|Publisher||Society of Petroleum Engineers||Language||English|
|Content Type||Journal Paper|
Tubing-Conveyed Perforating: Operating Experience
Bowler, G.V., Huffco Indonesia
|Journal||SPE Production Engineering|
|Volume||Volume 6, Number 2||Pages||195-198|
1991. Society of Petroleum Engineers
In 1981, Huffco Indonesia, a production-sharing contractor to Pertamina, was the first company in Indonesia to use tubing-conveyed perforating (TCP) in its well completions. This paper draws conclusions on the benefits of using a TCP technique over wire-line perforating techniques. Of the 75 TCP jobs done, less than 10% can be considered failures. The results achieved with various amounts of underbalance are compared with published results. TCP is considered the most effective perforating technique currently available.
Huffco Indonesia is the operator of an onshore production-sharing contract with Pertamina, the national oil company of Indonesia, in the Sanga-Sanga block in East Kalimantan, Indonesia. The discovery well, Badak Well 1, was drilled in late 1971. The well demonstrated that the anticlinal Badak field structure contained multiple stacked, deltaic gas and oil sandstone reservoirs between 4,500 and 11,000 ft [1370 and 3350 m]. Subsequent discoveries have resulted in three additional fields being brought to production: Nilam, Mutiara, and Wailawi. Total production is about 37,000 B/D [5880 m3/d] of oil and condensate and exceeds 1 Bcf [28.3 x 106 m3] of gas. The natural gas is supplied to the Bontang liquefied natural gas plant and to the Kaltim fertilizer plant. Currently, there are 111 wells in Badak and 95 wells in Nilam, with each field containing some 300 reservoirs. If the completion/recompletion process is carried out effectively by minimizing damage at the wellbore, then the work necessary (and therefore the cost) to maintain plant deliverability requirements can be minimized. The adoption of the TCP technique for completion is a key factor in achieving this objective.
Comparison of Perforating Techniques
Two distinct perforating techniques are available: wireline and tubing-conveyed. With the wireline technique, the charges are lowered into the well in a frame or carrier on a wireline and fired by a signal passing through the wireline. The wireline technique can be "through-tubing," meaning that the completion equipment is in place and the charges are lowered through the production string and out of the bottom, opposite where the interval is to be perforated. Alternatively, a casing gun, which is capable of using a larger carrier and therefore a larger charge, can be used to perforate the casing before the completion equipment is run. In TCP techniques, the perforating guns are attached to the bottom of the production string of the completion equipment or drillstream-test (DST) tools. The TCP guns are fired after running the completion equipment, setting the packer, and activating the firing head by applying annular pressure (Fig. 1), by dropping an impact bar (Fig. 2), or by using a "wet connect" (Fig. 3) where a wireline connection on the guns is used to place a timing device to activate the guns. Table 1 compares TCP and wireline perforating. Underbalance is the amount of differential pressure exerted by the reservoir formation into the wellbore. It can be adjusted by filling the production or test string with completion fluid or by emptying it by swabbing to a level where the required amount of underbalance is achieved. For TCP, the underbalance limitation is the mechanical integrity of the equipment, where up to 5000-psi [34.5-MPa] differential pressure can be achieved. Advantages are that no completion fluid enters the formation when the TCP is fired and the charge debris is expelled quickly from the formation, resulting in an effective cleanup. For wireline perforating, it is possible, with through-tubing perforating guns, to perforate with a limited amount of underbalance. The danger, however, is that a productive zone would carry the perforating tool up the tubing. To offset this problem, weight can be added to the bottom of the tool. For wirelines, the amount of underbalance generally is limited to hundreds of pounds per square inch, compared with thousands for TCP. With wireline perforating, the interval size is limited to 30 ft [9 m] per run. No limit exists with TCP. This advantage is especially useful in perforating commingled zones: the TCP technique can perforate the zones simultaneously, while wireline requires multiple runs if the zones fall outside the gun length. While it may be possible to make additional runs when the well is flowing, sufficient control of the well flow must exist to enable the wireline to be lowered into the hole without getting bailed up. In wireline perforating, firing is confirmed by inspection of the charge carrier where the gun is pulled out of the hole. With TCP, this confirmation cannot be made because the guns remain in the hole. Usually, because of the tremendous surge forces, especially with high underbalance, confirmation of firing is possible. This is not always the case, however, especially in a multizone completion when some zone charges may have fired, thereby masking failed charges. A technique to resolve this problem is to put a radioactive tag on a charge for each gun section. The guns can then be dropped off the tubing string alter firing and a gamma ray log run. Another major advantage of the TCP technique is the larger charge size and the depth of penetration achievable. Standard charts show the available charge size in grams, the hole diameter, and the depth of penetration. For example, a 37-g charge can provide a 26-in. [66-cm] penetration and a 0.56-in. [1.4-m] hole diameter in 7-in. [17-cm] casing. By comparison, a 2 1/8-in. [5.4-cm] enerjet provides 18.5-in. [47-cm] penetration and a 0.34-in. [0.9-cm] hole diameter in 5 1/2-in. [14-cm] casing.
TCP Firing-Mechanism Preference
The selection of the firing mechanism depends on the completion design and objective. For Huffco Indonesia, drop bar is now used almost exclusively for completions, with annular pressure firing used for DST's. Table 2 illustrates both the failure and trouble associated with each of the firing mechanisms expressed as a percentage of jobs run. In this case, "failure" means the guns did not fire; " trouble" means that the guns fired, but only after repeated attempts. For wet connects, the trouble was associated with the production logging tool or wireline used in association with the wet connect. The fewest failures and least trouble were associated with the drop bar. About 62% (47) of the samples selected were drop bar. Of the failures, three-fourths were caused by the firing pin design; when redesigned, the system proved more reliable. To confirm this reliability, 17 consecutive drop-bar-jobs carried out by another service company resulted in no failures or trouble. The remaining failures were the result of primacord. The trouble for the drop bar involved multiple drops of the bar. Multiple drops were necessary not only because of the poor firing-mechanism design but occasionally because of debris on the firing head, possibly resulting from the underbalance swabbing operation. For annular-pressure firing, the limited number of jobs carried out (11, or 15% of the sample) probably do not provide a reliable data base for extrapolation. In one case, the guns failed to fire; in a second, the guns fired after the packer was reset, which was classified as trouble.
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