Method of Cementing Well Casing Using a High Temperature Cement System

This invention relates to cement compositions and more particularly to cement compositions having resistance to elevated temperatures.

In the drilling of wells, for example oil wells, wells penetrating sources of geothermal energy and the like, it is standard practice to utilize a cement to hold the well casing in position and to selectively block or plug portions of the strata through which the well extends so as to prevent the escape of undesirable fluid into the well bore or the loss of drilling muds and the like. When so used, the cement, as an aqueous slurry, is pumped into the annular space between the bore of the well and the casing and permitted to cure so as to form a hardened mass which provides the reenforcing and plugging functions.

The cements utilized in drilling operations are formulated so as to be sufficiently slow setting to permit pumping and yet be sufficiently resistant to the elevated temperature and pressure conditions encountered in the wells. The American Petroleum Institute has promulgated specifications for testing cements to insure that they meet certain minimum requirements with respect to strength, permeability, setting time and the like. These cements are referred to as oil well cements.

To resist the temperatures and pressures normally encountered in wells, the oil well cements have been developed to provide the maximum physical properties under the severe temperature and pressure conditions encountered in relatively deep wells, such as oil wells. These cements are conventionally Portland-type cements to which have been added one or more various additives such as for example mica, blast furnace slag, alumina and various special reactive sands, which are designed to improve the mechanical strength and the thermal and chemical resistance of the set and hardened cement. Although satisfactory for conventional well operations, these cements have proven deficient particularly in the case of ultra-deep wells and geothermal wells where temperature in excess of 600.degree. F. (200.degree. C.) may be encountered. Under such conditions conventional cements quickly increase in porosity and lose compressive strength which may lead to a blowout. Such well blowouts are highly undesirable and can prove to be extremely dangerous, as well as costly to repair.

Accordingly, oil well cements, particularly those used in geothermal wells and in ultra deep wells, that is wells sunk to depths on the order of 25,000 ft. or more, must have the ability to effectively maintain an adequate compressive strength and density and low porosity, even under high temperatures and pressures and in the presence of steam and hot brine. In addition, since the useful life of a typical well is measured in terms of 10 to 30 years, a good oil well cement must operate under the aforementioned severe conditions for a substantial period of time, preferably as long as the life of the well. However, recent studies have indicated that oil well cements presently in use have effective lives of on the order of 5 to 10 years when utilized in geothermal wells, it was noted that the strength and permeability of the oil well cement were seriously deteriorated in a period of as short as 4 years thus raising the possibility of a well blowout and increasing the frequency of replacement and maintenance.

The present invention resides in an improved high temperature cement system, which, when set and hardened, maintains an effective compressive strength, density and low permeability even under the severe conditions of high temperature and steam or high temperature brines of the type encountered in geothermal wells. The cement composition is mixed with water, pumped into the well, allowed to set and harden and is otherwise handled in the usual manner for oil well cements. Thus no special handling or mixing of the cement or special training of personnel is required in order to achieve the benefits of the present invention.

More specifically the high temperature cement system of the present invention comprises a hardenable cement composition and an effective amount of a carbon-containing additive having a low volatile material content, preferably not more than 15 wt.%. The cement composition when set and hardened exhibits resistance to high temperature deterioration of its compressive strength and permeability.

The cement system of the present invention is utilized by forming a slurry of the cement system, pumping or otherwise introducing the slurry into a form or restricted area such as the annular space between a well casing and wall of the well bore, and permitting the slurry to cure into a solid mass. The cement mass formed from the cement system of the present invention has an improved resistance to the retrogression of its physical characteristics at high temperatures particularly in the presence of geothermal steam.

The carbon-containing additive may be incorporated in the cement during manufacture and bagging and thus the cement system may be delivered to the wellhead ready for use, thereby reducing handling and mixing problems that may be attendant with the use of separate additives. In addition to the foregoing advantages the cement composition of the present invention may be otherwise handled like a conventional oil well cement. Conventional cement additives normally mixed with or used with oil well cements may be incorporated in the cement system. These and other advantages and features of the present invention will become apparent from the following detailed description and as defined by the claims appended thereto.

It has been discovered that the density, compressive strength and permeability of cured cement compositions are less subject to retrogression over a period of time when the cement system, from which the cured cement mass is formed, includes carbon or a carboniferous material as an additive. Of particular interest is the ability of a cement mass formed from the cured cement system of the present invention to resist degradation of its structural properties when subjected to high temperatures, for example temperatures in excess of about 400.degree. F. (about 200.degree. C.) over a period of time. The cement system of the present invention is thus particularly useful as an oil well cement, especially for use in ultra-deep wells and geothermal wells where the finished hardened cement mass is to be subjected to substantial pressure, high temperatures and often steam and hot brine.

As used herein the term "cement system" is used to designate the cement and cement additives in dry form. The term "slurry" is used to denote the cement system when suspended in a liquid vehicle such as water, oil or a mixture of both. "Cement mass" designates the cement system in the set and hardened condition.

In accordance with the present invention the cement system comprises cement with which is admixed a carbon-containing material as a cement additive to improve the ultimate high temperature properties of the set and hardened cement mass. The cement additive is selected from carbon-containing materials which have a low volatile material content. Thus, high carbon content materials such as anthracite coal, calcined and uncalcined coke, and the like, are used as cement additives in the present invention. In these materials the carbon content typically ranges from about 85% to about 95%. Low carbon content materials, however, are also useful as a cement additive. Chief example of such a low carbon content material is burnt oil shale where the carbon content can be as low as 3 - 4%.

The carbon-containing cement additives used in the present invention are typified by having a low content of volatile material. Volatile material is used to designate lower boiling gases and liquids given off by the carbon-containing additive upon heating. For the purposes of this description, volatile matter is determined in accordance with ASTM method D 3175 and is the weight loss (minus the moisture) in percent of the carbon-containing material after heating to 950.degree. C. .+-. 20.degree. and being held at that temperature for several minutes. Although the effect of volatile matter in the additive on the properties of the cement mass is not fully understood, it is believed that the volatile matter can be vaporized at the temperatures to which the cement may be subjected in the well. The vaporized material apparently disrupts the cement structure and forms undesirable voids and pores in the cement mass which can lead to the eventual failure of the cement and thus offset any beneficial effects of the carbon-containing additive. Good results are achieved when the volatile material is not greater than about 15% by weight of the additive.

The amount of carbon-containing additive utilized in the cement system of the present invention will depend primarily on the carbon content of the additive. The proportion of carbon additive in the cement system must be effective to impart to the finished and hardened cement mass, resistance to retrogression of the ultimate compressive strength and permeability due to the effects of high temperature, pressure and steam, but less than an amount which will result in a reduction of the ultimate physical properties of the cement mass or which otherwise interferes with the curing of the cement system. Good results are achieved when sufficient additive is admixed with the cement to provide in the cement system a minimum carbon content of about 0.02% based on the weight of the cement. The maximum amount of additive utilized in the cement system is not critical and will depend upon such factors as the nature and carbon content of the additive, the type of cement utilized, well depth, the desired setting and curing rate and the like, provided, however, that the additive is not present in sufficient quantities to adversely effect the ultimate properties of the concrete mass. Normally, less than about 50 wt.% of additive is used in the cement system and excellent results are achieved when the additive comprises between about 5 wt.% and about 20 wt.% of the cement system.

As previously mentioned the carbon content of the additive can vary considerably, ranging from a low carbon oil shale to mature coals and coke. The non-fixed carbon portion of the additive must not interfere with the setting, curing and ultimate properties of the cement mass. Consequently the volatile matter must be low, preferably less than about 15 wt.% of the additive and the non-volatile material is preferably inert with respect to the cement. In many cases the non-carbon, non-volatile components of the additive are siliceous or clayey type materials which may be beneficial in the cement mass.

Among those additives highly preferred for use in the present invention both because of the desired effect on the set and hardened cement and also because of their relatively low cost are anthracite, calcined coke, green coke and burned oil shale. These additives, when added to cement in amounts equivalent to at least about 0.02 wt.% of carbon based on the weight of cement, are effective in improving the resistance of the set and hardened cement composition to the effects of high temperature. These additives, in finely divided form, are preferably admixed with the dry cement prior to forming the cement slurry although the additive may be introduced at the point of use by dry mixing with the cement or wet mixing in the cement slurry. The particle size of the additive is not deemed critical although good results have been achieved when the additive has a particle size ranging between about 12 mesh and about 40 mesh in the U.S.S. Sieve series.

The coke and anthracite additives typically comprise between about 87 wt.% and about 95 wt.% of fixed carbon and typically less than about 8 wt.% of volatile material. Burned oil shale, on the other hand, contains a much lower portion of fixed carbon (on the order of 3-4 wt.%) while the major portion is a combination of calcite, dolomite and siliceous materials. Burned oil shale is the shale recovered after thermal treatment of oil shale to extract the petroleum content thereof.

The choice of cement used in the cement system is dependent upon the desired ultimate physical properties of the set, hardened cement and the environment in which the cement is to be utilized. Normally, however, the cement will be a Portland-type oil well cement of one of the classes and types specified by The American Petroleum Institute (API) specification 10A. Depending on the well depth and environment in which the cement is to be used, it will be selected from one of the API classes A through J and will be formulated as an ordinary, moderate sulfate resistant, or high sulfate resistant type cement. The basis for selection of a particular class and type of cement for use in the cement system is well understood by those skilled in the art and is dependent, among other things, upon the depth at which the cement is intended to be used, the desired physical requirements and the chemical environment to which the cement mass is to be subjected. In addition, curing time is an important factor in selecting a particular class and type of cement. Typically, the cement utilized in the present invention will be class J which is formulated for oil well use at depths of from about 12,000 to 16,000 ft. under conditions of high temperature and pressure.

Besides the oil well cements discussed above, other types of cements, including conventional Portland construction cement, can also be utilized in the cement system of the present invention. For example, high alumina and pozzolan type cements, which are also noted for high temperature resistance and high durability under severe environments, for example contact by sea water, are useful. In addition, cements comprising mixtures of Portland cement and pozzolans or fly ash are also excellent cements for use in geothermal wells and the like.

The cement system of the present invention may also include any of the conventional additives utilized to alter the setting, curing or physical properties of the cement. These additives are used in the proportions ordinarily specified for conventional cement systems. For example, various retarders may be added to the cement composition to retard the setting of the cement thereby to render the cement slurry pumpable for a longer period of time than would be the case if no additives were used. Retarders are well-known in the art and do not per se form a part of the present invention, although for example, typical retarders include free lime, calcium sulfate (as gypsum or plaster of Paris), starches, cellulose products, sugars and the like.

Other additives include the so-called dispersing agents which aid in the workability of the cement slurry. Among such additives are surface active materials such as alkali salts of sulfonated fatty alcohols, salts of sulfonated hydrocarbons and fatty acids, carbohydrate salts and non-ionic surfactants such as the condensates of ethylene oxide with substituted phenols. These additives aid workability by permitting a reduction in the water-cement ratio, effect greater dispersion of the cement or effect air entrainment, all of which aid in the workability of the cement slurry.

Other additives include the water loss preventatives such as for example methyl hydroxy cellulose and ethyl hydroxy cellulose, which serve to prevent separation of the slurry water during pumping and setting of the cement.

In the utilization of the cement system of the present invention, the cement system, including cement and carbon additive, is admixed with a suitable liquid vehicle to form a pumpable slurry. Water is most commonly used and in the following examples the slurry is a water slurry. As mentioned, however, oil and oil/water mixtures can also be used.

In preparing cement slurries utilizing the cement system of the present invention the percent of water by weight of cement will typically range from between about 38 to about 46% to form a slurry of pumpable consistency. The slurry thus formed is pumped in conventional fashion into the annular space between the casing and the well bore to support and reenforce the casing and to prevent fluid migration between the permeable zones. It should be clear, however, that greater or lesser amounts of liquid may be used depending upon the desired consistency and density of the slurry as determined by the method of transporting and placing the slurry in position.

The following examples illustrate some preferred cement systems utilizing a carbon additive in accordance with the present invention. The resistance to thermal degradation achieved using the carbon-containing additive is also demonstrated. The examples, however, are not intended as limiting the scope of the invention as defined by the appended claims.

Test cement systems using API class J cement manufactured by the Universal Atlas Cement Company and distributed under the trademark "Unadeep" are prepared by dry mixing the cement and various carbon-containing materials as high temperature additives. The additives include anthracite, coke calcined at 2500.degree. F. and uncalcined coke. The cement additives are ground to a particle size of about 12 mesh to about 20 mesh (USS Sieve Series). Slurries are formed from the cement systems by the addition of water in an amount equal to 47% by weight of the cement. In addition a neat cement slurry is formed using only the class J cement and 47 wt.% water. The ingredients and proportions for each of the cement systems are summarized below:

Each of the slurries is poured into a mold, the cavity of which defines a cube having two inch faces. The mold is filled to overflowing and leveled off with a straight edge and the mold opening is sealed with a metal plate. The slurry containing mold is then placed in an autoclave and maintained at a temperature between 200.degree. and 300.degree. F. at 3000 psi for 24 hours in order to cure the slurry into a hardened cube. At the end of 24 hours the plate is removed from the molds and the cement cubes removed.

Each cube is tested for heat and steam resistance in an autoclave maintained at 464.degree. F. through which steam is continuously circulating. The steam is derived from water obtained from a geothermal well in California. The tests are conducted for 3 months, at the end of which period the cube is removed and the permeability, density and compressive strength measured. The compressive strength tests are conducted in accordance with API specification RP 10B, section 6, on a 1 inch .times. 1 inch core removed from the cube. The permeability tests are conducted utilizing a gas permeameter which consists of a pressure plate and o-ring which are sealed against a face of the cube and which is connected to a source of gas under pressure. The gas is led into the pressure chamber formed between the pressure plate and the face of the cube and the rate of pressure drop is measured. The pressure drop rate is then converted to permeability in millidarcies.

The results of the compressive strength tests and the permeability tests are summarized in Table A below.

All of the cubes, regardless of the cement system from which they are formed, have a permeability of less than about 1.0 millidarcies prior to the initiation of the steam tests. From the results summarized above it can be seen that the cement systems including carbon-containing additives form cement masses which exhibit substantially no measurable permeability. The neat cement system shows a substantial retrogression in permeability. With the higher rate of permeability, there is a substantially greater chance that the cement system of Example 5 will fail before the systems in Examples 1 - 4. Likewise while the compressive strength of all of the cement systems increases with age, the strengths of the cubes formed from the systems of Examples 1 - 4 averages an 88.5% increase after three months as compared to the 6% increase of the neat cement system of Example 5.

Cubes having two inch faces are prepared and tested in the same manner as described above in connection with Examples 1 - 5. The cement systems are made up of the class J cement, the water-loss inhibitor and a carbon-containing additive in the same proportions as in Examples 3 and 4 except that the carbon-containing additive in Example 6 is burned oil shale having a particle size of between 20 and 40 mesh. The additive in Example 7 is burned shale having a particle size of between 12 and 20 mesh. The burned oil shale is prepared from oil shale which has been heated to a temperature of 500.degree. C. to extract the oil and gas therefrom. The burned oil shale has a fixed carbon content of about 4 weight percent and the volatile material comprises less than 15%.

The cubes are subjected to 3 months of exposure to the geothermal steam and tested for permeability and compressive strength as in Examples 1 - 5. The results are summarized in Table B below:

From the foregoing it is seen that the difference in particle size of the carbon-containing additive had little or no effect on permeability or compressive strength of the cement after three months at high temperatures.

Thermally resistant cement systems are subjected to well simulation tests in accordance with the following procedure.

Tubing, one-fourth inch in diameter, prepared from J-55 casing steel is disposed in a 6.times.6.times.6 inch Berea sandstone core in which a bore 2 inches in diameter is formed to simulate an oil well casing in a well bore. The tubing is disposed along one side of the core and the space between the core and the tubing is filled with a cement slurry which is then permitted to set and cure under gradually increasing temperature (80.degree. F. to 464.degree. F. over 4 hours) to simulate the pumping of a cool cement slurry into a high temperature well. This is carried out in an autoclave constructed for the purpose through which the geyser steam of Examples 1 - 5 is circulated. Once at temperature the sample is maintained in the interior of the autoclave at a temperature of about 460.degree. F. for three months. At the completion of the three months period the autoclave is slowly cooled and the tubing and concrete mass are removed from the rock core for testing the permeability and compressive strength in accordance with procedures set forth in Examples 1 - 5.

Water slurries prepared for testing included; neat cement of the class J type (Example 7), class J cement and anthracite (Example 8) and class J cement and finely divided mica. The composition of the slurries and the test results after 3 months exposure to the high temperatures of the test are summarized in Table C below.

From the foregoing it has been shown that cement systems including a carbon-containing additive in accordance with the present invention demonstrate a resistance to degradation of permeability and a relatively greater compressive strength after exposure to high temperatures and steam as compared to cement systems without carbon-containing additives. Consequently the cement systems of the invention are particularly well suited for use in deep wells and geothermal wells where high temperatures are to be encountered. While the foregoing examples have demonstrated cement compositions using API class J cement it should be clear that other types and classes of cement may be used in the cement systems of the invention. Also, while the cement system has been described herein in connection with use as an oil well cement, the cement system of this invention will find use in other high temperature environments where resistance to degradation of strength and permeability are important, such as in furnaces, reactors and the like.

The cement systems of the present invention are preferably premixed at the point of manufacture so that the necessity of handling separate additives and the attendant increase in labor costs and the like is avoided.