Concentric Completions of Oil and Gas Wells

Abstract

Introduction This paper discusses the use of 1 ¼-in. tubing placed concentrically inside the normal flow string in both oil and gas wells. The use of this extra string provides an annular space which can be employed to perform a number of functions. The Union Oil Co. of California is probably one of the few companies still completing dual oil and gas wells concentrically, where feasible, rather than using parallel strings of tubing; i.e., Union uses the annulus between tubing and casing to flow the upper zones and the tubing annulus to flow the lower zones. In view of the economics, further consideration should be given to concentrically completing dual oil wells before deciding to incur the extra expense of larger hole, larger casing sizes and two strings of tubing required for parallel duals. Particularly this is true where past history of the two zones is available. Application to Oil Wells Numerous oil wells have been completed in this manner and have produced over a million bbl of oil on the lower zone and three-quarters of a million bbl on the upper zone over the past 17 years. In view of this history, it is hard to justify to management the extra cost of parallel string, dual completions. Of course, it is fully realized that concentric duals have a number of disadvantages. They are impractical where the wells are completed in open water and are subject to marine damage or where there is a need for artificial lifting of both zones. Also, operational problems arise in controlling items such as paraffin accumulation and sand production. However, with the development of new tools, techniques and materials over the past few years, some of the problems which made this type of dual completion so unattractive can now be alleviated. In one of the old fields, numerous wells were completed dually with 2-in. tubing inside of 5 ½-in. casing, producing from depths of approximately 6,000 ft. Natural water encroachment and declining reservoir pressure began to affect the upper zones in these wells, resulting in a need for artificial lift and higher gross rates in order to maintain the oil allowables. Experience on other wells in this zone also showed that at gross rates above 300 B/D, sand production became a problem. The lower-zone performance history showed that bottom-hole pressure was not declining because of a strong water drive and would not require any artificial lift for a number of years.