In wells that contain tubing, operators use small-diameter through-tubing guns. These systems consist of either expendable gun systems that leave debris in the wellbore after detonation or retrievable gun systems with a mounting strip that can be recovered after detonation. Through-tubing guns can be used in underbalanced conditions, in which hydrostatic borehole pressure is lower than formation pressure. After detonation, formation fluids flow into the well, flushing debris from the newly formed perforation tunnels. The well can be immediately flow tested or put on production. With through-tubing guns, operators can add perforations to producing intervals, or open new zones without the expense of removing the tubing. If the guns are to be retrieved after perforating, the well is usually perforated in an overbalanced condition, in which the borehole pressure is higher than formation pressure. If the well is perforated underbalanced with casing guns, the operator must kill the well to retrieve the guns.
Perforating guns are conveyed in the well using a variety of methods. Tubing-conveyed perforating (TCP) guns are attached to tubing and run in the well using a drilling or workover rig. TCP guns offer benefits such as leaving the tubing in place after perforating underbalanced, along with the improved performance and flexibility provided by using hollow-carrier guns. Because wells can be perforated underbalanced, flow to surface may be initiated immediately. Long intervals and widely separated zones can be simultaneously perforated using this method; other techniques require multiple trips into the well. One drawback to the use of TCP guns is that a drilling or workover rig is required to run the guns into and out of the well. If the guns are to be retrieved, the well must be killed.
Wireline-conveyed perforating has several advantages. For instance, operators have flexibility in choosing a gun system, and operations can be performed with or without a rig on location. Because the wireline cable provides communication between the downhole gun and the surface, wireline perforating offers accurate depth correlation. Through-tubing perforating almost always relies on wireline for conveyance. Limitations of wireline perforating include gun length and weight and wellbore geometry. Slickline perforating, which is becoming increasingly popular, offers a cost-effective and efficient alternative to conventional wireline perforating and TCP guns. However, slickline units do not provide power from the surface to set off blasting caps, and slickline perforating does not offer the same level of depth-correlation accuracy as wireline perforating.
Although operators consider many factors when designing a perforating program, the reservoir generally dictates which system will be used. For instance, formations that are prone to producing sand perform better with high shot density and large holes (Figure 4). Operators often perforate with large-diameter TCP guns that produce many holes per linear foot. Depth of penetration for these types of formations does not affect well performance. Formations damaged during drilling and completion, however, perform better with deep penetrations that extend beyond the damaged zone. Deeper penetration, however, comes with the disadvantage of smaller diameter perforation holes. Underbalanced perforating in wells with formation damage may also improve well performance.