US 4,793,409 A · Method and apparatus for forming an insulated oil well casing

Abstract

A method and apparatus for forming an electrically conductive externally insulated casing for an oil well of the type in which the casing carries electrical current to a primary heating electrode positioned downhole, using multiple prefabricated casing segments; each casing segment is a long steel pipe having a female thread coupling on one end and a male thread on the other end. Each segment has an insulator covering, over substantially all of its length, that has a figure of merit (erl)/.DELTA. of no more than 4.times.10.sup.8 so that the shunt impedance of the casing to ground can be kept substantially greater than the spreading resistance of the primary heating electrode. The preferred casing segment insulation is in two layers, including a hard, durarable inner layer subject to water degradation and an outer water-impervious layer usually applied as tape. The uninsulated portions of joints between segments are covered withan insulator cement in the field and then further covered with a water-impervious tape that overlaps the water-impervious layers on two segments.

Claims43

Dependent claims

A method of forming a casing in an oil well comprising an externally insulated electrically conductive casing employed as a conductor carrying electrical current to a heater electrode positioned downhole in the well in alignment with an oil producing formation, comprising the following steps:

(A) pre-assembling a plurality of casing segments, each casing segment comprising an elongated metal pipe, each casing segment having an electrical insulator covering on substantially all of its external surface;
the insulator covering having a figure of merit (e.sub.r L)/.DELTA. of no more than 4.times.10.sup.8, after extended immersion in water, wherein
e.sub.r =reaative dielectric constant of the insulator covering at 60 Hz,
.DELTA.=thickness of the insulator covering in feet, and
L=length of insulated casing in feet;
(B) inserting one casing segment partially into the well;
(C) joining another casing segment end-to-end to the one casing segment;
(D) applying electrical insulator material to the joint between the casing segments to afford a continuous external insulator covering approximating the electrical insulation characteristics of the insulator covering on each segment;
and
repeating steps B through D to complete an electrically conductive externally insulated casing down to approximately the depth of the oil producing formation.

7 more elements

In an electrically heated oil well comprising:

a well bore extending downwardly from the surface of the earth through one or more overburden formations and through an oil producing formation;
an electrically conductive externally insulated main casing extending from the surface of the earth down into the well bore to a depth adjacent the top of the oil producing formation;
an electrically conductive externally uninsulated primary heating electrode extending downwardly from the casing, through the oil producing formation;
at least one secondary heating electrode positioned within one of the overburden and oil producing formations;
and electrical power supply means connected to the primary electrode through the main casing and connected to the secondary electrode, for energizing the electrodes for conduction heating of a portion of the oil producing formation adjacent the well;
a casing which comprises a multiplicity of casing segments interconnected end-to-end, each such casing segment comprising:
an elongated metal pipe;
and an electrical insulator covering on substantially all of the external surface of the metal pipe;
in which ##EQU5## wherein:
G.sub.c =conductance of the insulator covering in mhos;
C.sub.c =capacitance of the insulator covering in farads;
R.sub.p =spreading resistance of the primary electrode in ohms;
and
.mu.=.pi. f, where f is frequency.

11 more elements

Dependencies are inferred from the claim text.

Method and apparatus for forming an insulated oil well casing

Life of this patent

Method and apparatus for forming an insulated oil well casing

Life of this patent

Abstract

Claims43