An Evaluation of Open Hole Zonal Isolation Solutions on Valhall
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The Valhall field, operated by BP Norge, is a mature field in the North Sea requiring more attention than before to be able to maintain a viable production rate. Keeping the rate sufficient is ultimately done, in simple terms, with two measures; drilling more wells and increasing the life time of the wells. However, doing these two things in the oil rich but weak chalk has proven to be more and more difficult over the years, and as with other maturing fields, Valhall is in need of new and more complicated methods of drilling and completing wells. Low fracture gradient and risk of breakthrough of injection water in producing wells are two of the main challenges met at Valhall today. One method that allows wells to be drilled in such conditions is by using open hole zonal isolation.Pumping cement is the normal method of providing zonal isolation between the casing or liner and the annulus. This paper focuses on the alternative methods to avoid cement by using open hole packers to seal between the reservoir liner and the formation. The packers available that are summarized have many different ways of providing zonal isolation, including inflating a rubber bladder with cement or mud, mechanical movement from applied or downhole pressure, rubber that swells in contact with a specific fluid or metal elements that are expanded by pressure. All have benefits and limitations that make them applicable in some situations and unsuited in others.Good open hole zonal isolation is vital for the ability to shut off unwanted water production and if achieved will improve well performance and hydrocarbon recovery at Valhall. Selecting the appropriate packers is not a straight forward task and requires considerations of the many variables of different wells. Stimulation method and formation weakness are amongst the parameters that will be most important when considering open hole packers. During stimulation, cooling and heating could result in movement of the reservoir liner and risk of leak-past and by-pass flow in the weak Valhall chalk, thus compromising zonal isolation.Several open hole packers on the market are applicable for Valhall, but swell packers are seen as the preferred solution due to self-healing capabilities, possibility for long elements and easy deployment. These properties are beneficial in meeting the challenges at Valhall.Further work is recommended for better understanding of the behavior of open hole packers downhole. This could include a geomechanical study of open hole completions in the Valhall chalk to find out if long swell packers will perform better than shorter packers and a review of long-term reliability of swell packers.