Expanding Cement - A New Development in Well Cementing
Authors: Parker P.N. 
Published: December 2007

PARKER, P.N., WAHL, W.W., MEMBERS AIME, DOWELL DIV. OF THE DOW CHEMICAL CO., TULSA, OKLA.

Abstract
Expanding cement is cement to which an expansive component is added. It is a true expanding cement, since the expansion occurs after the cement has set. As expansion occurs, the cement is restrained by the formation and by the casing so that expansion produces a self-stress in the cement. As casing diameter is reduced by temperature and pressure reductions, the restraint is removed and the self-stress is relieved. The cement thus maintains a shrink fit around the casing, and an expanded fit against the formation so that superior bonding is obtained. This paper presents an investigation of the properties and application of expanding cement for oil and gas well cementing. The cement studied is a mixture of normal API Class A portland cement and an expansive admixture. Laboratory results are given on the expansion characteristics and superiority of shear bond properties of expanding cement compared to those of conventional oil well cementing compositions. Field results, with bond log comparisons, are discussed on wells where satisfactory zone isolation had been difficult prior to using expanding cement.

Introduction
One of the most serious problems encountered when cementing casing in a well is the failure of the casing-cement and cement-formation bond. Communication between zones can be caused by such factors asinadequate mud removal resulting from channeling of the cement through the drilling fluid;poor bonding of the cement to the formation due to excessive mud filter-cake build-up;expansion and subsequent contraction of the pipe away from the set cement as a result of internal pressure and thermal stresses caused by hydration of the cement; andfailure of the cement itself due to contamination by drilling or formation fluids.
Migration of fluids from one zone to another is a major problem, particularly in gas storage wells where poor bonding can result in substantial gas losses, premature reservoir depletion and unsatisfactory stimulation operations. It has long been recognized that superior cementing operations could be achieved if expansion could be induced in oilwell cement systems.

Characteristics of Expanding Cement

Laboratory Procedure
To determine expansion or shrinkage characteristics of cement, slurries were mixed according to API procedures and then placed in standard 2-in. cube molds. Triplicate samples of each cement composition were prepared to establish and determine reproducibility of data. ASTM procedures were tried for measuring expansions and contractions of cement prisms and found to give poor reproducibility. Also, strain gauges were embedded in cement specimens, and this method also gave poor measurements. The method using direct measurements with a micrometer had the best reproducibility and accuracy. Specimens were cured in an autoclave pressure-curing vessel at 1,000 psi and 100F until the initial set of the cement was reached. Specimens were then broken out of the molds and dimensions measured to the nearest 0.001 in. with a micrometer caliper (Fig. 1). The specimens were then returned to the curing vessel and subjected again to 1,000 psi and 100F.

Expansion
Expanding cement consists of an expansive component interground with an API Class A or B cement.
JPT
P. 559ˆ