Highly Overbalanced Perforating
- John M. Dees (Dees Well Completions)
- Document ID
- Society of Petroleum Engineers
- Journal of Petroleum Technology
- Publication Date
- May 1995
- Document Type
- Journal Paper
- 395 - 397
- 1995. Society of Petroleum Engineers
- 1.2.3 Rock properties, 1.14 Casing and Cementing, 2.4.3 Sand/Solids Control, 3 Production and Well Operations, 2.5.2 Fracturing Materials (Fluids, Proppant), 2.2.2 Perforating, 4.1.5 Processing Equipment, 3.2.3 Hydraulic Fracturing Design, Implementation and Optimisation, 1.8 Formation Damage, 4.1.2 Separation and Treating
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This paper is SPE 30342. Technology Today Series articles provide usefulsummary information on both classic and emerging concepts in petroleumengineering. Purpose: To provide the general reader with a basic understandingof a significant concept, technique, or development within a specific area oftechnology. Journal of Petroleum Technology, May 1995.
This article briefly describes a new perforating technique that will resultin clean, open perforations. The well is normally stimulated when highlyoverbalanced perforation techniques are used. This perforation process improvesformation evaluation because the well flows better and is easier to test.
Underbalanced perforating is generally accepted as the petroleum industrystandard for perforating. However, a recent review of posperforation wellperformance indicated that inconsistent results are achieved with conventionalperforating techniques. Pressure buildup analyses after perforating oftenindicate a range of skin factors from -1.0 to more than +50. An averageperforation efficiency of 25% was observed in one perforation study.
On the basis of such results, the industry has been looking for a new methodto perforate a well that could improve perforation efficiency. Possiblestimulation techniques include propellants and explosives. Fig. 1 illustrateshow the pressure vs. time relationship varies for four separate stimulationmethods. Explosives or propellants apply a high pressure to the perforations inmicro- or milliseconds, respectively. Hydraulic fracturing loads theperforations very slowly. Overbalanced perforating loads the perforationsrapidly and maintains the pressure for a sufficient time to create cleanfractures at the tips of each perforation (Fig. 2).
A perforating jet exerts a stress of 4 to 5 million psi at the tip. Thishigh stress greatly exceeds all the principal rock stresses, and, consequently,a perforation hole and tunnel are formed. A reasonable estimate of the tunnellength can be made by considering such factors as charge specifications,in-situ rock properties, and wellbore conditions.
If a sufficient amount of overbalanced pressure is applied to theperforations after they are formed, two potential benefits can be gained.First, the prolonged application of high pressure will allow for somestabilization of the tunnel walls. Second, the action of the fluid used toapply the pressure can extend induced fractures created through eachperforation.
Erosion of the fracture faces can occur if the velocity is high enough.Fluid velocity alone may be insufficient to etch the induced fracture facesproperly in some cases. The addition of a reactive fluid, such as acid in acarbonate reservoir, or a scouring material, such as fracturing proppant, canimprove the etching patterns on the fracture faces significantly. The additionof a scouring material may also be beneficial in eroding and enlarging theperforating-entry-hole diameter through the casing and cement sheath of thewellbore.
Highly overbalanced perforating can be done with any of the currentlyavailable perforating guns. The preferred gun type is a hollow steel carrierbecause the gun body will retain most of the charge debris. Tubing-conveyedguns will allow for the application of the highest bottomhole pressure (BHP).Fig. 3 shows a typical tubing-conveyed setup. Before perforating, pressure isapplied to the wellbore that will result in a downhole pressure at least aslarge as the fracturing pressure of the formation. A typical applied pressuregradient is between 1.1 and 1.3 psi/ft. Ideally, the pressure gradient willexceed all the principal in-situ stresses. The rule of thumb for minimumapplied pressure is the formation fracturing gradient plus 0.4 psi/ft.
The pressure is applied by use of all liquid, all gas, or a combination ofthe two. The preferred method will incorporate a liquid column directly acrossthe perforation interval and up to a predetermined fluid level from thesurface. A gas column will be used above this liquid to provide the additionalpressure required to achieve the desired gradient. At the time of perforating,the expansion of the gas will translate directly into horsepower applied to theformation. Injection rates that are not attainable by practical means from thesurface can be achieved downhole. In instances where reducing surface pressuresis desirable, nitrogen has been used to achieve a predetermined surfacepressure, followed by pumping of a weighted brine water to complete the initialpressurization. This has reduced surface pressures by several thousand poundsper square inch on some well treatments.
The high rate of fluid displacement will exceed the capacity of theperforations to accept fluid. This event will last from 1 to 15 seconds untilthe pressure falls below the fracture-extension pressure. The breakdown isgreatly enhanced by performing additional pumping operations with additionalgas and/or liquid volumes. In some instances, a small stimulation treatment maybe done together with the breakdown.
The final fluid pumped into the reservoir can be tailored to enhance therelative permeability to the reservoir produced fluid. In low-pressure gaswells, only small liquid volumes of liquid (1 to 5 bbl) are required for thebreakdown. This small liquid volume can be dispersed easily by injecting gas,such as nitrogen or carbon dioxide. Surfactant blends that will leave theformation water-wet can be used in oil wells.
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