Effect of Perforations on Fracture Initiation
- L.A. Behrmann (Schlumberger Perforating and Testing Center) | J.L. Elbel (Dowell Schlumberger)
- Document ID
- Society of Petroleum Engineers
- Journal of Petroleum Technology
- Publication Date
- May 1991
- Document Type
- Journal Paper
- 608 - 615
- 1991. Society of Petroleum Engineers
- 2.2.2 Perforating, 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc), 1.2.3 Rock properties, 5.6.4 Drillstem/Well Testing, 2.5.2 Fracturing Materials (Fluids, Proppant), 4.3.4 Scale, 3.2.3 Hydraulic Fracturing Design, Implementation and Optimisation, 1.14 Casing and Cementing
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Summary. This paper describes perforation/fracture tests performed perforation/fracture tests performed in large sandstone blocks in a triaxial stress cell to determine perforating geometry and perforating-fracture procedures for optimal fracture procedures for optimal fracture initiation. Four-shot, 1800 phased perforating guns, steel casing, and perforating guns, steel casing, and oilfield cement were used. In one series of experiments, the casing was cemented and cured while under triaxial stress. Most tests were made with pore pressure, and vertical and horizontal wells were simulated. In general, the tests showed that (1) fractures initiate either at the base of perforations or at the intersection of the plane normal to the minimum horizontal stress that passes through the axis of the wellbore and the wellbore surface and (2) fracture initiation depended on perforation orientation with respect to the plane normal to the minimum horizontal stress and the properties and injection rate of the fracture fluid. All fractures reoriented into the plane normal to the minimum horizontal stress within one wellbore diameter; although multiple fractures were initiated, only primary single fractures propagated beyond one wellbore diameter.
Laboratory simulation of fracturing through cased and perforated wellbores generally has been performed with one or more of the following limiting conditions: (1) scaled tests (rate effects ignored), (2) artificial rock (cement, plaster of Paris, or hydrostone), (3) isolation of the perforations from the wellbore (no pressurization between the scaled casing and the rock), (4) no poroelastic effects (impermeable rock or poroelastic effects (impermeable rock or high-viscosity fracturing fluid), (5) artificially scaled perforations(no perforation damage), and (6) no pore pressure (impermeable or dry rock). Extrapolation of laboratory results to downhole situations must proceed with caution whenever any of these conditions are part of the laboratory experiment. For part of the laboratory experiment. For example, Condition 3 is unrealistic downhole and Condition 5 may apply only in special situations of high underbalance perforating. Conditions 4 and 6, however, may simulate a well with extensive wellbore and perforation damage. To the best of our knowledge, perforation damage. To the best of our knowledge, Warpinski's mineback experiments, in which annulus fractures were observed, were the only experiments conducted in the absence of these conditions. The experiments discussed in this paper were planned to evaluate the effect of performations on fracture initiation. Because of performations on fracture initiation. Because of the limiting-condition issues discussed above, fill-scale experiments were conducted in sandstone rock in a large triaxial stress frame that simulated downhole conditions.
The fracture-initiation experiments were performed in 27x27x32-in. sandstone performed in 27x27x32-in. sandstone blocks in Terra Tek's 8,psi triaxial stress frame. The circular frame and its top and bottom platens surround the rock and provide reactions for three independent pairs of flat jacks. Flat-jack efficiency was pairs of flat jacks. Flat-jack efficiency was measured at 91 %; all applied stress data use this correction. Access to the central wellbore is provided through the top loading plate. Pore pressure was applied by placing plate. Pore pressure was applied by placing the rock in a stainless-steel can with rubber seals on the top and bottom faces. The can allowed the placement of about 0.25 in. of bauxite beads around the four faces. The borehole was cored to 4% in., and 3-in. OD x 2 1/2 -in. ID) steel tubing was cemented in place. Through-tubing, 1 11/16-in. or in., 4-shot/ft (SPF), 180 degs phased guns with three or four shots were used. Table 1 gives the rock properties.
Three sets of tests were conducted (Table 2). Experimental techniques were enhanced with each additional set. Rock saturation and pore pressure were added to Set 2. An in-situ pore pressure were added to Set 2. An in-situ pore pressure gauge, a large 2.38-gal pore pressure gauge, a large 2.38-gal intensifier, and casing cemented and cured under stress were added to Set 3. The general test procedure was as follows. 1. Vacuum saturate the rocks with 3% brine, flowing brine from the uncased wellbore to the rock sides (Sets 2 and 3 only). 2. Cement casing into rock and allow to cure (Sets 1 and 2 only). 3. Place rock into test frame. For Set 3, casing was cemented in place before the frame was closed and cured while under stress. 4. Place perforating gun in wellbore. 5. Close frame, apply desired flat-jack and pore pressures; the wellbore is vented to atmosphere. 6. Fire gun. For Set 3, the casing cement cured for a minimum of 24 hours before the gun was fired. 7. Flow brine at 2,000 psi from the bead pack through perforations; measure flow pack through perforations; measure flow rate 8. Perform various prefracture procedures depending on test set. procedures depending on test set. 9. Fracture rock with red dye used in fracture fluids. 10. Remove, cut open, and examine rock.
Set 1, Torrey Buff Sandstone. Equipment failure prevented on of the specimens in this set. After perforation, the wellbore was flushed with brine, and on Tests 2 and 4, brine was injected at low pressure through the perforations to saturate the rock locally near the wellbore.
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