This paper documents an engineering study on sand production and its aftermaths on casing damage. Many wells were converted from production to water injection to maintain reservoir pressure and dispose waste water. Formation of precipitates across the injection interval required frequent washing. Inadvertently, the well washing involved reducing the well pressure rapidly. Consequently, solids were released from the formation into the wells. The well washing and associated solid production went unnoticed for an unknown long time until significant casing deformation encountered during a recent working-over. The significant solid production caused casing buckling near the perforation interval. It also activated a weak plane in the overburden, causing further casing damage. This paper will present relevant field data and engineering analyses to support the above conclusions. Field measures to improve the casing's resistance against the buckling are also described.
In the field, a casing is generally situated in a complicated geological environment. It is exposed to formation fluid that can be corrosive. It is mechanically loaded by in-situ stresses, fluid pressure, reservoir compaction or expansion. Along its path, it may traverse many complex geological structures such as faults, thin weak layers, weak bedding planes, weak 3-D bodies (salts), weak-strong interbeds, etc. The downhole temperature can be very high.
Besides the natural factors as described in the above, casing integrity may be compromised by human factors. For example, casing failure may have its origin right from the very beginning of a petroleum exploitation program. Inability to drill and maintain a gauged and stable borehole, a poor selection of casing hardware, a poor cement placement, an inappropriate perforation and a non-optimized well placement can all impact the casing integrity. This is further compounded by non-optimized reservoir stimulation/production strategies during the production life span.
The complex factors or processes described in the above, being it human or natural, can interact with each other. For example, chemical corrosion may create microcracks in the tubing. The microcracking process will accelerate upon stress loading. As another example, a deformed casing becomes oval. An ovalized casing causes stress concentration which makes the conventional design based on circular geometry invalid. Therefore, casing integrity design is an extremely complex issue. Nevertheless, from a practical point of view, it is always better to design or manage the complexities or uncertainties to prevent casing failures than to remedy a failed casing. This philosophy has repeatedly proven cost-effective in the worldwide industrial practices. Optimum casing integrity can be designed and/or managed if sufficient engineering conscience is executed.
The appropriate engineering conscience starts with characterizing the reservoir including its mechanical and eservoir engineering properties. It must also attend the production strategies so that the casing integrity is maintained in accordance with considerations for the production targets. When casing damage happens in the field history, it is critical to analyze the circumstances around the damage and draw lessons from them. All these tasks are executed in this paper.
Various fields in our assets experienced casing damage. Plan for increasing the production will accelerate the casing impairment trend.
