Design of Novel Composite Agent for Improving the Toughness of Oil-Well Cement Sheath
Authors: Yao Xiao , Hua Sudong 
Published: December 2007

Abstract
The fragile damage of cement sheath, induced by perforation and stimulation treatments, can bring formation fluid to cross flow and increase the damage rate of casing. The fragile damage can be avoided by using fiber-toughening agents. With ordinary fiber-toughening agents, the toughness of set cement is increased, but the Young's modulus is barely decreased and the elasticity is hardly improved. In fact, as the Young's modulus is decreased, the spread rate of stress wave produced by perforation in cement sheath is decreased. Because the spread rate of stress wave in cement sheath is decreased, the fragile damage of cement sheath is decreased. Therefore, a novel fiber-toughening agent, named PJ, was developed. It consisted of carboxylated nitrile rubber particles named J at 5.5% by weight of cement (BWOC) and a polypropylene fiber named P (0.2%BWOC). Its effects on the mechanical properties, the engineering properties and the microstructure of set cement and the compatibility with other admixtures were evaluated. Experimental results showed that both the elasticity and the toughness of set cement with PJ were increased markedly.The comprehensive engineering properties of the slurry with PJ (5.7% BWOC), drag reducer USZ (0.2% BWOC), filtrate loss additive F17B (1.2% BWOC) and expanding agent F17A (3% BWOC) met technical requirements of cementing operation. Good quality cement sheath with PJ was observed by the CBL /VDT logs (cement bond log/variable density log).

Introduction
Cementing is a process that slurry is placed into annulus between casing and cement sheath to isolate producible formation horizons and support casing[1]. Although cement is one of the most important cementing materials, set cement has inherent frangibility. During the development of an oil well, cement sheath is exposed to high energy convergence of perforating charge which brings 3000 to 4000MPa impact pressure[2]. Cement sheath will crack if the radial tensile stress exceeds its limit. The cracks in cement sheath may extend with further well stimulations, even lead to the total failure of the isolation and bring formation fluid to cross flow. During the production of an oil well, casing may expand radially and axially because of high pressure and high temperature resulting from hydraulic fracturing treatments, high drawdown pressures and steam injection. The expansion stress brings cement sheath to crack, or brings the bond interface between the external surface of casing and cement sheath to fail. The bond interface is named the second interface. If cement sheath fractures or bond interface fails, the damage rate of casing will be increased because of severe corrosion on casing, and the output of the oil well will be reduced remarkably. Additionally, the toughness of cement sheath needs to be improved further in the ever increasing number of intricate wells, such as horizontal wells, inclined wells and thermal production wells. A usual measure to improve the toughness of cement sheath is using fiber-toughening agent. However, many overseas and Chinese researches show that, the toughness of the set cement with ordinary fiber-toughening agent is increased, but the Young's modulus is barely decreased and the elasticity is hardly improved [3∼8]. In fact, as the Young's modulus is decreased, the spread rate of stress wave produced by perforation in cement sheath is decreased. Because the spread rate of stress wave in cement sheath is decreased, the fragile damage of cement sheath is decreased. Therefore, it is necessary to develop a novel agent that is able to improve both the elasticity and the toughnessof set cement, while doesn't affect the engineering properties of slurry. The influences of J, P, and PJ on the mechanical properties of set cement were studied. The effect of PJ on the engineering properties of slurries, and the compatibility of PJ with cement and other admixtures were investigated. The function mechanism of PJ was discussed. The field performance of the slurry with PJ was evaluated by CBL /VDT logs.

Experimental Program
Materials Used: Class G oil-well cement was used in the study. Rubber particles J and fiber P were used as the agents. The properties of J and P are shown in Table 1. Filtrate loss additive F17B and expanding agent F17A were used to evaluate the compatibility of admixtures. Drag reducer USZ (0.2% BWOC) was added into all samples (except neat slurry) to improve the fluidity of slurries.