Abstract

Salt is usually added to oil well cement systems to overcome compatibility issues between well cements and salt containing formations as well as salt related durability issues with cement sheath. Several studies on the impact of adding salt to oil well cement have been documented in the literature; however, the impact of compression on salt cement sheath such as during hydraulic fracturing has not been fully investigated. This study focuses on investigating the impact of compression (cement’s compaction) on the petrophysical and mechanical properties of wellbore cement containing salt and their potential impact on long term wellbore integrity issues. A unique bench-scale physical model, which utilizes expandable tubulars, was used to simulate the compaction of a previously cemented casing under field-like conditions. The impact of the compression on the cement’s petrophysical and mechanical properties were quantified by measuring the porosity, permeability and hardness of 1in x 2in cores drilled parallel to the orientation of the pipe from the compacted cement sheath. The acquired results indicate that the compaction of the cement sheath resulted in a reduction in porosity and permeability of the cement sheath and an increase in the hardness of the cement sheath after compaction. Furthermore, the results confirm reduction in the detrimental effect of salt on the strength and stiffness of the cement after compaction..

1. INTRODUCTION

Cementing forms an integral part and is often regarded as one of the most critical steps in oil well completions. It is used to fill the annulus between the casing and the formation and between two consecutive casing strings. The main objectives for cementing oil-wells are to isolate the well from its surrounding in order to prevent fluid migration, protect the casing from corrosion and provide mechanical support for the casing string. The properties of the cement slurry and its behavior depend on the components and additives of the cement slurry design [1]. As a result of the development of offshore fields and drilling activities through salt formations, seawater has become widely used for cementing purposes especially in the US Gulf of Mexico with well-known massive salt formations often in excess of 10, 000ft thickness [2]. There are several reasons for adding salt to oil well cement such as more desirable mechanical properties, better compatibility with salt formations and offsetting of bulk hydration shrinkage. However, salt could adversely affect admixture performance and lower the ultimate compressive strength of the cement [3]. Calcium chlorides in particular, have been used to shorten cements’ setting time and calcium chloride and sodium chloride to a lesser extent have been shown to leach calcium hydroxide leading to chemical changes in the cement resulting in loss of strength and corrosion of the casing [4]. The presence of salt in oil well cements presents a danger to the cement. Although the exact mechanism of salt damage to oil well cement have not been fully understood, it has been shown that salt, which is a mild acid, lowers the pH of the cement and attracts more water into the pore structure of the cement leaving room for expansion leading to fracture initiation and propagation [4].

This content is only available via PDF.
You can access this article if you purchase or spend a download.