3336 Logon T.L., Mavor M.J., Khodaverdian M. Optimizing and Evaluation of Open-Hole Cavity Completion Techniques for Coal Gas Wells. / 9346. - The 1993 International Coalbed Methane Symposium (May 17-21, 1993, Birmingham-Jefferson Civic Center, Birmingham, Alabama, U.S.A). Volume 2. - Alabama: University of Alabama, 1993. - pp.609-622.
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 ќЌ—ѕ≈ “:  лючевое слово - self-propped fractures (само-поддерживающийс€ процесс разрушени€).

An innovative completion technique that has been used to effectively complete coal gas wells is an open-hole completion in which the coal is encouraged to slough into the wellbore. During the completion process, the wellbore is effectively linked to the reservoir due to the creation of numerous multi-directional self-propped fractures. The technique has been commonly called an "open-hole cavity completion." However, the cavity is a by-product of the process and not the primary objective of the completion. A more suitable terminology for the technique is "dynamic open-hole completion." The majority of the dynamic open- hole completions have been performed in the Fruitland Formation of the San Juan Basin, Colorado and New Mexico. Dynamic open-hole completed coal gas wells in some areas, but not all, produce at significantly greater rates than wells completed using other techniques such as hydraulic fracturing. Because of the success in the San Juan Basin, dynamic open-hole completions have been attempted in other basins including the Piceance, Powder River, Arkoma, Uinta, and Black Warrior. This paper presents a conceptual model and a working hypothesis concerning what takes place in the coal reservoir during the dynamic open-hole completion process. Based upon this model and hypothesis, techniques to: i) optimize the completion, ii) evaluate the effectiveness of the technique, and iii) how to determine when to terminate completion operations will be presented.

INTRODUCTION (см. оригинал) Coal natural gas wells typically require stimulation resulting in effective wellbore to reservoir linkage to achieve economic gas production rates. The objective of a dynamic open-hole completion is to: i) effectively link the open-hole wellbore with the undamaged reservoir, ii) create multi-directional self-propped fractures in the reservoir, and iii) to intersect the natural fracture systems within the coal. A by-product of the dynamic open-hole completion procedure is an enlarged wellbore caused by multiple pressure surges that encourage the friable and relatively low strength coal to slough into the wellbore. In this process, near wellbore damage is removed, multi- directional self propped fractures –ис.1 (увеличить) are created, and the enlarged wellbore may become linked to the natural fracture system within the reservoir. During the open-hole completion process, it is hypothesized that failure occurs in the coal due to shear and tensile stresses creating numerous multi-directional tensile, shear and extension fractures. These fractures stimulate production by effectively linking the wellbore to a large pre-existing natural fracture surface area within the coal gas reservoir.


–ис.2 (увеличить) The tensile failure mechanism is a result of the injection process where the wellbore pressure is increased to a pressure greater than the minimum wellbore stress concentration near the well and the minimum principal stress away from the wellbore in the coal. The orientation of the tensile failure zone is parallel to the maximum horizontal stress direction and the zone contains numerous connected parallel fractures. These tensile failure induced fractures may become self-propped. –ис.3 (увеличить) Also during the injection period, the increase in pore pressure near the wellbore causes the wellbore to decrease in size. The decrease in the wellbore diameter may result in a stress reduction away from the wellbore and cause tensile failure initiation away from the well in multiple directions. [11] It is hypothesized that these tensile fractures may extend 30 to 60 m (100 to 200 feet) from the wellbore on each side of the wellbore. This hypothesis is based upon limited laboratory data. Additional laboratory and field research are needed for confirmation.

–ис.4 (увеличить) In contrast, shear failure is a result of active loading when the wellbore pressure is depressed, as during the production period of the completion operations or during under-hydrostatically balanced drilling operations. In this case, active shear failure zones are created that are oriented perpendicular to the maximum horizontal stress direction and perpendicular to the tensile failure zone orientation. The concept of active shear failure is similar to that for wellbore breakout which has been extensively researched and documented in the literature. [12,13,14] –ис.5 (увеличить) The fractures in the shear zone may also become self- propped. Based upon laboratory measurements in sandstone rock types, the shear zones may extend several wellbore diameters or upwards of 7.5 m (25 feet) from the wellbore. The orientation of the tensile and shear failure zones is illustrated in Figure 1.


–ис.6 (увеличить) Techniques need to be developed to clean out the wellbore more effectively and rapidly to reduce completion costs. One technique that may increase the cleaning efficiency is a side jetting tool placed in the drill string, as shown on Figure 10. The objective of the jetting tool is to create turbulent flow in the cavities where coal has accumulated. During conventional clean out operations, air and small slugs of water are circulated down the drill string and up the annulus. The flow direction in the annulus is vertical with the high velocities in the in-gauge wellbore and low velocities in the cavities. –ис.7 (увеличить) When the velocity of the fluid decreases, rock particles transported by the fluid are dropped at the base of the cavities. If the base of a cavity is in a zone of high permeability, the cavity will be self cleaning due to the inflow of water and gas from the reservoir. However, if the base of the cavity is not in a zone of high permeability, coal will accumulate in the bottom of the cavity. This accumulation can block the lower portion of the wellbore, as is illustrated in Figure 10. –ис.8 (увеличить) In addition, if the base of the cavity deteriorates, wellbore cleaning and bridges at the base of the cavity become more of a problem and will require additional time and expense to remove. Therefore, the completion operations should be terminated as soon as expected fluid production rates are achieved and the wellbore is stabilized. Extended operations typically result in increased wellbore problems and costs, without enhancing fluid production.



–ис.9 (увеличить) Х The objective of a dynamic open-hole completion is to: i) effectively link the open-hole wellbore with the undamaged reservoir, ii) create multi-directional self- propped fractures in the reservoir due to tensile and shear failure, and iii) to intersect the natural fracture system within the coal.

Х The enlarged wellbore "cavity" is a by-product of the operation and is not the primary cause of the increased production rates observed.

–ис.10 (увеличить) Х The agreement between pre- and post-completion permeability estimates has given us confidence in the potential for estimating the applicability of dynamic open-hole completions and the prediction of the post- completion well performance based upon pre- completion tests. Analysis of data that results in estimates of the absolute permeability greater than 20 md suggests that dynamic open-hole completion procedures can be used.

Х Although dynamic open-hole completions have been successful in some areas, the technique can be improved, thereby enhancing the productivity, reducing completion costs, and potentially broadening the reservoir types where this technique can be successfully applied.

–ис.11 (увеличить) Х One technique that may be used to complete multiple coal seams is to increase the pore pressure and thus the near wellbore stresses in the zone that is accepting the air by initially injecting water ahead of the air injection volume. The increase in the near wellbore stress inhibits continued injection into the zone and increases the ability to inject into other coal zones that have not been affected by past injection/surges.

Х The injected volume for each pressure/surge, should be determined based upon the reservoir volume required to i) pressurize the wellbore to create shear failure, and ii) pressurize the fractured zones to propagate the fractures. As the completion continues, the fluid volume required increases due to the increasing wellbore volume as the result of coal sloughing.

Х The average total time required to complete a dynamic open-hole completion operation is approximately 10 days at an average daily cost of approximately $8,000 ($US) per day. During the completion operations, the majority (65.2%) of the operation is spent cleaning out the wellbore while only 22.3% of the total time is spent conducting the injection/surges. Techniques need to be developed to more effectively and timely clean out the wellbore to reduce completion costs. One technique that may increase the cleaning efficiency is a side jetting tool placed in the drill string.

Х Generally, completion operations are discontinued when successive injection/surge operations do not result in increased fluid production rates. A more quantitative approach is based upon estimates of the fluid productivity of the reservoir once the wellbore and the natural fracture system are linked.


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