Multiple-Coal-Seam Well Completions in the Deerlick Creek Field

Abstract

Summary The multiple-coal-seam completion work conducted during 1982–85 at theDeerlick Creek field entailed a large number of different completion methodsatleast six major design stages. The overall result suggests that there is nosingle "best" or "worst" design, but probably many of each. Motivation to change completions came not from unsatisfactory production butfrom a strong desire to reduce costs. Such reductions were production but froma strong desire to reduce costs. Such reductions were attributed primarily tothe successful application of less expensive, conventional methods. Introduction Multiple-seam well-completion work began in the Deerlick Creek field in Nov.1982 and ended in Nov. 1985. Well completions initially reflected contemporarytechnical concepts of the emerging coalbed-methane-gas industry. During thisperiod, multiple-seam wells were completed openhole. Later attempts to improveon multizone work involved a change to cased-hole, slotted completions. AtDeerlick, slotted-casing completions provided little improvement and in factintroduced additional complications. A basic change in the approach tocompletion design began in Feb. 1984. This new approach emphasized simpler, more cost-effective field operations, not the view that coal completionsnecessitate a unique set of completion criteria. During this period, completionoperational costs were the lowest recorded for the project. The last stage ofwell-completion development at Deerlick can be characterized by a generatedeffort to tailor completion designs to individual coal groups. Typicalcompletions included perforation designs geared specifically to perforationdesigns geared specifically to closely spaced, thick coal seams or to severalfar-spaced, thin coals. Baffle fracturing operations were introduced to allowcompletion designs to be individualized for each of the three coal groups. Overall results indicate that many different completion types produce gaseffectively and that conventional methods can be used successfully to reducecoalbed-methane well-completion costs. From 1982 until 1985, mostcoalbed-methane wells in the eastern U.S. were completed in a single coal bedwith some type of openhole technique. Although this completion could notrecover gas from all the coal seams in a well, single-seam work was simple andcost-effective. The technical risks associated with multizone completions werenot understood at that time, and few operators had attempted to develop themultiple-completion technology. In Nov. 1982, TRW Inc. began to develop theDeerlick Creek field (Fig. 1). In 1986, Taurus Exploration Inc., based inBirmingham, AL, assumed operating responsibility for the project. The coal bedsat Deerlick Creek project. The coal beds at Deerlick Creek field are not deepenough to make mining worthwhile. Three coal groups occur at depths greatenough to contain gas in commercial quantities: the Pratt, Mary Lee, and BlackCreek. In this field, five to eight individual coal seams are completed withineach well (Fig. 2). The developed area is structurally complex, containingnumerous northwest- to southeast-trending normal faults traversed bynorth-northeast-trending "linear" features. Regional dip is to thesouthwest, but local dip can be strongly to the southeast within individualfault blocks. Early Completions In Nov. 1982, multiple-seam well completions at Deerlick reflectedcontemporary technical concepts in this new facet of the coalbed-methaneindustry. By necessity, operations were patterned after unrefined and untestedfield techniques available to the project through other operators' experiences.project through other operators' experiences. This passive approach towell-completion design dominated field development through Jan. 1984. Duringthis period, multiple-seam wells were completed either by openhole techniquesor by jet cutting vertical slots through casing. Openhole Techniques. Wells were completed openhole by isolating each zone(containing one to three individual coal seams) with an inflatable packer aboveand a sand plug below the zone and hydraulically plug below the zone andhydraulically stimulating with gel water and sand proppant through a tubingstring (Fig. 2). proppant through a tubing string (Fig. 2). Popular practicesincluded overflushing, Popular practices included overflushing, changing from12/40- to 12/20-mesh sand, and jetting the coal face before hydraulicfracturing. The field operations necessary for multiple openhole completions wereextremely difficult and costly; up to 2 weeks were required to complete eachwell. One of the most severe problems encountered involved the use ofinflatable packers, which had to be removed after each treatment. Also, adequate settings frequently could not be achieved because of holeirregularities, which caused leaks around the packer. JPT