NZ616315B2 - Use of methylhydroxyethyl cellulose as cement additive - Google Patents

Abstract

Disclosed is the use of methylhydroxyethyl cellulose as an additive in cement slurries in the treatment of gas wells to prevent or reduce the occurrence of gas channeling. In addition to acting as a gas control additive, methylhydroxyethyl cellulose may also reduce fluid loss, minimise free fluid and stabilise foam during cementing of wells. d stabilise foam during cementing of wells.

Description

APPLICATION FOR PATENT TITLE: USE OF HYDROXYETHYL CELLULOSE AS CEMENT VE SPECIFICATION Field of the ion This ion relates to the use of methylhydroxyethyl cellulose (MHEC) as an additive for cement itions. The MHEC may be used as a multipurpose additive for use to prevent and/or control gas channeling, control fluid loss, minimize free fluid, improve slurry stability and/or stabilize foam.

Background of the Invention During construction of a well penetrating a subterranean formation, a rotary drill is typically used to bore through the subterranean formation to form a wellbore.

Once the wellbore has been d, a pipe or casing is lowered into the re. A cementitious slurry and a displacing fluid, such as a drilling mud or water, is pumped down the inside of the pipe or casing and back up the outside of the pipe or casing through the annular space between the exterior of the pipe or casing and the wellbore.

The cementitious slurry is then allowed to set and harden.

A primary function of the cementing process is to restrict fluid movement between the subterranean formation and to bond and support the casing. In addition, the cement aids in protecting the casing from corrosion, preventing blowouts by quickly sealing formations, protecting the casing from shock loads in drilling deeper wells, g off lost circulation or thief zones and forming a plug in a well to be abandoned.

Cementing operations r provide zonal isolation of the subterranean formation and help prevent sloughing or erosion of the wellbore. In addition to their use in oil gas wells, cementitious slurries may be used to cement pipes or casings within geothermal wells, water wells, injection wells, disposal wells and storage wells.

In addition to selectively isolating particular areas of the wellbore from other areas of the wellbore, cementitious slurries may further be used for other purposes. For instance, cements may be used in remedial operations to repair casing and/or to e formation isolation as well as in g off perforations, repairing casing leak/s (including leaks from damaged areas of the casing), plugging back or sealing off the lower section of a wellbore, etc.

Cementitious es for use in such applications contain hydraulically active s which set and develop compressive strength due to a ion reaction.

Physical properties of the set cement relate to the x-ray amorphous structure of the calcium-silicate-hydrates formed during hydration. For example, conventional Portland s form an interlocking network of, for example, cium silicate, dicalcium silicate, alcium aluminum ferrite hydrates, interspersed with calcium sulfate and calcium hydroxide ls. These crystals interconnect to form an interlocking structure which provides both flexural strength and a degree of resiliency.

Gas channeling in a cement composition is a common problem in the oil and gas industry. When a cement slurry is first placed in the annulus of an oil or gas well, it is the hydraulic fluid that exerts hydrostatic pressure on the sides of the well. Initially the hydrostatic pressure of the cement composition is great enough to keep gases that are naturally occurring within the reservoir in situ. But as the slurry of cement composition sets, it goes h a transition stage changing from liquid to solid. During this transition stage, the cement composition exerts less and less hydrostatic pressure on the well. It is in this transition stage that the cement composition is susceptible to formation gas entering into the cement sheath. The gas entering into the cement sheath produces ys filled with gas. As the cement hardens, the pathways become channels in the hardened cement composition. ling in a cement composition weakens the structure.

Another common problem in well cementing is the loss of liquid fluid from the cementitious slurry into porous low pressure zones in the formation surrounding the well annulus. Fluid (liquid and/or gas) loss is undesirable since it can result in dehydration of the cementitious slurry. In addition, it may cause the formation of thick filter cakes of cement solids. Such filter cakes may plug the wellbore. In addition, fluid loss can damage sensitive formations. l fluid loss is desired therefore in order to provide better zonal isolation and to minimize ion damage by fluid invasion.

Controlling gas in light weight cements, especially at low temperatures, has also been an industry problem for a number of years e the additive systems that are generally used or employed are better suited for heavier or higher density cements.

Common additives used to control fluid loss and gas migration from the slurry to the porous permeable formation include hydroxyethyl cellulose (HEC), carboxymethylhydroxyethyl cellulose (CMHEC), acrylamidomethylpropane sulfonic acid (AMPS), polyethyleneimines, e butadiene rubber latexes and polyvinyl alcohol.

Further, microparticulate additives, such as silica fume, may be used in combination with such additives to make the cement composition less ble. Such materials work best, however, in cement compositions that have a high cement density and a low water to cement ratio. The lower the cement density and the higher water to cement ratio, the greater the quantity of water soluble or film—lbiming ves that are required to reduce gas migration to an able level and keep channeling to a minimum. The lower the cement y, therefore, the greater the quantity of traditional additives that are required. This quantity increases to a point that is cost prohibitive for lower density cement compositions.

Alternative additives for controlling fluid loss and gas migration have therefore been .

[OOOlOA] nce to any prior art in the specification is not, and should not be taken as, an acknowledgment or any form of tion that this prior art forms part of the common general knowledge in New Zealand or any other j urisdiction. [00010B] As used herein, except where the context requires otherwise, the term "comprise" and variations of the term, such as "comprising , comprises" and "comprised", are not intended to exclude other additives, components, integers or steps.

Summarv 0f the Invention Methylethylhydroxy cellulose (MHEC) may be used in the cementing of wells, including oil, gas, water, injection, disposal, storage and geothermal wells. The use of MHEC in cement slurries prevents and/or reduces the occurrence of gas channeling. In on, MHEC controls fluid loss, minimizes free fluid, improves slurry stability, and stabilizes foam.

In an embodiment, MHEC is used in cement slurries as a gas l agent.

In one embodiment, MHEC is used in cement slurries as a free fluid control agent or extender for cementitious slurries of low density.

In another embodiment, MHEC is used in cement slurries as a foam stabilizer.

In another embodiment, MHEC is used in cement slurries to retard the effect on thickening times in the slurry.

In another embodiment, MHEC is used in cement slurries to impart multiple effects and thus serves as a multipurpose additive. As such, MHEC may replace several additives conventionally t in cement slurries. Optimum placement of a cementitious slurry into the wellbore may therefore be effectuated by use of a multipurpose additive such as MHEC in the cement slurry.

The MHEC may be added to a cement slurry in dry form, in dry form suspended in oil-based carrier fluids, or in dry form mixed in a water based medium.

The use of MHEC in cement slurries provides an economical design of cement systems reduces potential incompatibilities between combinations of additives in the cement slurry and fies operations in the field.

Brief ption of the Drawings In order to more fully understand the drawings ed to in the detailed description of the present invention, a brief description of each drawing is presented, in which: is a graph showing the results of gas model g for a cementitious slurry containing methylhydroxyethyl cellulose (MHEC); and is a graph showing the results of gas model testing for a cementitious slurry containing hydroxyethyl cellulose (HEC).

Detailed Description of the Preferred Embodiments MHEC may be used in a cement mix (1) to t the ence of gas- channeling and/or gas migration during cementing of a well; (2) as a density-reducing extender; (3) to l fluid loss, (4) to minimize or limit free fluid and improve slurry ity and/or (5) to stabilize foam within the cement slurry. For instance, MHEC has been found to l gas channeling, minimize free fluid, improve slurry stability and stabilize foam at temperatures up to about 180° F.

The itious slurries may have a density less than or equal to 17.0 pounds per gallon (ppg) and typically less than or equal to 15.0. In another embodiment, the cement slurry may an ultra low-density slurry, typically ranging from about 13.0 ppg to about 6.0 ppg or less.

Gas channeling occurs when the hydrostatic pressure exerted by the cement column decays to a pressure below the pore re of an oil or gas g reservoir.

The pressure decay within the cement column is due to cement volume losses due to hydration and from fluid loss to permeable strata. These effects cause the cement to be self-supporting and therefore unable to it full fluid hydrostatic pressure. Gas migration can sly compromise the hydraulic integrity of the cement sheath and can cause safety ms at the surface due to lack of zonal isolation. Gas ling is reduced and/or minimized by use of MHEC in the cement mix.

As gas migration within a wellbore may occur in a variety of downhole re circumstances, it has been found that the density of the cement slurry may be reduced using MHEC. Reduction in cement slurry density is often required in order to place the slurry into the well without exceeding the formation fracture pressure. MHEC in the cement mix thus may function as a cement extender and aid in the lowering of the y of the slurry, thereby allowing for a lower density slurry to be used. MHEC allows for the addition of water without compromising the stability and free fluid control of the system.

MHEC further controls fluid loss which is important in controlling viscosity and thickening time of a cement composition. Fluid can be lost from cement compositions when the cement filtrate, the fluid phase of the cement composition, seeps into the permeable walls of a formation. When cement filtrate moves into the ble walls, a layer of solids deposit on the wall. Under differential pressure, cement slurries lose filtrate to permeable formations. The loss of filtrate from the slurry can impact the ability to place the cement due to dehydration and subsequent viscosification of the slurry. Loss of fluid, either internally to ion, or externally due to fluid loss, reduces the y of the cement to transmit full hydrostatic pressure to a point where the cement becomes self-supporting and unable to control formation pore pressure. When this occurs, the cement is susceptible to the movement of gas or other fluids into the annulus between the casing and formation.

MHEC in the slurry zes free fluid, which improves slurry stability.

Free fluid is water that separates from the cement ition after it is placed in the wellbore. Free fluid tends to migrate up within the cement column because the free fluid is less dense than the particles in the cement composition. Minimizing the free fluid in the cement composition makes the density of the top of the column of cement composition equal to that or close to that on the bottom so the column is homogeneous, whereas free fluid causes the column of cement composition to be light on top and very heavy on the bottom.

Supernatant water breakout is free fluid that has broken out of a cement slurry.

Controlling supernatant water breakout of a slurry with MHEC thus limits possible pathways for formation fluids to penetrate through the cement sheath and cause channeling, especially under ed conditions.

The presence of the MHEC in the slurry may r function as a stabilizer when a foaming agent and gas, such as nitrogen, are introduced to the cement. Such gases are sometimes added to a cement composition containing a surfactant or foaming agent to create a foam which reduces further the density of the system. The foam that is created is basically a series of bubbles in close proximity with one another. These materials tend to be unstable and coalesce into much larger s and ultimately break out. MHEC stabilizes the foam, keeping the foam bubbles at about the same diameter and making a much more homogeneous foam.

Cementitious materials, suitable for use in the cementitious slurry, include als with lic properties, such as hydraulic cement, slag and blends of hydraulic cement and slag (slagment), which are well known in the art. The term "hydraulic cement" refers to any inorganic cement that hardens or sets due to hydration.

As used herein, the term "hydraulically-active" refers to properties of a cementitious material that allow the material to set in a manner like hydraulic cement, either with or without additional activation. Hydraulically-active cementitious materials may also have minor amounts of extenders such as bentonite, gilsonite, and cementitious materials used either without any iable sand or aggregate material or admixed with a granular g material such as sand, ground limestone, the like. th enhancers such as silica powder or silica flour can be ed as well. Hydraulic cements, for instance, e Portland cements, aluminous cements, pozzolan cements, fly ash cements, and the like. Thus, for e, any of the oilwell type cements of the class "A-H" as listed in the API Spec 10, (1st ed., 1982), are suitable hydraulic cements. In addition, the cementitious material may include silica sand/flour and/or ng agents including hematite or barite.

Mixing water is utilized with the dry cement composition to produce a fluid pumpable slurry of suitable consistency. API Spec 10, Second n, June 1984 which is known in the cement industry, describes an approved apparatus and method for measuring the consistency of cement slurries in terms of Bearden units of consistency (Be). A pumpable slurry should measure in the range from about 2-20 Be and preferably be in the range from about 5 to 11 Be. Slurries thinner than about 5 Be will tend to have greater particle settling and free fluid generation. Slurries thicker than about 20 Be become increasingly difficult to mix and pump.

Depending upon the particular slurry and intended conditions of use, mixing water is utilized in the slurry of the present ion in the range from about 30 to 150 weight percent based upon the dry weight of cement and preferably is in the range of about 35 to 90 weight percent.

The cementitious slurry of the invention may further n conventional additives used in the cementing of a gas or oil wellbore such as suspending or thixotropic agents, th retrogression additives, bility reducers, weighting materials, and anti-settling agents, etc.

The combination of the slurry and MHEC produces a slurry exhibiting low fluid loss, minimal free fluid, excellent solids support, and unexpected gas migration control. Low fluid loss is accomplished by establishing a low bility filter cake in the presence of differential pressure against a permeable medium. Low fluid loss for a light weight cement is less than 500 cc per 30 minutes using the API fluid loss test. Low fluid loss for a cement having greater than about 14 pounds per gallon density is less than about 50 cc per 30 minutes using the API fluid loss test.

The determination of the amount of MHEC to add to a cement slurry to create the gas-tight design described herein can be based on the parameters of a particular well such as temperature and pressure. In an embodiment, from about 0.05 to about 1.50 percent by weight of cement (BWOC) of MHEC is used in the .

A preferred amount of MHEC may be determined for the particular temperature and pressure parameters of a particular well by running a series of tests described and incorporated by reference herein. First, to produce gas-tight designs using the multi-functional additive of this ion, it is necessary to add sufficient additive to lower the API Fluid Loss to level below approximately 500 cm /30min rate for a low y cement composition or approximately 50 cm /30 min rate for a high density cement composition. A test for determining the API Fluid Loss may be found in API Recommended Practice 10B, Twenty-Second Edition, December 1997, and is incorporated herein by nce.

Sufficient MHEC is also desirably added to minimize the free fluid content of the gas-tight design to below approximately 0.2 mL. A test for determining the free fluid content of a cement may be found in API Recommended Practice 10B, Twenty -Second Edition, December 1997 and is incorporated herein by reference. It is necessary to add a sufficient amount of additive such that the gas-tight design exhibits minimal sedimentation. A test for determining sedimentation may be found in API Recommended Practice 10B, Twenty-Second n, December 1997, and is incorporated herein by reference. For gas control designs, the m desirable density differential between the top sample and bottom sample, as described in API Recommended Practice 10B, -Second Edition, er 1997, should be no greater than 0.2 Lb/gal. Once it is determined that a ular amount of additive will result in: an API Fluid Loss level below approximately 500 cc/30min rate; free fluid content of the gas-tight design to below approximately 0.2 mL, and a minimal sedimentation, gas flow model testing as described herein may be performed to determine the fluid loss, transition time, and permeability to resist gas intrusions.

The slurry may further n a set retarder in order to delay the set time of the cement composition. Such set retarders are particularly useful when the cement composition is exposed to high subterranean temperatures. In addition to being capable of delaying the set time of the cement composition, the set retarder also functions to extend the time the cement composition remains pumpable after the cement composition is mixed and placed into the well. When present, the set retarder may be present in an amount between from about 0.1 to about 5 percent BWOC. Suitable set retarders e glucoheptonates, such as sodium glucoheptonate, calcium glucoheptonate and magnesium glucoheptonate; lignin sulfonates, such as sodium lignosulfonate and calcium sodium ulfonate; gluconic acids gluconates, such as sodium gluconate, calcium gluconate and calcium sodium ate; phosphonates, such as the sodium salt of EDTA phosphonic acid; sugars, such as e; hydroxycarboxylic acids, such as citric acid; and the like, as well as their blends.

MHEC (and optional cementing additives) may be added to cement compositions by any methods know to one of ordinary skill in the art. One preferred method to add the additive of this invention to cement is through liquid additive systems.

Water based and oil based ves can be added to cement compositions by injecting or placing the additive into displacement tanks on a cementing unit. The additive may be d to fall into the mix water, se, and then be used to mix with cement.

Another preferred method is to add dry form additives of this invention by dry ng them with the cement at a cement bulk facility. The blend of dry cement and additive can then used to form a slurry.

MHEC differs primarily from other commonly used additives because it controls annular gas. It may be used however strictly as a fluid loss additive or as an extender. For instance, MHEC may be used as a density reducing extender since it permits the use of a large amount of water thereby lowering the density of the cement composition. Further, MHEC may be used strictly to control fluid loss of the cement composition which is the integral part of any gas migration control phenomenon or process within the cement slurry composition. Since MHEC minimizes free fluid and improves slurry stability (important parameters in combating gas migration), it is particularly useful as a urpose cement. Its numerous functions all contribute to the ability to control gas migration. Adding ers, fluid loss additives, and free fluid control agents separately is not required when using MHEC in the cement mix.

The following examples are rative of some of the embodiments of the present invention. Other embodiments within the scope of the claims herein will be nt to one skilled in the art from consideration of the description set forth herein. It is intended that the specification, together with the examples, be considered exemplary only, with the scope and spirit of the invention being ted by the claims which follow.

All percentages set forth in the Examples are given in terms of weight units except as may otherwise be indicated.

EXAMPLES Examples 1-13. Cementitious slurries of desired density were prepared by mixing neat Joppa Class H Portland cement, optionally mixed with fly ash, with fresh tap water at room temperature. To the slurry was added methylhydroxyethyl cellulose (MHEC) and, optionally, sodium lignosulfonate ("SLS") as set retarder. (Comparative slurries were also prepared using, in place of MHEC, HEC from one of three suppliers.) The resultant slurry was kept agitated by onal stirring. The free fluid content of the slurries was determined in accordance with procedures in API ended Practice 10B-2, First n, July 2005. The amount of fluid loss was determined at a designated temperature in accordance with procedures in API Recommended Practice 10B, Twenty- Second Edition, December 1997, incorporated herein by reference. Standard API viscosity reading readings were taken on a Fann 35 viscometer, at 120° F, 160° F or 170° C. The s are tabulated in Table I : Table ITable 1 Example Density Cement Additive SLS ature ' Rheologies N0. . [ppg] [BWOC] [BWOC] [OF] 300/2(é0/0100/6/3 (Cozmp. Joppa H HEC— 1, EX.) 1.0 120 4 12.6 JoppaH HEC—1, 120 ~100 268 20/13/9/1/1 @ 120 (Comp. 1.0 EX.) 12.6 Joppa H HEC—2, 120 132 216/184/141/30/21 (Comp. 1.0 @ 120 EX.) 6 12.6 Joppa H HEC—3, 120 74 187/162/125/25/17 (Comp. 1.0 @ 120 EX.) 7 12.6 50/50 MHEC, 160 slight 22 133/105/72/25/22 FA/H 1.0 trace @ 160 (V/V) 8 12.6 50/50 HEC—1, 160 4 503 47/37/25/9/8 @ (Comp. FA/H 1.0 160 EX.) (V/V) 9 12.6 50/50 HEC—2, 160 150 253/205/144/31/24 (Comp. FA/H 1.0 @ 160 EX.) (V/V) 1" ....l (Comp. F/H (V/V) 1.0 @ 160 EX.) 11 14.5 50/50 MHEC, 0.3 170 192/148/105/61/50 FA/H 0.4 @ 170 (V/V) 12 14.5 50/50 HEC—1, 0.3 170 ~50 418 30/22/15/1/1@ (Comp. FA/H 0 .4 170 EX.) (V/V) 13 14.5 50/50 HEC—3, 0.3 170 trace 195 254/197/128/20/13 (Comp. FA/H 0 .4 @ 170 EX.) (V/V) Table I rates that improved results are ed when MHEC is used in cement es. In particular, Table I establishes lower fluid loss with a MHEC containing cement slurry compared to the HEC containing cement slurry with minimal free fluid.

Examples 14-16. Cementitious slurries of 15.6 ppg density were prepared by mixing at room temperature 50/50 (v/v) fly ash/Joppa Cement H cement mix with fresh tap water. To the slurry was added 0.2% BWOC polynaphthalene sulfonate dispersant admixture (commercially available as CD-32 from Baker Hughes Incorporated) and 0.2 gallons per sac (gps) of an ammonium salt of ethoxylated alcohol sulfate foaming agent, commercially available as FAW-20 from Baker Hughes Incorporated. r, to one slurry was added 0.2% BWOC MHEC and to another slurry was added 0.2% BWOC HEC. The stability of the foam was determined starting at a 15.6 ppg density and foamed down to 11.6 ppg. The results are shown in Table II. Further, the density of the cured cement was determined in a BP settling tube at 120°F and the results shown in Table III, below. The tests were conducted in accordance with the protocol set forth in API ended Practice 10B-2 and ISO 10426-2.

Table II Table III Examples 16-17. Cementitious slurries having a density of 15.0 ppg were prepared by mixing neat Joppa Class H Portland cement and fly ash (50:50 v/v) with fresh tap water at room temperature. To one slurry was added methylhydroxyethyl cellulose (MHEC) and to the second slurry was added yethyl cellulose (HEC).

Sodium lignosulfonate ("SLS") as set er was further added to one of the slurries in ance with Table IV below: Table IV The gas volume and pore pressure of the cementitious slurries were determined over an extended period of time. These were ined by a Gasflow Model which was used to simulate the well configuration where the cemented annulus is between pressurized gas sand and a low pressure permeable zone. (In a typical well uration where the cementitious slurry in the annulus between the casing and the formation, the cement is exposed to a highly permeable gas zone and a lower pressure permeable zone.

Hydrostatic pressure on the unset cement keeps gas intrusion from occurring. During the cement hydration, the hydrostatic pressure is relieved and the cement pore pressure may decrease below the gas reservoir pressure and allow gas to intrude the cement column.

The gas may penetrate to the well surface or to another lower pressurized permeable zone.) A 3 inch e diameter by 10 inches long ess steel cylinder contained the itious slurry. A (325/60) mesh stainless steel screen or a core was fitted at the bottom of the test cell. A back pressure regulator connected to the bottom assembly represented the lower pressure permeable zone. The top of the er consisted of a head arrangement that allows for introduction of the pressure on top of the piston simulating hydrostatic pressure. Also, a traveling piston fitted with a 325 mesh screen or a core represented the high pressure formation. The Gas-flow Model is commercially available from Baker Hughes orated as Part Number 51030-2. The pore re and gas weight of the slurry temperature of 150° F were determined over a period of time and are demonstrated by the graphs shown in (the slurry of Example 16) and (the slurry of Comparative Example 17) wherein the gas volume is the amount of gas that enters the cell displacing the filtrate. For a successful test to take place, this volume must be less than the filtrate volume. The cement pore pressure is the pressure recorded by the transducer located on the side of the test cell. As the cement sets, it loses the ability to transmit measurable hydrostatic pressure to the transducer. As the cement sets the cement pore pressure falls. If gas communication through the cement column occurs, the cement pore pressure will rise after an initial decline. A continuously ing pore pressure indicates zero gas flow through the cement column. The s therefore demonstrate control of gas migration of the MHEC versus the HEC containing slurry.

From the foregoing, it will be observed that numerous variations and modifications may be ed without departing from the true spirit and scope of the novel concepts of the invention. It is intended that the specification, together with the es, be considered exemplary only, with the scope and spirit of the invention being indicated by the claims which follow.

Claims (5)

1.l. A method of cementing a pipe or casing in a gas well which comprises: (a) introducing into the gas well a cementitious slurry comprising methylhydroxyethyl cellulose (MHEC), wherein the MHEC is present in the itious slurry in an amount sufficient to reduce or prevent the occurrence of gas channeling in the wellbore during the cementing of the pipe or casing; and (b) allowing the slurry to harden to a solid mass.

2. The method ot‘claim 1, wherein the y ol’the cementitious slurry is less than or equal to about 17 ppg.

3. The method of claim 2, wherein the density of the itious slurry is less than or equal to about 13.0 ppg.

4. The method of any one of claims 1—3, wherein the amount 01" MHEC in the cementitious slurry is between from about 0.05 to about 1.50 percent by weight olicement.

5. The method of any one ol‘claims 1—4, wherein the amount of lluid loss 01‘ the cementitious slurry is less than about 500 cm