Sand Consolidation by Enzyme Mediated Calcium Carbonate Precipitation

Abstract Sand production from poorly consolidated reservoir formations has been a persistent problem in the petroleum industry. Sand production can cause erosion and corrosion to downhole and surface equipment's and loss of production. Several technologies are used to reduce sand production effects and subsequently maintain well production and safe operations. Such techniques include completion techniques, and in-situ chemical consolidation methods. The enzyme urease induced carbonate precipitation (EICP) is a reversible and environmentally friendly technique that can be used for sand consolidation. In EICP, urease enzyme catalyses the hydrolysis of urea in an aqueous solution, which results in ammonia and carbonic acid production. In the presence of calcium ions, the carbonate ions precipitate as calcium carbonate. It has been reported that urease enzyme starts losing its activity above 65 °C and thus this technology can only be applied in reservoirs with temperatures up to 65 °C. This study addresses an improved EICP method where protein is added and the technique can be applicable at high temperature reservoirs. Two EICP solutions were prepared, EICP control solution (solution 1) which contains urease enzyme, calcium chloride and urea and modified EICP solution (solution 2) which consists of urease enzyme, calcium chloride, urea and protein. Test specimens were made by mixing sand with EICP solution and allowed to cure at different temperatures ranging from 25°C to 130 °C. Additionally, XRD analysis was performed to identify the type of calcium carbonate polymorph. SEM imaging was carried out to visualize the morphology of the calcium carbonate precipitation in the sand specimens. Specimens treated with the solution containing protein (solution 2) had a high consolidation strength. As the temperature increases the strength of consolidation decreases in specimens treated with solution 2 and 1. However, the strength of consolidation of specimens treated with solution 2 that contains protein was considerably greater at all temperatures (up to 130 °C), than the strength of specimens treated with solution 1. Moreover, XRD analysis revealed that 70% of the calcium carbonate polymorph in solution 2 was calcite (which is the most stable polymorph). SEM images show that in the specimens treated with solution 2 the calcium carbonate precipitates at inter-particle contacts. The impact of these results include the use of the EICP protein technique as a downhole sand consolidation method in high temperature reservoirs. Furthermore, the addition of protein in the EICP solution can lead to a reduction in the concentration of substrate and enzyme required to achieve sand consolidation, and subsequently reduction in undesirable ammonium chloride. These advantages enhance the potential use of the EICP protein system for sand consolidation in high temperature reservoirs.