Pushing Portland Cement Beyond The Norm Of Extreme High Temperature
- Dale Robert Doherty (BJ Services Company) | Andreas Brandl (BJ Services Company)
- Document ID
- Society of Petroleum Engineers
- IADC/SPE Asia Pacific Drilling Technology Conference and Exhibition, 1-3 November, Ho Chi Minh City, Vietnam
- Publication Date
- Document Type
- Conference Paper
- 2010. IADC/SPE Asia Pacific Drilling Technology Conference and Exhibition
- 2.2.3 Fluid Loss Control, 4.1.5 Processing Equipment, 1.6 Drilling Operations, 4.3.1 Hydrates, 4.1.2 Separation and Treating, 1.14.3 Cement Formulation (Chemistry, Properties), 5.9.2 Geothermal Resources, 1.14 Casing and Cementing
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For a recent project in Australia, Portland cement was modified to withstand contact with temperatures above 800 °C. This High Temperature (HT) cement system is applied to cement casing strings used as injection and production wells in a process known as Underground Coal Gasification (UCG). In its simplest form, this process involves drilling a production well and an injector well into an existing coal seam. Once drilling and cementing of the wells are complete the coal is ignited underground, and air and water are then introduced through an injector well. The air reforms with the combustion materials and forms a synthesis gas (syngas) containing carbon monoxide, hydrogen and methane (as a minor component), that moves under pressure through the coal seam to the production well, where it travels uphole to the downstream facility. Since coal typically resides at shallow depths, it was necessary to have cement that would set at relatively low bottomhole static temperatures of less than 39 °C and have the ability to withstand the extreme temperatures of the advancing combustion front, which can exceed 800 °C.
Portland cement with a silica source has been used in geothermal wells in which production temperatures can reach 380 °C or more, but recent testing showed that it can completely disintegrate around 450 °C. The final HT cement system design easily tolerates the elevated dry temperatures at the combustion front and maintains compressive strength in normal API compressive strength (CS) tests at 350 - 450 °C. The slurry mixes and pumps in the field as one would expect any slurry properly prepared to do while following API methods. Fluid loss control was adjusted with common liquid additives. Since the welfare of our planet is dependent on our ability to develop more environmentally friendly alternatives, the UCG process may become the preferred method to extract the energy contained within coal. The new HT cement system design enables a cost-efficient, logistically simple cementing method/material for this application where none existed previously.
UCG is becoming a favored technique for extraction of the energy contained within a coal seam. This method has been in use for decades and provides a viable alternative to conventional mining by reducing the surface footprint, emissions and energy costs. An operator in Australia is utilizing this method to produce syngas to fuel power plants near the coal seams. One recent attempt, utilizing a Calcium Aluminate Phosphate (CaP) system to cement the casing strings in place, resulted in costly remedial work plus many hours of non-productive time (NPT). Although the CaP cement could withstand the temperatures generated by the combustion front and the high production temperatures, it is not ideally suited for everyday cementing operations. Common cementing additives used for Portland cement-based systems do not work for CaP cement, and contamination of CaP with Portland cement residues in a cementing unit can cause unpredictable setting times. Therefore CaP systems must be handled separately, which requires advanced planning. The expensive logistics and manufacturing, as well as the fact that CaP cements are not available everywhere, significantly increase their costs as compared with Portland cements. The attempt to cement the wells with the CaP resulted in failure of this material to set at the low static temperatures associated with these depths, causing a lack of zonal isolation. The consequence was gas communication with the surface.
Search for Alternative Material
Due to the CaP cement failure, the operator began looking for an alternative material to cement these wells. The new cement system needed to be able to set up at relatively low bottomhole static temperatures of less than 30 °C, then provide zonal isolation along the majority of the wellbore at average production temperatures of 350 °C. The ultimate goal would be for the cement sheath to maintain integrity at the combustion front, where temperatures can reach 1000 °C. Successful isolation at the combustion front would provide an insulating barrier so that catastrophic failure of the sheath would not cause a chain reaction and propagate up the cemented annulus.
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