Superiority of Using Polyurethane Foam Injection Over Nitrogen Freezing Technique as Core Preservation Method: Experimental Approach
Authors: Sulaiman A.D.I. , Maikobi A.A. , Chinezelum O.S. , Abdulsalam S. 
Published: March 2010

Abstract
This research is aimed at proving the superiority of using polyurethane foam injection over the nitrogen freezing technique as a preservation method for core samples. Controlled experiments comparing core samples preserved by polyurethane foam injection, otherwise known as the resin method, and those preserved with the nitrogen freezing technique were conducted. Three core samples were preserved by the resin method and three other samples were frozen using nitrogen. Their weight losses with time under various conditions were monitored. A graphical method was adopted for the comparative analysis. The results showed that core samples preserved with polyurethane foam injection exhibited higher stability under every condition over a longer period of time than did the ones preserved with the nitrogen freezing technique.

Introduction
The purpose of core preservation is primarily to maintain the original core fluids and distribution of fluids in cores as sampled, while conserving their mechanical integrity. Effective preservation of core improves the quality of data obtained through core analysis and prevents changes in the rock, such as mineral oxidation and clay dehydration.
Coring is an integral part of the reservoir life-cycle process, with cored wells selected to verify or provide maximum information for the current geological, engineering or production model of the reservoir. Good handling and preservation of core is necessary in order to obtain good results from core analysis. Through core analysis, we can understand the reservoir rock properties that determine hydrocarbon production, the variation in these properties and how these properties affect ultimate recovery. When core is recovered, it must be laid out, measured, described and prepared for shipping and preservation. Core samples of reservoir rock from a well located 30 km offshore of the Niger Delta were used for the experiment.
A number of preservation methods have been put into practice. These include quick freezing, wrapping of core with plastic, use of plastic sacks that can be sealed, sealing the core samples into metals or plastic tubes, the resin method, etc.(1) However, freezing of rocks is the most widely-used technique for both handling and preservation of unconsolidated and slightly consolidated cores in the Niger Delta fields. The effect of freezing on cores has been discussed intensively in the literature(2–6) and will not be discussed here. Also mentioned above, the resin method is one of the numerous ways of preserving the mechanical integrity of unconsolidated cores. However, this method is not much applied in the Niger Delta. This paper will explain the advantages of using the resin method over the nitrogen freezing technique in preserving cores at a high temperature and a humid environment like the Niger Delta.

Methodology
After drilled to the chosen coring point, the hole was circulated clean and a 22.35 cm (8.8 in) core bit was fitted into the Bottomhole Assembly (BHA). The core bit was then lowered into the bottomhole and 9.14 m (30 ft) of the formation was cored. The core was pulled out of the hole to the shoe and eventually to the surface on the catwalk with utmost care (see Figure 1).