Development of a suitable binder for salt mine drilling

The objective of this study was to investigate the performance and characteristics of cement-brine mixtures for the sealing of boreholes in salt-bearing formations, under underground conditions at an ambient temperature of 8 °C. The target formulation was expected to achieve a compressive strength of at least 10 MPa after 72 hours, exhibit sufficient pumpability, and demonstrate defined expansive behaviour in order to counteract salt dissolution and ensure a reliable borehole seal. At the outset, the current state of the art was reviewed, and the client's prior findings were assessed. It was found that conventional cement formulations, when combined with saturated brine, typically exhibit delayed setting behaviour and reduced strength development. These observations were confirmed through a literature review. However, the available studies primarily focus on cementing applications at greater depths and elevated temperatures and are therefore only partially transferable to the intended use case.
The experimental program comprised a systematic series of laboratory tests using saturated NaCl brines. The cement - brine mixtures were modified with varying dosages of accelerator, superplasticizer, and the expanding agent SRA 150. Additionally, the influence of brine temperature (30 °C, 40 °C, and 50 °C) was evaluated. Neither the addition of admixtures nor thermal modification of the brine led to a significant improvement in early strength to the target level of ≥10 MPa after 72 hours.
A key finding emerged from the comparison of Marsh funnel times. Identical mixtures exhibited significantly higher flow times when prepared with the Eirich laboratory mixer than when mixed and pumped using the HÄNY injection pump applied on-site at Salinen Austria. This indicates that Marsh funnel measurements are of limited relevance for assessing field pumpability. As a result, the focus of performance evaluation was shifted towards compressive strength development. Further testing included a comparative study of KCl- and NaCl- based systems with varying dosages. KCl containing formulations consistently achieved higher strength values. However, for economic and logistical reasons, the client selected a diluted NaCl solution with 16 % concentration for further optimization, as this brine was readily available on site. Notably, diluted NaCl brines outperformed corresponding mixtures made with pure salt (NaCl) dissolved in water.
In the final phase of optimization, the influence of accelerator dosage was assessed via thermocouple testing and compressive strength measurements. The results showed that a dosage of 2% was sufficient to achieve reliable early strength. Increasing the dosage to 3% yielded only marginal additional improvement.
To evaluate the effect of the expanding agent SRA 150, which consists mainly of CaO, the contact bonding strength was measured and found to increase significantly using 5% SRA 150 compared to without SRA 150.