Recent Advances in Coal Gas-Well Openhole Well Completion Technology
- M.J. Mavor (Tesseract Corp.)
- Document ID
- Society of Petroleum Engineers
- Journal of Petroleum Technology
- Publication Date
- July 1994
- Document Type
- Journal Paper
- 587 - 593
- 1994. Society of Petroleum Engineers
- 5.4.2 Gas Injection Methods, 1.6 Drilling Operations, 1.8 Formation Damage, 5.6.4 Drillstem/Well Testing, 4.1.2 Separation and Treating, 6.5.2 Water use, produced water discharge and disposal, 3.2.3 Hydraulic Fracturing Design, Implementation and Optimisation, 4.6 Natural Gas, 5.4 Enhanced Recovery, 5.8.6 Naturally Fractured Reservoir, 5.8.3 Coal Seam Gas, 1.11 Drilling Fluids and Materials, 2.5.2 Fracturing Materials (Fluids, Proppant), 5.1.2 Faults and Fracture Characterisation, 2 Well Completion, 3 Production and Well Operations, 2.2.2 Perforating, 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc), 1.10 Drilling Equipment, 1.14 Casing and Cementing, 4.3.4 Scale, 5.2 Reservoir Fluid Dynamics, 5.8.7 Carbonate Reservoir, 4.1.5 Processing Equipment
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Distinguished Author Series
This paper reviews the current applications of openhole techniques in theSan Juan basin. Examples of recent advances and details for the use of thetechnology are documented in many of the references. The range of reservoirproperties required for successful implementation of the technology isincluded. Hypotheses proposed to explain the success of the openhole wells arepresented. The paper concludes with a discussion on future advances required toimprove current completion procedures and the potential applicability of theprocess to other reservoir rock types.
Production of natural gas from coal seams has reached commercial levelsduring the past 2 decades in the San Juan basin of Colorado and New Mexico andthe Warrior basin of Alabama. Because of the susceptibility of the naturallyfractured coal to formation damage, the techniques used to drill and completewells have a dramatic influence on fluid productivity. The greatest gas andwater production rates have been achieved from the Fruitland Formation of theSan Juan basin in wells completed with dynamic openhole techniques commonlyreferred to as openhole cavity completions. The dynamic completion procedureconsists of gas and water injection into the reservoir followed immediately bya controlled blowout of the well that is repeated over 1 to 2 weeks. In manycases, the dynamic openhole wells outperform adjacent wells completed withcased-hole hydraulic fracturing by an order of magnitude. Typically, therelative performance ratio between the two completion types is three-fold.Formation damage surrounding the cased-well induced fractures is the primarycause of the productivity differences between the completion types.
Dynamic Openhole Completion Procedures
The state-of-the-art coal gas-well openhole completion technique allows coaland other rock to collapse into the wellbore during a controlled blowout,resulting in an enlarged wellbore in the coal zones. "Openhole cavitycompletion" is often used because measured diameters of enlarged wellboreshave ranged from that of the bit diameter to 4.8 m [16 ft]. "Dynamicopenhole completion" more accurately describes the technique becausecreation of a cavity is a byproduct of the process, not the primary objective.The objective of a dynamic openhole completion is to link the wellbore with theundamaged natural fracture system of the reservoir. During the process,damaged, near-wellbore coal and other rocks are removed; multidirectional,self-propped fractures are created that intersect pre-existing naturalfractures; the near-wellbore aperture of pre-existing natural fractures may beincreased and retained; and the enlarged wellbore may intersect naturalfractures. The wells produce gas and water at rates controlled by the normalcomponents of Darcy's law (e.g., the pressure differential into the enlargedwellbore and the absolute and relative permeabilities of the reservoir).
An excellent example of the improved gas productivity of openhole wellscompared with that of cased, fractured wells can be taken from the NortheastBlanco Unit (NEBU) in the San Juan basin (see Fig. 1). The NEBU No. 403 Ropenhole well has produced at rates exceeding 200 000 std m3/d and 91 m3/d[7,000 Mscf/D and 570 STB/D] of gas and water, respectively, at a bottomholepressure (BHP) of 6400 kPa [930 psia]. The NEBU No. 403, some 61 m [200 ft]from NEBU No. 403 R, was completed in the same coal gas reservoirs withcased-hole, crosslinked gel fracture stimulation techniques. The maximumproductivity of the cased well was 31 000 std m3/d and 64 m3/d [1,100 Mscf/Dand 400 STB/D] of gas and water, respectively, at a BHP of 500 kPa [800 psia].Table 1 lists the locations of these and other wells discussed.
Numerous other examples can be selected to illustrate the success of theopenhole completions within a region of the San Juan basin referred to as theopenhole "fairway." According to a recent study, the average fairwayopenhole well recovers 5.9x107 std m3 [2.1 Bscf] of gas in 3 years compared to1.4x107 std m3 [0.5 Bscf] of gas from cased, fractured wells during the sametime. More than 600 openhole wells are on production, accounting for 73% of thecoal gas production from the basin. More than 920 openhole wells have beendrilled, accounting for 33% of the San Juan coal gas wells.
Fig. 2 compares average gas production rates from openhole and cased,fractured wells located in and outside the fairway over the first 3 years ofthe well life. The average gas production rate of fairway openhole wells ismore than five times greater than that of the cased, fractured wells after 3years. In addition, the openhole well productivity has not reached the maximumproductivity that is expected as water is depleted from the natural fracturesystem. The relatively constant gas production rate of the fractured wells iscommonly associated with formation damage. In contrast, outside the fairway,the cased, fractured wells outperform openhole wells, but the productivity ofboth types is less than that within the fairway. The productivity differencesbetween inside and outside the fairway are believed to result from thedifference in the absolute permeability of the natural fracture system,although other properties, such as gas content, also differ. In thegreater-permeability reservoirs, the openhole wells outperform the cased,fractured wells. Cased, fractured wells located in lower-permeabilityreservoirs are less susceptible to damage. Therefore, casing and fracturingwells can be the preferred completion technique.
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