Perforating for Heavy Oil Cold Production
In North West Saskatchewan and North East Alberta, Canada (continued pg.3)

by Kirby Hayes

Casing Integrity

There is no way to put numerous large diameter holes into casing without lowering its strength. As an example, 69 shots per meter (SPM) .8" at 120 by 60 degree phased holes reduces the casing’s strength by approximately 20 %. Casing strength is decreased the closer holes are on the same plane. Suggestions to minimize casing strength loss are: perforate less, re-perforate cautiously with a different phased product (the re-perforated shots will not be oriented relative to the existing perforations), (logistical complication), try not to place them in the perforating interval when setting casing (another possible logistical complication). If it is, as suspected, that casing damage is a result of subsidence of the overburden caused by producing large quantities of sand, there is not much that can be done when the earth starts to move. If in fact these phenomina are determined to exist in a large number of cold produced fields, it will have enormous negative implications for heavy oil.  After such aggressive perforating practices are carried out - 26 SPM of 1" holes - the area of the perforations only represents approximately 2.3% of the original casing area. Casing that is not sufficiently supported can experience moderate to severe damage when perforated.

Cement Integrity

The cement sheath through the perforated interval will be damaged (having been shot full of holes). Figures 10 to 13 illustrate the damage is contained primarily to the perforating interval. Some of the differences in the cement bond logs is due to the difference in fluid level from the before to the after perforating passes. Severe damage to cement has been experienced in previously steamed and then aggressively re-perfed situations. Severe casing damage will occur when the cement sheath is not supported by the well bore or formation.

Re-perforating

Re-perforating is an accepted, widely used and, as demonstrated in figure 14, highly successful work over technique. In situations where existing perforations are deemed inadequate for CP (i.e. .5"EHD at 13 SPM) it may be imperative to re-perforate. In situations where casing integrity is in question it might be more prudent to perforation wash, chemical stimulate (figure 15), use sand acids, sand bail, pump to surface, use fluid flushes, super flushes (fluid stimulation), use stable foam clean outs or utilize propellants to unplug perforations. The problem could conceivably be out in the formation and not in the perforation. In this case, re-perforating, although it may stimulate the well back into production, although it may stimulate the well back into production, will further decrease casing strength.

Case studies

Variables such as production philosophies, production techniques, economics (wells being shut in), geology, geography, geometry, completion practices, drilling practices, drainage geometry and other such things that may or may not be apparent make product comparisons cumbersomely difficult.

The 69 SPM vs. 26 SPM study results in Figure 16 and 17 demonstrate that a significant increase in flow area did not produce a positive impact on production volumes. The results from this test should not be interpreted to exclude the 69 SPM product from use in heavy oil. The thin pay of some Colony zones showed dramatic results from utilizing this product (Figure 18). The greater flow area’s effect of less pressure draw down on the production of water needs more experimentation. The study, because of it’s relative short time span, was not able to conclude whether there was a reduction in work over frequency. There was no difference in work over frequency in the short term after the wells were completed.

The results from the study done in Figure 19, though the brief span of data (two months) and did not hold true for cubic meters of production per meter of perforations (figure 20), makes this conclusion very dubious. The fact that these two studies contradict each other (i.e. one indicates more perforations and flow area did not effect production and the other indicated it did) proves: 1) that more variables need to be eliminated 2) obviously further study is needed and 3) that products should not be evaluated on such a limited basis.

Conclusions:

 When perforating a heavy oil well that is going to be subjected to common cold production practices, the following is recommended:

• Closely review all pertinent data, sources of information and considerations such as open hole logs, cores of the well(s) in question or cores from the area, area geology, area geophysics, economics, logistics, and most importantly (if applicable) past production experience to ensure the development of a congruent exploitation, drilling, completion and production strategy. 14
• Choose perforating products and perforating and completion techniques congruent to the strategy and consider the implications to production, logistics and economics.
• Large diameter, high density, closely phased perforations be utilized, shot under balanced and covering the entire zone to provide a geometrically thorough disturbed zone, minimize plugging, enhance initial sanding and sustain production.
• Use compatible fluids that do not promote sand fall out
• Clean out possible perforation, formation and drilling solids debris
• Pump to surface or put on pump to ensure sand movement is continuous
• Track and document the results

 In the spirit of the example in the introduction, encouragement of new and different products, techniques, technologies and business practices is crucial to the full exploitation of heavy oil cold production.  Participation in and support of forums to introduce, exchange and evaluate new innovations will greatly expedite the quest.

 © 1998 Kirby Hayes