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by Kirby Hayes
Debris
The achievement of large EHD has not been without detrimental side effects. In order to achieve a large EHD in the casing a larger exit hole in the carrier is produced. This allowed substantially more shrapnel (mostly case material approximately ½" square pieces from the spent explosives) to exit the carrier than was previously experienced with smaller EHD producing charges. This was lessened by different means by different manufactures. One method was to design a charge with a case (zinc) that would powder after the jet was formed. Another utilized a jet that would develop after exiting the carrier resulting in a small exit hole (limiting the debris entering the well bore) while still achieving a large EHD in the casing wall. Still another applied a stronger material that would leave the case relatively intact preventing it from exiting the carrier through the exit hole.
The other cause of debris is carrots. The last particles of the liner forming the tail of the jet, travel at a lower velocity. This lower velocity allows these particles to fuse, forming what is referred to as a carrot. In smaller EHD charges, carrots are virtually eliminated by use of disintegrating metal liners (DML) and powdered metal liners (PML). The parabolic shape of BH & EBH liners makes the use of PMLs difficult to manufacture. The use of PML material in BH type charges also have reduced EHD because of the way that that liner breaks down during detonation and the speed of the resulting jet. By using a solid liner material like copper brass or pfinodal with its high yield strength, a higher mass is induced into the jet stream which tends to produce a larger EHD. The parabolic shape on the liner also controls the speeds of the jet stream which in turn controls the shape of the jet. Although the use of brass liners has greatly reduced the number and size of carrots experienced, they have not completely eliminated the problem. Recently there has been discussions/inquiries into the extremely unconsolidated nature of the reservoir partly being the cause of forming more carrots. In more consolidated conditions, the liner material is deposited along the circumference and length of the perforation. In extremely unconsolidated conditions, the liner material has more opportunity to collide and fuse into carrots. Clearly more documented study is needed to understand this phenomena.
Penetration
The effects of extremely unconsolidated formations on entrance holes and the formation of carrots is not understood. The effects of these formations on penetration is equally a mystery. The lower the compressive strength of a target, the farther the penetration; is an accepted fact up to the point where that compressive strength fails to maintain the shape of the jet. At this point the jet starts to disperse. To illustrate this effect, refer to the exit hole of the target in figure 6. The recent introduction of some perforating products with PEN of a mere 5.9" proves that far less penetrations is needed than was once thought (i.e. PEN 12"). The presence of drilling damage might be reason to consider products with further penetration (DP charges). There is no evidence, except open hole caliper logs, though, of what that penetration has to be (drilling fluids have been discovered as far off as offset wells). Charge performance modeling indicates the probable penetration through most well bore wash outs. The argument that a larger volume of disturbed area will aid initial sand production might be a further reason for using DP charges. When considering the use of DP charges or the highly technically termed practice of "salt and peppering"4 consider the resultant decrease of flow area.
Shot Density and Phasing
It is extremely logical that if solids are produced and those solids have a high tendency to plug perforations, we should therefore create more perforations to sustain production longer. If there is a quality control issue with perforating products, higher shot densities will increase the odds of effective perforations.iv Heavy oil reservoirs are not homogeneous, therefore, to enhance the number of worm holes that propagate from perforations and to increase the odds of accessing the preferential direction of propagation, higher densities and close phasing are recommended. v vi Higher shot densities and close phasing aid sand flow initiation by a creating a geometrically thorough disturbed zone around the well bore.vii viii ( Refer to figures 7 and 8)
Conveyance
Hollow steel carriers5 (HSC) both re-useable and expendable retrievable (ERHSC) are utilized in this area. Rigidity, robustness, and weight all aid in conveyance through highly viscous fluids. HSC minimize casing damage by containing most of the concussion. Perforating in a true under balanced condition,6 avoiding problems caused by severe and immediate sand inflow suspended in high viscous fluids (getting down with subsequent runs and getting stuck), perforating longer intervals (i.e. the entire zone7 8 and safety (in very active areas etc.) are factors contributing to the growth of pressure activated (PA) TCP operations. The use of mechanical collar locators (currently an under utilized method) when PA TCP perforating large intervals >6m in wells <600m is an economical option where accurate depth control is not imperative (i.e. +-1m).
Logistics and economics
Constantly complicating field operations, are the choices of completion products and techniques. These decisions are based primarily on logistics and economics. The low margins experienced in heavy oil and the high efficiencies of field operations demanded by tight margins dictate the prioritizing of logistics and economics. They should not , however, overlook the long term effect they may have on production. It should be the objective of a completion strategy to ensure continuous and sustained production. The prevention of work over operations not only has favorable implications on economics, but it might make sustaining production possible. It has been widely experienced that cold production, once interrupted, is difficult and sometimes impossible to re-establish.
The products and techniques that have idiosyncratic effects on economics, logistics and possibly production are: HSC vs. ERHSC, TCP vs. wire line, higher shot densities, MCL, clean out techniques, completion fluid and multiple well completion (the performance of a single service on multiple wells enabling reductions on mobilization charges). The effects of these variables on long term production has not been rigorously tracked. The magnitude of these effects may vary from well to well, field to field and area to area.
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