Abstract
An urgent problem of geothermal energy source development is how to cut down the production costs. The use of temporary sealing materials can reduce the costs associated with the circulation lost by plugging, and increase the production by self-degradation. Based on the utilization of starches as self-degradable additives in the medical field, this paper investigated the effects of three kinds of starches, namely corn starch (CS), hydroxypropyl starch (HPS) and carboxymethyl starch (CMS) on the properties of alkali-activated cement (AAC). In addition, the thermal properties of starch, the compressive strength and microstructures of the cement with starch were tested, to evaluate the potentiality of starch as self-degradable additive for geothermal cement. The analysis showed that: (1) all the starches have the effect of increasing the apparent viscosity, prolonging the setting time and reducing the static fluid loss of alkali-activated cement; (2) the addition of starch increased the number of pores in 200 °C-heated cement, facilitated the leaching process, and thus promoted the self-degradation; and (3) among the three starches, CMS has the most potential as a self-degradable additive.
Highlights
Geothermal reservoirs are usually constructed in fractured formations, which results in the lost circulation problem during drilling operations
All the tested starches have the effects of increasing the apparent viscosity, prolonging the setting time, and reducing the fluid loss of activated cementitious materials (AAC)
carboxymethyl starch (CMS) presented the greatest volume expansion, as the alkaline CMS increased the alkalinity of filtrate-treated samples, and the stronger alkaline conditions promoted the swelling of starch granules
Summary
Geothermal reservoirs are usually constructed in fractured formations, which results in the lost circulation problem during drilling operations. Lost circulation leads to increased material waste and the time cost of non-drilling. The cost associated with the lost circulation usually accounts for more than 20% of the total costs for drilling in geothermal development [1,2,3]. Another feature that geothermal reservoirs have is that the reservoirs contain corrosive gases, such as CO2 and H2 S [4]. Alkali-activated cementitious materials (AAC) are resistant to high temperature and acid corrosion, compared with the failure of Portland cement [5]. The gels, not significantly affected by exposure to elevated temperatures, protect the materials from acid corrosion [6,7]
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