Sand Jet Perforating Revisited
- J.S. Cobbett
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- March 1999
- Document Type
- Journal Paper
- 28 - 33
- 1999. Society of Petroleum Engineers
- 1.8 Formation Damage, 3.2.3 Hydraulic Fracturing Design, Implementation and Optimisation, 1.2.3 Rock properties, 3 Production and Well Operations, 2.4.3 Sand/Solids Control, 2 Well Completion, 5.6.4 Drillstem/Well Testing, 1.14 Casing and Cementing, 1.6 Drilling Operations, 2.2.2 Perforating, 1.11 Drilling Fluids and Materials, 2.4.5 Gravel pack design & evaluation
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Though sand jet perforating is not a new technique, it is one that has been almost forgotten, the last SPE paper on the subject being published in 1972. Unlike explosive perforating, which is literally a "one shot" process, sand jet perforating uses a high velocity jet of abrasive fluid to cut through the casing, cement and deep into the formation, enabling pressure, pumping time and other parameters to be varied to maximize penetration. Sand jets can penetrate much deeper than explosives, and offer a cost-efficient, safer and better-targeted alternative to hydraulic fracturing to bypassing deep near-wellbore damage.
This paper is based mainly on experience in Lithuania, where, in 1995, joint venture oil companies first started field operations to complete development of small oil fields found in the west of the country (see Fig. 1) during the Soviet era, but which had been considered as too small to develop for the Soviet Union, with giant oil fields to the east. Wells, some of which had produced on test at over one thousand barrels per day, had been left with heavy mud across open perforations, often for more than a decade. When it proved impossible to get these wells to flow again using (western) tubing conveyed explosive perforators (TCPs), sand jetting was used, as has been the almost universal practice in Lithuania. The first well sand jet perforated by a joint venture company, which had yielded less than one barrel per day with (western) TCPs, gave over 900 barrels per day when perforated with sand jets.
Subsequently, one of the best producers in Lithuania, which had already been sand jetted once, was reperforated using more advanced techniques. Coiled tubing was run through the xmas tree and completion to enable the well to be sand jet perforated, with oil as the carrier fluid, underbalanced, with the well flowing throughout, resulting in a doubling of production to 800 BOPD. Though sand jet perforating is, at least theoretically, available from the main pumping contractors, it is almost unknown outside North America and the former Soviet Union, the main technical references (Refs. 1-5) being over 30 yr old. Sand jet perforating does, however, provide a cost-effective means of passing deep formation damage and should form part of the armory of any practicing petroleum engineer. This paper aims to remind engineers of this, review the technology and suggest appropriate applications.
Most of the oil and gas produced today comes from wells with cased, cemented and perforated completions. Nowadays, the perforations are almost always made using shaped-charge explosives, either run on electric line or tubing conveyed (TCP). In addition to punching holes through the casing, the main purpose of perforating is to pass the "damaged zone," near to the wellbore, where drilling and completion operations have caused a reduction in permeability. This damaged zone, often quantified as "skin" from well-test analysis, typically extends from a few inches to a few feet into the formation. With modern drilling and completion techniques, including improved drilling fluids and "well-productivity-friendly" drilling practices, the depth of the damaged zone and the degree of damage (i.e., the contrast in permeabilities between damaged and undamaged formations) can be minimized, enabling modern perforating techniques, particularly underbalanced perforating with deeply penetrating low-debris TCP guns, to obtain maximum productivity, and hence maximum value, from newly drilled and completed wells.
Though most new wells can be perforated effectively with explosives, there remain exceptions. Where perforating performance is significantly reduced, usually because the formation is very hard, it may be impossible to get adequate penetration with explosives. In some wells, explosive perforation may be ineffective at passing a very deep damaged zone, perhaps due to poor drilling and completion practices or in old wells that have been left with drilling mud across open perforations. As explosives are (literally) a "one shot" process, it is not practical to reperforate many times to improve penetration, as the chances of reperforating along (and thus extending) the original perforation tunnels would be minimal. Where increased penetration has been required, this has usually been via hydraulic fracturing.
In contrast to explosive perforation, perforating by high pressure fluid jets is not a one shot process. Unlike explosives, the amount of energy, and hence rock destruction, that can be achieved at one point is not fixed but can be increased by, for example, pumping for more time or at a greater pressure. Though hydraulic jet perforating with clear water or brine is used (see Refs. 5 and 6), more often sand has been added to the fluid to improve the penetration rate. For this reason, and because the present author has no experience in using clear fluids, this paper will refer to "sand jet perforating" only.
Most of the technical papers on sand jet perforating have been written by engineers employed by pumping companies (for example, Refs. 1-3, 6-9) and concentrate on the mechanical aspects of the perforating process. This paper will comment also on the productivity, and in particular the cleanup, of sand jet perforated wells.
Technology, Equipment, and Operational Techniques
Details of the technology, equipment and operational techniques are well described in the references at the end of this paper and will only be summarized here. Sand jet perforating uses a jet of fluid, with sand added to improve penetration rates, pumped at high pressure through a jet nozzle to cut through the casing, cement and into the formation. Though other fluids, including diesel and crude oil, have been used, usually the fluid used is clear water or a brine of sufficient weight to ensure that the well remains dead. If appropriate, the fluid may be inhibited, for example, with KCl to prevent clay swelling. A sand jet perforating tool (see Fig. 2), run on the end of the treating work string, carries one or more tungsten carbide jet nozzles. High pressure pumps on the surface, together with a sand blender, are used to mix and pump the water and sand.
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