Efficient corrosion inhibition of the metal tubular goods is an important issue in the design of matrix acid stimulation fluids. In fact the increased application of matrix treatments in the field has paralleled the introduction of new corrosion inhibitors. Corrosion inhibitors can be a very significant cost item in matrix treatments. Further testing of novel corrosion inhibitors is required, especially for higher temperatures and for the more sophisticated steels used in oil field operations, to ensure that sufficient corrosion protection is achieved for the treatment time envisaged. The conventional test procedure is the coupon test in which a small coupon is exposed to the stimulation fluid in a pressurised, heated autoclave for a fixed length of time. After the fixed time the sample weight loss and degree of pitting is measured. This procedure has several disadvantages: - the cut edge exhibiting different (excessive) corrosion behaviour than the metal surface
- several samples have to be used to evaluate the effect of time
- the stimulation fluid is stirred intermittently
A new testing method was therefore developed which is based on the principle of hydrogen gas evolution by the reaction of the acid with the steel surface, i.e. the corrosion test cell. The cell incorporates a metal test coupon that consists of a ring of regular oil field tubing. The cell can be rotated preventing liquid/steel contact in the heating phase; upon reaching thermal and pressure equilibrium the cell is inverted and the corrosion process starts by the reaction of the acid with the steel. The evolved hydrogen results in a pressure increase that is translated into a corrosion loss; the latter can be used for corrosion rate determination. The advantages of this measurement are:
- only the inside of the tubing is contacted by the stimulation fluid
- continuous corrosion loss measurement
- continuous stirring of the stimulation fluid
- the conventional weight loss/pitting measurement is made at the same time.
Corrosion inhibition in matrix stimulation treatments was until recently not a major concern because most of the wells were equipped with conventional carbon steels for which sufficient experience and thus data exist to potentially guarantee effective corrosion protection. However, the introduction of high alloy steels, e.g. 13 Cr and 22 Cr, and the increase in the number of stimulation treatments in high temperature environments (100 – 160 C) made it necessary to establish for those conditions the corrosion protection provided by the available and newly developed corrosion inhibitors. The coupon test method was not seen as an optimal procedure for a rapid evaluation of corrosion inhibition in the oil field and it was, therefore, decided to develop an alternative experimental procedure which could easily be applied for last minute testing and would result in a fairly accurate corrosion inhibition measurement. This procedure should enable testing under realistic conditions of the effectiveness of the corrosion inhibitors and intensifiers present in the stimulation fluids and simultaneously result in the minimal concentrations required. The corrosion inhibitor concentration is related to treatment cost, environmental impact and potential adverse effects when too high corrosion inhibitor concentrations are used, e.g. wettability changes and emulsion problems.