Certain downhole operations involve placement of elements in a downhole environment, where the element performs its function, and is then removed. For example, elements such as ball/ball seat assemblies and fracture (frac) plugs are downhole elements used to seal off lower zones in a borehole in order to carry out a hydraulic fracturing process (also referred to in the art as “fracking”) to break up different zones of reservoir rock. After the fracking operation, the ball/ball seat or plugs are then removed to allow fluid flow to or from the fractured rock.
Balls and/or ball seats, and frac plugs, may be formed of a corrodible material so that they need not be physically removed intact from the downhole environment. In this way, when the operation involving the ball/ball seat or frac plug is completed, the ball, ball seat, and/or frac plug corrodes away. Otherwise, the downhole article may have to remain in the hole for a longer period than is necessary for the operation.
To facilitate removal, such elements may be formed of a material that reacts with the ambient downhole environment so that they need not be physically removed by, for example, a mechanical operation, but may instead corrode or dissolve under downhole conditions. However, while corrosion rates of, for example, an alloy used to prepare a corrodible article can be controlled by adjusting alloy composition, an alternative way of controlling the corrosion rate of a downhole article is desirable.
Corrodible materials may include those having a high activity on the saltwater galvanic series, such as a magnesium alloy adjusted for corrosion rate. It has been found that adjusting the amount of trace contaminants in a magnesium alloy can have a significant impact on the corrosion rate of such alloys (Song, G. and Atrens, A., “Understanding Magnesium Corrosion: A Framework for Improved Alloy Performance,” Adv. Eng. Mater. 2003, 5(12) pp. 837-858). For example, metals such as nickel, iron, copper, calcium, etc., may be added to magnesium to increase the corrosion rate and other metals such as zirconium, yttrium, etc. may be added to decrease the corrosion rate. Balancing the amounts of such additives to achieve a desired bulk corrosion rate can in this way control overall corrosion of articles made from the alloy; however, such an approach requires preparation of multiple batches of alloy, requiring high batch-to-batch reproducibility and precise, reproducible control of metal additives or contaminants in the alloy.
There accordingly remains a need for controlling the overall corrosion rate of magnesium alloys for use in downhole articles without need for fine adjustment of alloy composition and with improved corrosion control.