Abstract

In the Salton Sea Field of Imperial County, California, the operating company successfully applied foamed calcium aluminate phosphate (CaP) cement to achieve long-term zonal isolation in a geothermal well that presented a corrosive carbon dioxide (CO2) environment. Weak formations along the wellbore of the shallow well required tight management of drilling-mud weight and cement density to help avoid circulation loss during drilling and cementing. This paper (1) describes the case history of Del Ranch Well 14, (2) describes the cement, (3) discusses the job design, and (4) compares/contrasts the performance of conventional cements to that of CaP cement.

CO2 is a common element in downhole fluids, whether naturally occurring in ground waters or the result of CO2 injection processes. When CO2 comes into contact with the Portland cement traditionally used to cement well casings, it produces a deterioration phenomenon, called carbonation, in the cement. Over time, the loss of cement caused by carbonation can (1) cause serious damage to downhole tubulars and (2) destroy zonal isolation integrity, resulting in costly remedial services or even abandonment of a well.

CaP is specially formulated cement that is both CO2 and acid resistant. It comprises four basic components: calcium aluminate cement, sodium polyphosphate, Class F fly ash, and water. CaP has been laboratory tested and proven at temperatures as low as 140°F and as high as 700°F. Under test conditions that cause Class G and H cements and latex-containing Portland cements to lose up to 50% of their weight, CaP cement's properties are only slightly affected and might actually improve.

Some sections of the cement sheath were cemented with foamed slurry to provide added protection against formation breakdown.

Introduction

Geothermal wells are generally shallow, with the wellbore intersecting weak zones. Wellbores penetrate different levels of rock, and layers of sediment that exist at different temperatures have variable constituents (water, gas, brine, etc.) and differing pressures and physical attributes. Cementing the surface and production casings for the life of the well requires application of extraordinary slurries.

There are several disadvantages to cementation with the conventional Portland cement in geothermal wells where harsh environments include high acidity and high temperature. A few include:

  • • Portland cement is based on calcium hydroxide and calcium silicon hydrates, ingredients that react chemically with an acidic environment, disintegrate the cement, and destroy its cement-like properties.

  • • Portland cement has low tensile strength and resiliency (i.e., is brittle and not likely to deform without failure). Under stress, such as high temperature, it can crack and buckle.

Portland cement is subject to corrosion by carbonic acid, which develops when cement comes in contact with CO2, especially at high temperatures. The corrosion process reduces the Portland cement-sheath volume, increasing the incidence of annular and casing communication of well fluids, hydrocarbons, and CO2 to the surface and from one zone to another.

Operators have attempted to prevent Portland cement volume loss by adding products such as fly ash and/or latex to improve the cement corrosion-resistant properties. These reduced Portland systems could not withstand corrosive effects of water saturated with CO2; Fig. 1 illustrates this failure by showing weight loss of CaP cement and Portland cement systems at 140°F in a solution of carbonic acid and sulphuric acid. This aggressive fluid was used in this laboratory test to accelerate the effects of long-term, "real-life" experience (Brothers 2006).

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