Effect of Perforations on Fracture Initiation

Abstract

Summary. This paper describesperforation/fracture tests performed perforation/fracture tests performed in large sandstone blocks in a triaxialstress cell to determine perforatinggeometry and perforating-fractureprocedures for optimal fracture procedures for optimal fracture initiation. Four-shot, 1800 phasedperforating guns, steel casing, and perforating guns, steel casing, and oilfield cement were used. In oneseries of experiments, the casing wascemented and cured while undertriaxial stress. Most tests were madewith pore pressure, and vertical andhorizontal wells were simulated. Ingeneral, the tests showed that(1) fractures initiate either at the baseof perforations or at the intersectionof the plane normal to the minimumhorizontal stress that passes throughthe axis of the wellbore and thewellbore surface and (2) fracture initiationdepended on perforation orientationwith respect to the plane normal tothe minimum horizontal stress andthe properties and injection rate ofthe fracture fluid. All fracturesreoriented into the plane normal to theminimum horizontal stress within onewellbore diameter; although multiplefractures were initiated, only primarysingle fractures propagated beyondone wellbore diameter. Introduction Laboratory simulation of fracturing throughcased and perforated wellbores generally hasbeen performed with one or more of thefollowing limiting conditions:scaled tests(rate effects ignored),artificial rock(cement, plaster of Paris, or hydrostone),isolation of the perforations from thewellbore (no pressurization between thescaled casing and the rock),noporoelastic effects (impermeable rock or poroelastic effects (impermeable rock or high-viscosity fracturing fluid),artificially scaled perforations(no perforation damage), andno pore pressure (impermeable ordry rock). Extrapolation of laboratory results todownhole situations must proceed withcaution whenever any of these conditions arepart of the laboratory experiment. For part of the laboratory experiment. For example, Condition 3 is unrealistic downholeand Condition 5 may apply only in specialsituations of high underbalance perforating. Conditions 4 and 6, however, may simulatea well with extensive wellbore andperforation damage. To the best of our knowledge, perforation damage. To the best of our knowledge, Warpinski's mineback experiments, inwhich annulus fractures were observed, were the only experiments conducted in theabsence of these conditions. The experiments discussed in this paperwere planned to evaluate the effect ofperformations on fracture initiation. Because of performations on fracture initiation. Because of the limiting-condition issues discussedabove, fill-scale experiments wereconducted in sandstone rock in a large triaxial stressframe that simulated downhole conditions. Test Fixture The fracture-initiation experiments wereperformed in 27×27×32-in. sandstone performed in 27×27×32-in. sandstone blocks in Terra Tek's 8, psitriaxial stress frame. The circular frame andits top and bottom platens surround the rockand provide reactions for three independentpairs of flat jacks. Flat-jack efficiency was pairs of flat jacks. Flat-jack efficiency was measured at 91 %; all applied stress datause this correction. Access to the centralwellbore is provided through the top loadingplate. Pore pressure was applied by placing plate. Pore pressure was applied by placing the rock in a stainless-steel can with rubberseals on the top and bottom faces. The canallowed the placement of about 0.25 in. ofbauxite beads around the four faces. Theborehole was cored to 4% in., and 3-in. OD × 2 1/2 -in. ID) steel tubing was cementedin place. Through-tubing, 1 11/16-in. orin., 4-shot/ft (SPF), 180 degs phased guns withthree or four shots were used. Table 1 givesthe rock properties. Experimental Procedure Three sets of tests were conducted (Table2). Experimental techniques were enhancedwith each additional set. Rock saturation andpore 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 understress were added to Set 3. The general test procedure was as follows.1. Vacuum saturate the rocks with 3%brine, flowing brine from the uncasedwellbore to the rock sides (Sets 2 and 3 only).2. Cement casing into rock and allow tocure (Sets 1 and 2 only).3. Place rock into test frame. For Set 3, casing was cemented in place before theframe was closed and cured while understress.4. Place perforating gun in wellbore.5. Close frame, apply desired flat-jackand pore pressures; the wellbore is ventedto atmosphere.6. Fire gun. For Set 3, the casing cementcured for a minimum of 24 hours before thegun was fired.7. Flow brine at 2,000 psi from the beadpack through perforations; measure flow pack through perforations; measure flow rate8. Perform various prefractureprocedures depending on test set. procedures depending on test set. 9. Fracture rock with red dye used infracture fluids.10. Remove, cut open, and examine rock. Experimental Results Set 1, Torrey Buff Sandstone. Equipmentfailure prevented on of the specimensin this set. After perforation, the wellborewas flushed with brine, and on Tests 2 and4, brine was injected at low pressure throughthe perforations to saturate the rock locallynear the wellbore. JPT P. 608