Abstract

A series of tests to describe the effect of numerous holes in casing on the mechanical crush resistance has been run in casing and in L-80 steel tube models. The tests were run with big holes and normal hole sizes for perforations in phasings of 0 degrees, 180 degrees, 120 degrees, 90 degrees, and 60 degrees. Shot density was varied in the model tubes from 4 to 36 shots per foot. The actual perforated casing was at 12 shots per foot. All of perforated casing was at 12 shots per foot. All of the perforated casing and tubing sections were compared to strength remaining in unperforated pipe of the same specimen.

The tests were carried out in a compression machine under a controlled rate of loading with strain gauges attached at critical points along the plane of expected failure. The casing and tube plane of expected failure. The casing and tube specimens were loaded between steel platens. The data for the test are plotted in the form of load vs deflection. Strain gauge data are also presented. The information generated includes observations of the effect of entrance hole size, phasing (lateral hole separation), shot density (linear hole separation at a specific phasing), and effect of the position of the plane of perforations to the plane position of the plane of perforations to the plane of application of load.

Introduction

This project was implemented to establish how much lateral and axial crush resistance is lost after casing is perforated at shot densities from 4 to 36 shots/ft. The research used both 7 in. N-80 and P-110 casing perforated in cement targets and L-80 tubing machined to model the 7 in. casing on a 1 to 3.33 scale. The modeled parameters included scaled diameter, length, wall thickness, and perforation hole size. The casing was perforated with perforation hole size. The casing was perforated with jet charges. The holes in the tubing were drilled to model a 0.37 in. diameter hole, and 0.75 in. and 0.9 in. diameter big holes. An additional goal of the study was to determine the effect of perforation phasing on casing strength loss. The phasings used phasing on casing strength loss. The phasings used were the available commercial phasings of 0, 120, 90 and 60. The methods of applying load in these tests are intended to roughly simulate earth shifts such as plastic formation flow and stresses produced around salt domes and in unconsolidated formations. These tests do not attempt to model the effects of fluid pressure, either on unperforated or perforated sections.

TEST PROCEDURE

An upper test limit of 60,000 lb for the available Tinius-Olsen compression machine necessitated the use of tubes to model the casing, particularly in axial compression tests. A joint of L-80 grade tubing was cut and machined to provide the 11.5 in. long axial test pieces and the 6 in. long pieces for the lateral crush tests. L-80 grade pipe was selected for its similarity to N-80, one of the most commonly used grades, and because the requirements for the L-80 series pipe are more closely controlled than in N-80. Elongation tests on tensile specimens from L-80 tubing and N-80 and P-110 casing are shown along with the global stress vs strain curves of Figures 1 and 2. These tensile strength specimens were from the same joint of pipe as the perforated sections discussed later in the report. The compressional tests were worst possible case, point loading tests made by placing the model tubes or the actual sections of 7 in. casing between the steel platens of the Tinius Olsen compressional device and platens of the Tinius Olsen compressional device and deforming the casing by application of load at a rate equivalent to 0.00017 in./sec platen travel in unopposed loading or an application of 256 lbs/sec in an infinitely opposed condition. Application of the point-to-point loading in a steel-or-steel fashion does not take into account the effects of a concrete sheath or conforming borehole that would spread out the stress over a larger area.

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