CEMENTING Casing strings are cemented to isolate zones and support the casing © 2001 PetroSkills LLC, All Rights Reserved Cementing Not the cement trucks used in to cement casing 2 © 2001 PetroSkills LLC, All Rights Reserved Cementing 3 © 2001 PetroSkills LLC, All Rights Reserved Cementing 4 © 2001 PetroSkills LLC, All Rights Reserved Cementing Oil well cementing equipment 5 © 2001 PetroSkills LLC, All Rights Reserved Cementing Cement is made of Limestone and clay or shale mixed in the right proportions Each run may be slightly different due to impurities 6 © 2001 PetroSkills LLC, All Rights Reserved Cementing Cement is heated in a rotary kiln to 2600 to 2800 degrees F 7 © 2001 PetroSkills LLC, All Rights Reserved Cementing What comes out of the kiln is called clinker 8 © 2001 PetroSkills LLC, All Rights Reserved Cementing The clinker is ground with a controlled amount of gypsum to make portland cement 9 © 2001 PetroSkills LLC, All Rights Reserved Cementing API classes of cement 10 © 2001 PetroSkills LLC, All Rights Reserved Cementing The classes used today are primarily Class G, H, C and A Class C and A are used on shallow casing strings Class A is used in some areas because it is very similar to construction cement and can be obtained locally 11 © 2001 PetroSkills LLC, All Rights Reserved Cementing Class C and A are already ground with an accelerator for shallow casing strings 12 © 2001 PetroSkills LLC, All Rights Reserved Cementing Class G and H are basic cements Class G is a finer grind and requires more mix water resulting in a lower density G is more common around the world 13 © 2001 PetroSkills LLC, All Rights Reserved Cementing Cement must be placed in wells ranging from shallow to very deep 14 © 2001 PetroSkills LLC, All Rights Reserved Cementing Additives are used to adjust cement properties and tailor the cement to specific needs 15 © 2001 PetroSkills LLC, All Rights Reserved Cementing Density The density of cement must be varied from high density to low density depending upon the application 16 © 2001 PetroSkills LLC, All Rights Reserved Cementing 17 © 2001 PetroSkills LLC, All Rights Reserved Cementing Lightweight additives or extenders are used to decrease the density of cement Excess mix water can be used to decrease the density to a limited extent Excess water increases thickening time, increases free water and reduces compressive strength 18 © 2001 PetroSkills LLC, All Rights Reserved Cementing Bentonite is the most common light weight additive Bentonite will tie up extra mix water reducing density Light weight, filler cements have as much as 12% bentonite 19 © 2001 PetroSkills LLC, All Rights Reserved Cementing Adding bentonite thickens the cement slurry and it must be thinned by adding a thinner or friction reducer 20 © 2001 PetroSkills LLC, All Rights Reserved Cementing 1.4 3 CLASS A LITEPOZ3 50:50 LITEWATE CLASS A W/ DIACEL 1.2 1 Cost ($/ft3) 2 0.8 1.5 0.6 1 0.4 0.5 0.2 0 0 Neat 15.6# 4% Gel 14.2# 2% Gel 14.8# 8% Gel 13.3# 6% Gel 13.7# 12% Gel 12.6# Neat 15.2# 10% Gel 12.9# Cost per cubic foot 21 4% Gel 13.6# 8K Lite 12.4# 2% Gel 14.2# Compressive Strength © 2001 PetroSkills LLC, All Rights Reserved 10% 13.2# 4% Gel 14.2# 30% 11.7# 20% 12.4# 40% 11.0# Compressive Strength (1000 psi) 2.5 Cementing Pozzolans are siliceous materials which will react with lime and water to form a compound having the ability to act as a cementing material Natural pozzolans are volcanic ash Artificial pozzolans include glass, furnace slag and a residue collected from the chimneys of coal burning power plants called “fly ash” 22 © 2001 PetroSkills LLC, All Rights Reserved Cementing Pozzolans are slightly cheaper and lower density Diatomaceous earth or Diacel D has been used to reduce slurry density but is more expensive than bentonite Perlite is volcanic glass bubbles that has some times been used in geothermal wells because of its insulating properties 23 © 2001 PetroSkills LLC, All Rights Reserved Cementing Perlite is considerably more expensive Gilsonite and kolite are used to reduce density; however, their primary function is as a lost circulation material Gilsonite is a black, lustrous asphalt Kolite is crushed coal 24 © 2001 PetroSkills LLC, All Rights Reserved Cementing Foamed cements are also used to reduce the density of the slurry In a foamed cement, nitrogen is added to the cement mixture Very low densities can be obtained with foamed cement but they are more expensive 25 © 2001 PetroSkills LLC, All Rights Reserved Cementing At times, the density needs to be higher than neat cement and additives are used to increase the density Table 7 in the book shows some of the more common additives for increasing the density of a cement slurry 26 © 2001 PetroSkills LLC, All Rights Reserved Cementing Hematite is one of the more common additive for high density cement due to its high specific gravity For smaller increases in density, barite can be used Barite is ground fine and requires more mix water to keep the slurry pumpable Sand can be added to increase the density due to low mix water requirements 27 © 2001 PetroSkills LLC, All Rights Reserved Cementing Densified slurries can be used up to 17.5 ppg A densified slurry is produced by reducing the mix water and adding a dispersant to make it thin enough to pump Salt can be used to increase the density of a slurry Salt increases the density of the liquid phase 28 © 2001 PetroSkills LLC, All Rights Reserved Cementing At low temperatures, it would take too long for the cement to set up so accelerators are added to the cement 29 © 2001 PetroSkills LLC, All Rights Reserved Cementing Accelerators Decrease the thickening time of cement for shallow, low temperature applications 30 © 2001 PetroSkills LLC, All Rights Reserved Cementing As a rule of thumb, accelerators are inorganic compounds 31 © 2001 PetroSkills LLC, All Rights Reserved Cementing Calcium chloride is the most common accelerator It is used in concentrations from 1 to 3% 32 © 2001 PetroSkills LLC, All Rights Reserved Cementing A little salt will accelerate 33 © 2001 PetroSkills LLC, All Rights Reserved Cementing A lot of salt will retard the cement 34 © 2001 PetroSkills LLC, All Rights Reserved Cementing 35 © 2001 PetroSkills LLC, All Rights Reserved Cementing Retarders Increase the thickening time of cement for deeper, hotter applications Typically retarders are organic compounds 36 © 2001 PetroSkills LLC, All Rights Reserved Cementing 37 © 2001 PetroSkills LLC, All Rights Reserved Cementing CMHEC is a common retarder One of the most common retarders is calcium lignosufonate Sodium Chloride is a retarder at high concentrations As bottomhole temperatures change, the type of retarder will change 38 © 2001 PetroSkills LLC, All Rights Reserved Cementing Friction loss additives are used to thin the cement slurry Organic acids Lignosulfonate Alky aryl sulfonate Phosphate Salt 39 © 2001 PetroSkills LLC, All Rights Reserved Cementing Lost circulation material Granular material such as gilsonite, kolite, perlite and walnut hulls 40 © 2001 PetroSkills LLC, All Rights Reserved Cementing Do not use wood fibers typically used in drilling fluids Organic compounds can retard the cement 41 © 2001 PetroSkills LLC, All Rights Reserved Cement Job Cement Head Common cementing equipment includes Rig Floor Ground Level Drilling Fluid Float shoe Float collar Centralizers Cementing head 42 Cement Centralizer Casing © 2001 PetroSkills LLC, All Rights Reserved Float Collar Float Shoe Figure 9 Typical cementing equipment. Cement Job Top Plug Top Plug Released Cement Displacement Fluid Bottom Plug Spacer Fluid Drilling Fluid Float Shoe A 43 Bottom Plug Ruptured Float Collar Shoe Joint Plug Bumped B © 2001 PetroSkills LLC, All Rights Reserved C Cement Job Cement must support the pipe and isolate zones Most cement mixtures will accomplish that objective The difficulty is in getting the cement all the way around the pipe 44 © 2001 PetroSkills LLC, All Rights Reserved Cement Job Farris related the tensile strength of the casing to how well it supported the pipe 9 5/8” Casing 1 Foot Cement 45 © 2001 PetroSkills LLC, All Rights Reserved 5 1/2” Csg Cement Job Unset cement will support the pipe One psi tensile = 8 to 10 psi compressive 46 Cement Age Force to Break 1’ Cmt Tensile Strength Length of Pipe 1’ Will Support 1.83 100 0 5.8 Soft Cmt 2.33 137 0 8.0 Soft Cmt 3.08 325 0 19.1 Initial Set 3.66 1,000 4 58.8 Cement Stiffening 4.42 4,550 8 267.7 Final Set 5.50 5,000 12 ---- Could Not Break Bond © 2001 PetroSkills LLC, All Rights Reserved Remarks Cement Job The industry assumes that if the cement has a compressive strength of 500 psi it will support the pipe The 500 psi includes a safety factor from two to five 47 © 2001 PetroSkills LLC, All Rights Reserved Cement Job Cement must also isolate the zone Hydraulic bond strengths were tested in the laboratory and the hydraulic bond varied with the roughness of the pipe Sandblasting the casing was popular for a while along with Ruff Coat (resin and sand coated pipe) 48 © 2001 PetroSkills LLC, All Rights Reserved Cement Job There was no correlation between hydraulic bonding and compressive strength The trick is to get the cement all the way around the pipe 49 © 2001 PetroSkills LLC, All Rights Reserved Cement Job There are a number of things we can do increase the odds of getting a good cement job Centralization of the casing Pipe movement - rotation and/or reciprocation Drilling fluid condition Hole conditions Displacement velocity Spacer fluids Mud - cement density differences Contact time 50 © 2001 PetroSkills LLC, All Rights Reserved Cement Job Centralization We have know for more than half a century that centralizing the casing will aid in getting a good cement job For a non-Newtonian fluid, the velocity on the narrow side of the annulus is slower than the velocity on the wide side of the annulus It is the “Popcorn Theory” 51 © 2001 PetroSkills LLC, All Rights Reserved Cement Job Eccentric annulus promotes channeling Centralizers are used to keep the pipe in the center of the hole The point between the centralizers is the hardest to centralize 52 © 2001 PetroSkills LLC, All Rights Reserved Cement Job The degree of centralization is commonly termed % standoff Rb Rc C D % S tan doff x100 Rb Rc 100% is perfectly centralized and 0% would be touching the wall of the hole 53 © 2001 PetroSkills LLC, All Rights Reserved Cement Job An API centralizer is designed to support one joint of casing at 90 degrees The sag between the centralizers is the concern 54 © 2001 PetroSkills LLC, All Rights Reserved Cement Job The sag between the centralizers increases as the length of the casing increases to the fourth power 4 Wb Sin I 2 L u Cosh u u 1296 u avg Dmax 4 u 2 E I Sinh u 1 36 T L2 2 u 55 E I © 2001 PetroSkills LLC, All Rights Reserved Cement Job As a rule of thumb, one centralizer per two joints in vertical wells and one centralizer per joint in directional wells High angle or horizontal wells may require two centralizers per joint for adequate centralization Smaller hole clearances will require more centralizers 56 © 2001 PetroSkills LLC, All Rights Reserved Cement Job Pipe movement It has been shown that pipe movement can increase the displacement efficiency Pipe movement is rotation and/or reciprocation Pipe rotation helps the most 57 © 2001 PetroSkills LLC, All Rights Reserved Cement Job Drag forces associated with rotation will pull the cement into the narrow side of the annulus Rotation requires specialized equipment Some liner hangers can be rotated 58 No Rotation Flowing Cement Gelled Mud © 2001 PetroSkills LLC, All Rights Reserved Rotation Cement Job Reciprocation does not require any extra equipment and is free Reciprocation has a tendency to break the gel strengths of the mud in the narrow side of the annulus The pipe should be reciprocated from the time the pipe reaches bottom until the cement plug bumps 59 © 2001 PetroSkills LLC, All Rights Reserved Cement Job Pipe sticking may be a problem Land the casing if it starts to stick Might not reciprocate when using a subsea wellhead or mud line suspension hanger because it can be very expensive if the pipe is stuck off bottom 60 © 2001 PetroSkills LLC, All Rights Reserved Cement Job Drilling fluid condition Higher viscosity mud is harder to displace Reduce the viscosity of the mud prior to cementing by adding water (or thinners in a weighted mud) Move the casing The absolute minimum volume to circulate is bottoms up or casing volume, whichever is greater 61 © 2001 PetroSkills LLC, All Rights Reserved Cement Job The thicker the mud, the longer you should circulate Hole condition A clean hole allows the ease of running casing and getting centralizers to bottom Washouts are hard to cement If washouts are a problem, change the mud to try and minimize the washouts 62 © 2001 PetroSkills LLC, All Rights Reserved Cement Job If you are getting adequate cement jobs, don’t spend more money on the mud Displacement velocity Is turbulent, laminar or plug flow better? 63 © 2001 PetroSkills LLC, All Rights Reserved Cement Job Plug flow is the lowest annular Turbulent Flow velocity (30 to 90 feet per minute Laminar Flow (10 to 30 m/min), depending upon cement Plug Flow properties) 64 © 2001 PetroSkills LLC, All Rights Reserved Cement Job Laminar flow should be avoided since it promotes channeling Some cement studies touted plug flow while some indicated that turbulent flow was better Figure 13 summaries the studies 65 © 2001 PetroSkills LLC, All Rights Reserved Cement Job 66 © 2001 PetroSkills LLC, All Rights Reserved Cement Job It is better to pump thin cement slurries at higher flow rates If ECD or hole size prevents higher flow rates, consider plug flow For plug flow, you may have to take the U-tube effect into account 67 © 2001 PetroSkills LLC, All Rights Reserved Cement Job Spacer fluids Cement and mud are not compatible and should be kept separate A spacer fluid is used to separate the cement and mud in the annulus The spacer fluid should be compatible with both the cement and the mud 68 © 2001 PetroSkills LLC, All Rights Reserved Cement Job For water based muds, water is a very good spacer Water is a thin fluid that will easily go into turbulent flow It is compatible with both cement and mud It is cost effective The spacer volume should equal at least 500 ft (150 m) in the annulus 69 © 2001 PetroSkills LLC, All Rights Reserved Cement Job In weighted muds, the spacer may have to be weighted with barite Can add products from service companies to enhance displacement Mud-Cement density difference There is little affect on displacement efficiency The cement should be at least 0.5 ppg (60 kg/m3) greater than mud density to prevent movement after the pump stops 70 © 2001 PetroSkills LLC, All Rights Reserved Cement Job Directional wells More difficult to get adequate centralization Will need to run more centralizers than a vertical well 71 © 2001 PetroSkills LLC, All Rights Reserved Cement Job There may be a cuttings bed on the bottom of the hole Try to remove the cuttings bed before cementing Cuttings Bed 72 © 2001 PetroSkills LLC, All Rights Reserved Cement Job Free Water Channel Free water content should be zero to minimize the possibility of a free water channel on the high side of the hole 73 © 2001 PetroSkills LLC, All Rights Reserved Cement Job Summary If cementing is a potential problem, try to minimize the size of the washouts during the drilling process especially across the intervals to be isolated. Unfortunately, that may not always be possible Centralize the casing. Take into account dogleg severity and hole inclination in centralizer placement 74 © 2001 PetroSkills LLC, All Rights Reserved Cement Job Circulate the hole and condition the mud prior to cementing. The mud should be thinned as much as possible while still maintaining adequate lifting capacity and density Reciprocate (or rotate in critical situations) the casing while circulating and cementing. Unless the casing starts to stick, the casing should be reciprocated until the plug bumps 75 © 2001 PetroSkills LLC, All Rights Reserved Cement Job Pump a thin spacer fluid in front of the cement. The spacer should be compatible with both the drilling fluid and the cement Pump a thin cement slurry at high displacement rates. Use plug flow techniques only when high rate displacement is not possible 76 © 2001 PetroSkills LLC, All Rights Reserved Cement Job Use the most economical slurry possessing satisfactory properties Do not hold pressure on the casing after the plug has bumped unless the float equipment does not hold. Keeping pressure on the casing while the cement sets can cause a micro annulus 77 © 2001 PetroSkills LLC, All Rights Reserved
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