Abstract

A cement which expands (or at least does not contract) under downhole conditions can prevent microannular flow and can make bond log interpretation easier.

A comprehensive laboratory investigation of cement expansion using the most common cement expansion additives, calcium sulfate and sodium sulfate, was performed. Four API classes of cement were investigated from 75 deg. to 200 deg. F at pressures from atmospheric to 5000 psi. The slurries were cured in air, water, oil, and 100 percent humidity environments. The cements were mixed with three water contents. The expansion measurements were taken for ten days. The expansion measurements were of the type which measure the exterior dimensions of a cement sample as it cures.

The various curing environments are meant to simulate the curing environments experienced downhole. For instance, wells drilled with oil base mud would leave the wellbore area inundated with oil and thus the cement would be exposed to oil and not water during the curing period.

Several new additives for cement expansion have recently been introduced from the cementing service companies. Now that a thorough background study on existing materials has been completed, new materials can be more easily evaluated.

Introduction

A cement which expands (or at least does not contract) under downhole conditions can prevent microannular flow and can make bond log interpretation easier. A laboratory investigation of cement expansion using two common cement expansion additives, calcium sulfate and sodium sulfate, was performed and is the subject of this report This investigation considered cement expansion under various simulated downhole conditions with a variety of cements.

Several new expanding materials are currently being sold by the cementing service companies. With a background study completed on the conventional expanding additives it will be much easier to evaluate the effectiveness of these new materials.

The type of expansion tests performed in this study can determine cement slurry expansion, but cannot identity cement slurry contraction.

Background

Cement expansion has been used for many years as part of the overall process of obtaining a competent cement sheath around the casing process of obtaining a competent cement sheath around the casing during primary cementing. Cement expansion is normally desirable but not always considered when designing a cement job. Two reasons for this are:

  1. there is no standard test for cement expansion and

  2. the expansion additives which exist have drawbacks and temperature limitations.

Two common cement expansion additives are calcium sulfate hemihydrate and sodium sulfate. sodium sulfate causes cement expansion but normally not to the extent that calcium sulfate hemihydrate does. However. calcium sulfate hemihydrate causes a viscosity increase in slurries and also tends to make the slurries very thixotropic. This can be unacceptable in areas where friction pressures must be minimized. Calcium sulfate hemihydrate also slows compressive strength development The maximum temperature for expansion with calcium sulfate has been shown to be less than 200 deg. F.

This laboratory investigation shows how neat cement and cement with conventional expanding agents behave under a variety of conditions. Cement expansion can help prevent the formation of a microannulus which can allow gas to flow to unwanted areas in an annulus. A positive cement expansion can "snug" the cement up against the casing positive cement expansion can "snug" the cement up against the casing and the formation. It can act to compress mud filter cakes which were deposited during the drilling process. Having a tight seal between the cement and the pipe and the cement and the formation prevents annular communication via a microannulus. Then, the only prevents annular communication via a microannulus. Then, the only place where communication can occur is thru the matrix of the cement place where communication can occur is thru the matrix of the cement or thru mud or solids channels left due to poor displacement by the cement slurry itself.

Microannuli make bond log interpretation difficult. A microannulus can cause a bond log to be misinterpreted as a poor cement job. If a bond log shows less bonding than it should, the log should be run a second time under sufficient pressure to close a microannulus if one exists. Once the microannuli is closed, then the bond log can correctly show the quality of the cement sheath. If there is no microannulus, both logs should be nearly identical.

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