1) PURPOSE OF CEMENT SEAL UNITS:
SEAL UNITS prevent fluid movement at the interfacebetween the outer surface of the casing and the inside ofthe cement sheath. In spite of the improved practice ofreleasing pressure on the casing after the top plug bumps,investigations by different research and developmentgroups confirm that leakage at this interface is not onlypossible, but probable, when subjected differentialpressures often encountered in completing or producinga well. Inter-zonal communication at this point, like“channeling” in the remainder of the annulus, will preventefficient depletion of the producing zone, or causestimulation work (fracturing, acdizing, etc) to beineffective due to uncontrolled fluid migration in theannulus.
2) Cement Seal Units – General Comments
The Cement Seal unit is designed to block off possiblegas or fluid migration along the casing-cement interface,commonly referred to as “micro-annulus”
1) Micro-Annulus Evidence and Consequences:
The presence of a “micro-annulus” between the outercasing surface and the cement sheet is very commonand can lead to:
A) Inter zonal Communications:
B) Gas Pressure On The Annulus
C) Leakage of Stimulation Medial Up the Annulus:
D) Gas Storage Well Leakage:
2) Causes of Micro-Annulus:
The creation of a micro-annuls during or after thecementation is aided by:
A) Chemical reaction of the cement: During the setting or hardening process of the cement thechemical reaction generates heat, causing the casing bodyto slightly expand. Upon completion of the setting processthe cement cools which causes the casing to contract again.This leaves a possible micro-annulus since the cement isincapable of “flow back” to the casing body.
B) Hydrostatic Pressure Reduction inside the Casing:Drilling mud used to displace the cement is usually replacedwith a lighter production fluid. In production strings thiscauses a reduction of the hydrostatic pressure inside thecasing resulting in slight contraction of the casing body away from the cement sheet.
C) Exterior Casing body contamination:The exterior casing surface may be covered with millscale, corrosion inhibitor or an oil film (when oil basedmuds are used), all of which negatively effect the cementbonding to the casing.
D) Cement Shrinkage:When cement goes through the hardening process thereis a reaction which leads to an overall shrinkage of thetotal volume. This chemical contraction is split betweena less than 1% external volumetric shrinkage and a 4 to6% internal contraction. The reduction in cement volume can break the cement bond creating a micro annulus. Theeffect of shrinkage is more pronounced in large casingand open hole sizes.
3) Cement Seal Unit Features:
A) The Cement Seal Unit is a simple and inexpensivetool which has been developed from the philosophy thatmicro-annulus occurrence often cannot be prevented butcan be sealed off.
B) The unit functions somewhat as a “micro-annulus packer” through pressure sealing against the casing andthe existing cement. Gas and/or fluid migration is blockedoff in both directions.
C) Extensive laboratory tests as well as fieldapplications have proven the effectiveness of the units.Even though in well applications its effectiveness becomesapparent only after a period of time. For example, delayedand low oil-water ratio’s, no annular gas pressure build-upand measurable decreases in stored gas losses.
4) SPECIAL DESIGN FEATURES
In that each “seal unit” contains opposing “Sealingelements”, it stops fluid movement from both above andbelow. The “Seal Unit” O.D is approximately casingcoupling size to permit their use in both conventional andclose tolerance annuli. Seal Unit with hinged “back-uprings” are available for use on upset tubing and casing.The pipe on which it is to be installed requires specialsurface preparation (mill varnish removal, sandblasting,etc.) to improve its effectiveness. The “sealing elements”used both the “Standard” and the “Extreme Temperature”units are produced in special-shaped molds using customelastomer compounds developed for this application(possessing the essential properties of strength, abrasionand chemical resistance, not hardening with age orenvironment, etc.).
5) SEAL UNITS ARE RECOMMENDED FOR:
Depletion type producing zones.
Wells which must be fractured or acidized.
Multiple completion wells (one multiple casing strings).
Producing zones with water or gas levels, or permeable zones.
Gas storage wells.
Wells to be stimulated by steam or other thermal process.
SOLUTION TO CEMENT-CASING MICRO-ANNULUS
Suppose, after cementing a micro-annulus 0.01 inchwide is left between the cement and 7 casing. If this micro-annulus extends from the producing formation through animpermeable shale 20 feet thick to an underlying watersand, how much water will channel from the water sand to the production formation if there is a differential pressureof 100 psi resulting from the production interval? Tocalculate the permeability of fracture, the following
|relationship is used:|
|Where||k = 54 x 106W2 (1)|
|k = Permeability (Darcys)|
|w = Fracture width (Inches)|
|Solving in our case|
|k = 54 x 106 x .012|
|k = 5400 Darcys|
Note that a micro-annulus 0.01 inches wide has a permeabilityof 5400 darcys or 5,400,000 milli-darcys.Therefore to calculate the flow, Darcy’s equation for flowthrough a porous medium be used.
|q = 1.127 KA (P1 – P2)|
|Where||q = Flowrate (Bbls/Day)|
|K = Permeability (Darcys)|
|A = Micro-annulus area (Square Feet)|
|P1 = Pressure of water sand|
|(PSI)P2 = Pressure of producing interval (PSI)|
|u = Viscosity of salt water(Centipoises)|
|L = Length of micro-annulus (Feet)|
Assume the micro-annulus is completely around the casing. Also, let’s use a viscosity of .49 centipoises for the salt water
q = 1.127 x (5400) x 7 x 77 x .01) (100)
.49 x 20 x 12 x 12
q = 83 bbls. of water per day
Since the flow rate is directly proportional to the width ofthe micro-annulus and the differential pressure, an increase ineither of these variables would result in an increased flow rate.
The magniture of 83 bbls per day channeling withdifferential pressure of only 100 psi and micro-annuluswidth of 0.01 inch is alarming. Remedial action is a necessity.
6) HOW SEAL UNITS WORK:
Each seal unit consists of two opposing cup-type“sealing elements” held in place and reinforced by twosteel “back-up rings and a center “spacer band”. The innerand outer “lips” of each “sealing element” provide amechanical seal between the pipe surface and the hardenedcement surrounding it. As a pressure differential developsat the casing cement interface it causes the “cupped” innersection of the opposing “sealing element” to expand and seal.
7) Seal Unit Operating Instructions:
The rubber rings for seal-units area available for two temperature ranges:1) Up to 275°F
2) Up to 400°F.
When the unit will be used in relatively deep applicationswhere the bottom hole temperature may not be exactly known,it is advisable to use the high temperature version.
The placement of the seal-units on the string is basedon zone spacing, pressure and formation types. For positivezone separation one unit is placed above and below eachzone, with a centralizer or turbolizer installed 3 to 5 foot below each unit. In case of critical zone separations twounits are installed on each side of the zone with a centralizer or turbolizer between the seal-units.
In liner applications where liner top leakage is likelyto occur, one or two seal units are placed in the lineroverlap area.Important Note: Although the Cement Seal Unitresembles a stop collar in many ways, it should NOT beused as a stop device.
8) Case History
More than 700 cement seal units of different sizes (171 pc of 9 5/8”, 542 pc of 7”, and 18 pc of 4•”) wereused and installed in the casing of 72 wells (gas and oilproducers wells) in one major off shore operating companyin the Arabian gulf since 1999.Nineteen (19) wells had been tested for zonal isolationafter primary cement operations, the results proved nocommunications between zones in 18 wells. Sixty three (63) wells are producing gas or oil withoutany indication of annulus pressure or zonal communicationsince year 2000.
Number of SCU
Engineer Said Zaki Elzeghaty, Weatherford Middle East Cairo-Egypt, (ISO/API &SPE Member)
This paper was presented in the 15th Middle East Oil & Gas Show and Conference (MEOS 2007) on March 14th in Manama, Kingdom of Bahrain