking is the definition of lithofacies from logs by the use of specially defined electrofacies.