Abstract

In the never ending search to reduce drilling cost the ability to reduce the time spent in analyzing well logs and making well completion decisions in a timely fashion is very important. The ability to make Quick Look wellsite decisions has proven elusive in the Neuquen basin due to the complex lithology encountered in the basin.

New generation logging tools now exist that make dependence on Archie's equations and all the variable parameters that are needed by them not a requirement in reaching sound decisions in the early step of new well completion. Through the comparison of the mineralogy free porosity provided by Nuclear Magnetic Resonance tools and the highly water filled dependent porosity available from the Dielectric tools a method has been found to make a quick look well site qualitative interpretation.

In combination with Rw and Rwa overlay methods we are able to confidently make decisions much more quickly and save considerable rig time.

The objective of this paper is to demonstrate a visual comparison of the two porosity indicators and their use to highlight probable productive zones. A Rw and Rwa overlap method is also demonstrated in confirmation of the method.

Introduction

New technology logging tools for the formation evaluation of exploratory wells, such as Nuclear Magnetic Resonance and Multiple-channel induction, has created a need to make their interpretation easily available. Western Atlas and the West Region Exploration Department of YPF SA have designed a new log presentation by combining different logs and adding "Quick-Look" processing. This helps in identifying hydrocarbons zones and facilitates the decision making process at the wellsite The reward is doubled:

  1. save in well completion costs and

  2. reduce the risk of bypassing productive zones.

New Technology

NMR. A recent addition to the formation evaluation services has been the Nuclear Magnetic Resonance tool. It measures mineralogy-independent effective porosity. NMR tool does not measure micro-porosity neither shale porosity that, normally, affects other porosity logs. The latest generation of the NMR tools combined with recent advances in logging techniques allow taking measurements at TE less than 1.2 ms which guarantees measuring micro-porosity and CBW (Clay Bound Water). This permits the discrimination between effective and total porosity. It also measures the Transversal Relaxation Time (T2) which can be associated with apparent pore throat and radius size distribution. Irreducible volumes and rock permeability can be determined from these measurements.

Experience tells us that gas affects the effective porosity measurements by reducing its value and, to a lesser extent, the T2 distribution which alters permeability calculations. Even though these effects can be corrected in post-processing, it is very useful to have reliable information at the wellsite. There exists several NMR tool designs such as the models having the signal emission and recording section mounted on a pad, and the more conventional mandrel-type with different diameters (6" and 4.5"). The present work has been performed with the mandrel-type tool. The volume investigated has a toroidal shape with a diameter ranging from 14" to 16" and a thickness of 1 mm approximately.

MChl. Multiple Channel Induction measures formation resistivity at different radius of investigation (10, 20, 30, 60, 90, and 120 inches for the most modern designs).

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