Rediscovering the Reservoir by Using New Logging Techniques in Mature Fields
Authors: Liu Zhipeng , Kwong Sean , Hull Rob , Hunt Ian Robert 
Published: May 2016

Mature fields account for 70% of global oil and gas production. However, in many of these fields logging data has not been collected in years. During this time, new logging techniques and technology have advanced to the point that it may be cost effective to re-log these wells and ‘rediscover’ the reservoir; providing new resources and vastly improved economics.

Introduction

Logging was invented in 1926 by Conrad and Marcel Schlumberger. Their initial array was used for prospecting for metal deposits. However, the brothers adopted that surface technology to subsurface applications. On September 5, 1927, a crew working for Schlumberger lowered an electric sonde or tool down a well in Pechelbronn, Alsace, France creating the first well log. In modern terms, the first log was a resistivity log that could be described as 3.5-meter upside-down lateral log (Wikipedia). The introduction of the transistor and integrated circuits in the 1960s made electric logs vastly more reliable. The 1970s brought new types of logs and computers. The new logs included combo logs where resistivity and porosity logs were recorded in one pass in the borehole. Computerization allowed much faster log processing, and dramatically expanded log data-gathering and interpretation capacity.

However if you look at these early logs, you will notice that most of them were performed while the wells were being drilled. Since most mature fields in the US were drilled in the 1950s and 1960s, these logs are only on paper and having been interpreted once, were stuffed in boxes and left to disintegrate. According to some sources, the log count for the US is nearly 350,000. How many of these are so old that logging the well again using new technology would add value?

With the advent of software, old paper logs were digitized into images. This process was heavily used in the 1980s and 1990s by both International and National Oil Companies. The process was to lay the log on a digitizing table and trace the log image. The log image was then processed by a software package specifically developed to interpret that type of log. This process did provide benefit and add value as some early bypass zones were identified. Furthermore as technology continues to advance so does the ability to ‘see’ further into the reservoir. Crystals and tools are now available that can provide a comprehensive look deep into the reservoir through casing and tubing while letting the operator view the data in real time as well.

Obtaining New Logs

Drilling new wells to obtain logs is one solution but can be costly in terms of return on investment. Compounding issues such as enhanced oil recovery through water flood and tertiary gas or CO2 change the original reservoir fluids from a simple two-phase environment of a single-density saline water and oil, to that of a mixed salinity and hydrocarbon system. Using Archie's equation and localized methods for production log based on resistivity and porosity methods, and then tying them back to offset combo production logs can be risky. Additional salinity-independent sensors must be run but this further increase costs and thus increases the risks of recovering the investment.