Abstract:
A process for killing and suspending mixtures of hydrocarbonaceous fluid production, particulary oil, in a formation containing same which minimizes formation damage. First, a &#34;spacer volume&#34; of liquid containing a surfactant is directed into the formation&#39;s productive interval. Thereafter, a solidifiable pumpable gel mixture is placed via a wellbore into the formation&#39;s productive interval and also within said wellbore. Said mixture solidifies in the formation and forms a gel plug within the wellbore. Thereafter, a light weight cement is placed over said gel plug effectively &#34;Killing&#34; and suspending the production of hydrocarbonaceous fluids.

Description:
FIELD OF THE INVENTION 
     This invention is directed to a method for minimizing formation damage when a well is &#34;killed&#34; and production is suspended. 
     BACKGROUND OF THE INVENTION 
     When production intervals are completed in exploratory wells, it is common practice to &#34;suspend&#34; wells for a period of time to allow construction of pipelines and gathering facilities in conjuction with field development. In remote locations (offshore, e.g.), it may even be necessary to plug and abandon zones found productive until development wells can be drilled. 
     In many cases, it is not known how long a well may be suspended. Therefore, thorough measures are taken to isolate the productive interval from the surface. For example, following a flow test of the zone indicating productivity at commercial rates, the zone will be &#34;killed&#34; with completion fluid, a bridge plug set above the perforated interval, and a cement plug placed on top of the bridge plug. Two or three additional bridge plug/cement plug combinations may be placed above the interval to insure zone isolation during suspensin. 
     When the suspended well is re-entered to open the interval to production, the plugs must be drilled out. Frequently this results in substantial loss of drilling fluid to the zone. This is especially true if the zone was stimualted (fracturing or acidizing, e.g.) as part of the initial well test program. Once the plugs are drilled out, the zone is opened to production. Usually, a lower rate than the initial test rate is observed because of damage which occured during re-entry. 
     Thereofre, what is needed is a method which will allow a producing well to be &#34;killed&#34; and suspended without causing undue formation damage. Utilization of said method would result in maintaining the producing rate near that initially determined. Well re-entry costs would be reduced, and formation damage minimized. 
     SUMMARY OF THE INVENTION 
     This invention is directed to a method for &#34;killing&#34; and suspending oil production in a well which results in a reduction in formation damage. In the practice of this method, a &#34;spacer volume&#34; of liquid containing a surfactant is directed into the formation&#39;s productive interval. Thereafter, a pumpable solidifiable gel mixture is placed into the wellbore substantially at the formation&#39;s productive interval. Subsequently, the gel mixture solidifies after entering said productive interval of the formation while causing a solid gel plug to form in the wellbore substantially in the area of the wellbore&#39;s productive interval. Said solified gel mixture within the formation and the wellbore is sufficient to withstand environmental conditions int he formation depths, including pressures. As a result of the solidifed gel mixture in the formation and gel plug within the wellbore, oil and other hydrocarbonaceous fluids cease to flow from the formation into said wellbore, thus &#34;killing&#34; the well. 
     Thereafter, the depth of the top of the solid gel plug is determined so that a desired amount of light (low density) concrete can be placed over said gel plug. Next, a desired amount of a light concrete is placed over said gel plug in an amount sufficient to suspend the production of hydrocarbonaceous fluids from said formation to the surface. 
     It is therefore an object of this invention to eliminate the need for a kill or completion fluid when &#34;killing&#34; a well. 
     It is another object of this invention to eliminate the need for a bridge plug when well production is suspended. 
     It is yet another object of this invention to minimize formation damage resultant from loss of drilling fluid which in prior art methods often entered the productive interval of a formation thereby decreasing the formation&#39;s permeability. 
     It is a yet further object of this invention to reduce well re-entry costs upon termination of the well suspension period. 
     It is a still yet further object of this invention to maintain the productino rate substantially near the initial production rate upon termination of the well suspension period. 
     It is a still yet even further object of this invention to pump the required materials into the wellbore without the need to circulate them into and out of the wellbore as was required in prior art methods. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is schematic representation of a prior art method for &#34;killing&#34; and suspending oil production in a well. 
     FIG. 2 is schematic representation of the method which is disclosed herein. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     When &#34;killing&#34; a productive interval of a well, as shown in FIG. 1, a kill fluid 14 is generally injected in wellbore 12. Said kill fluid enters formation 10 via perforations 16 &#34;drowning&#34; the productive interval in formation 10. Thereafter, a bridge plug 18, which is generally of a metallic construction, is caused to bind securely against wellbore 12. Once bridge plug 18 is securely bound, a cement plug 20 hardens and with the bridge plug causes the productive interval of wellbore 12 to be closed to the surface and thereby suspends oil production into said wellbore. To more effectively sucure the productive interval against oil production, multiple cement plug/bridge plug combinations can be utilized in wellbore 12 as is shown in FIG. 1. 
     In the practice of this invention, a spacer volume of a liquid is pumped directly into the wellhead. The spacer volume of liquid is used to clear the formation and well of materials which might interfere with adherence and solidification of the gel mixture. Generally, about two wellbore tubing volumes of fluid will be sufficient to clear and condition said formation and wellbore. Fluids which can be used to make up the spacer volume of liquid comprise completion brine, formation brine, potassium chloride, and fluids wth similar chemical composition. Placement of the spacer volume of liquid into the formation also substantially inhibits production of hydrocarbonaceous fluids when performing the subsequent steps of this invention. As is known to those skilled in the art, the pumping or injection rates utilized should be maintained below those rates which will create a pressure sufficient to fracture the formation. The spacer volume of liquid utilized can comprise a surfactant, preferably an aqueous surfactant, in a concentration of from about 0.10 to preferably about 1.0 volume percent. Some surfactants which can be utilized are mentioned in Clampitt, U.S. Pat. No. 4,300,634, which is hereby incorporated by reference. Other compounds which can be utilized include methanol and isopropanol. When used, these compounds should be incorporated into the spacer volume liquid in an amount of from about 5 to about 50 volume percent, preferably about 20 percent. Upon incorpation of these compounds in the spacer volume of liquid which is injected via the wellbore 12 into formation 10, said compounds have the ability to minimize fluid retention, and condition the formation for greater receptivity of a solidifiable pumpable gel mixture. 
     Afterwards, as is shown if FIG. 2, a pumpable solidifiable gel mixture is directed into wellbore 12, usually by pumping into the wellhead. Said gel mixture is allowed to flow down wellbore 12 in formation 10 until it comes in contact with the productive interval of formation 10. The injection or pumping rates should be maintained below those pressures which will cause the formation to fracture. At the productive interval, said gel mixture enters the productive interval of formation 10 via perforations 16. Sufficient solidifiable gel is allowed to enter said productive interval thereby closing off said interval to production of hydrocarbonaceous fluid mixtures, particularly oil. Additional solidifiable gel material is allowed to enter wellbore 12 which material contacts said productive interval until said gel has filled said wellbore 12 above the productive interval. 
     The solidifiable gel material subsequently forms a solid gel plug 24 within wellbore 12. It als forms a solid formation gel 22 in the productive interval of formation 10. Gel plug 24, upon solidification, is of a composition and strength sufficient to support a cement plug 20 thereabove in addition to forming an impermeable barrier to the flow of fluids from the productive interval. Cement plug 20 is comprised of a light cement. Representative cements are foamed cements and a light (low density) cement sold by Haliburton under the Spherelite tradename. Similar cement compositions are disclosed in U.S. Pat. Nos. 3,902,911 and 4,120,360 which are hereby incorporated by reference. Upon hardening, the cement forms a further impermeable barrier to the flow of fluids from the productive interval of formation 10. As will be understood by those skilled in the art, quantities of &#34;spacer&#34; volumes of liquid 26, said gel mixture, and light weight cement will vary depending on formation parameters encountered. Also, a solidified gel plug, and a cement plug can be alternated in wellbore 12 until a sufficient number have been placed depending on the productive interval depth and length of the suspension period. 
     After the gel plug and light-weight cement plug have set, a pressure test is applied to the plug combination to insure competency and the desired plugging effect. Upon confirmation of the competency and plugging effect, the distance from the wellhead to the final light-weight cement plug is determined for re-entry at a later date. This determination can be made by running a dummy or wireline into wellbore 12. When it is desired to commence production of hydrocarbonaceous fluids from said formation, cement plug 20 is drilled out and gel plug 24 caused to be removed. Said removal can be accomplished by mechanical or chemical means. Since the productive interval of formation 10 is closed from wellbore 12 by the solidified gel, any pressurized drilling fluid utilized to remove cement plug 20 from wellbore 12 will not enter the productive interval and damage it. 
     Since a kill fluid is not utilized in this invention, hydrocarbonaceous fluids, particularly oil, are produced through said productive interval at substantially the initial production rate once production resumes. Re-entry costs are substantially reduced since more costly drill bits are not required as a metallic bridge plug is not used. 
     One method of making a suitable pumpable mixture is discussed in U.S. Pat. No. 4,33,461 issued to Muller on June 8, 1982 which is hereby incorporated by reference. The stability and rigidity of the gel plug 24 will depend upon the physical and chemical characteristics of the gel plug. As is known to those skilled in the art, gel plug 24 should be of a stability and rigidity which will withstand the weight of cement plug 20 and environmental well conditions. 
     Often, it will be necessary to increase the density of the pumpable gel to obtain the desired stability and rigidity therein. To accomplish this a solid non-reacting material can be added to the pumpable gel mixture. Preferred non-reacting solid materials include solid rock salt, calcium carbonate, and suitably crushed mollusk shells, such as oyster shells. 
     Other gel mixtures can be used to obtain the desired stability and rigidity. A preferred mixture used to obtain the desired stability and rigidity, for example, is a mixture of hydrogen guar crosslinked with transitional metals and ions thereof. The purpose of the transitional metal ions is to provide increased strength, stability and rigidity for the gel plug stem 22. 
     Hydropropyl guar is placed into the gel mixture in an amount of from about 0.70 to about 10.0 weight percent of said mixture. As preferred, hydropropyl guar is placed in said mixture in about 7.2 percent by weight of said mixture. 
     Metallic ions which can be used in the pumpable gel mixture include titanium, zirconium, chromium, antimony and aluminum. The concentration of these transitional metals in the pumpable gel fluid will of course vary depending upon the environmental nature of the wellbore and formation. Although the exact amount of the metals required will vary depending on the particular application, it is anticipated that the metals should be included within the pumpable gel fluid in amounts of from about 0.005 weight percent to about 0.50 weight percent, preferably about 0.10 weight percent of said fluid. 
     It is often desirable, depending on the kill and suspension period, to have a gel stem plug 22 which will withstand a temperature range from about 300° F. to about 450° F. for from about 0.5 of a day to about 4 days. A thermally stable solid gel plug 24 and formation gel 22 can be obtained by mixing into the pumpable gel mixture a chemical known as an oxygen scavenger (such as sodium thiosulfate or short chain alcohols such as methanol, ethanol, and isopropanol), preferably sodium thiosulfate. The concentration of the oxygen scavenger utilized, of course, will depend upon the thermal stability desired to be obtained for the gel plug 24 and formation gel 22. However, as preferred, it is anticipated that the concentration of the oxygen scavenger in the pumpable gel mixture will be from about 0.10 percent by weight to about 0.75 percent by weight, preferably 0.50 percent by weight. 
     Cement plug 20 can be removed from wellbore 12 by drilling. However, the formation gel 22 and gel plug 24 can also be removed in several ways. Several variations are provided for. One variation, which can be utilized to facilitate removal of the gel plug 24 from wellbore 12 and formation gel 22 is to form a solid gel plug 24 or formation gel 22 containing a gel breaker. This gel breaker, included in the gel mixture, is selected from a group of chemical compounds which can break down the solid gel at temperatures of less than from about 60° F. to about 250° F. Generally, this breakdown will occur within from about 2 hours to about 24 hours depending upon type and concentration of breaker added. Chemicals satisfactory for use as gel breakers, and which are incorporated into the gel mixture, include enzymes and oxidizing agents (such as sodium persulfate) suitable for breaking down the solid gel. Other gel breakers sufficient for this purpose are discussed in U.S. Pat. No. 4,265,311 issued to Ely on May 5, 1981, which is hereby incorporated by reference. These chemicals are readily available from chemical suppliers and with the exception of enzyme breakers are sold under their chemical names. Enzyme breakers can be obtained from oil field service companies. The concentration of the gel breaker incorporated into the gel mixture will vary from about 0.01 weight percent to about 0.10 weight percent, preferably about 0.05 weight percent of the gel mixture. Upon cooling to a temperature of from about 60° F. to about 150° F., the gel breaker will breakdown the solid gel causing it to liquify which will facilitate removal of gel plug 24 and formation gel 22. 
     Another method for breaking the gel is to contact the solidified gel with a mineral acid for removing cement plug 20 after a suitable or desired time interval. In those instances where it is undesirable to have a gel breaker incorporated into the gel mixture to remove the solid gel plug 24 or formation gel 22, it is preferred to use hydrochloric acid of a strength sufficient to solubilize the solid gel plug 24 and formation gel 22 without attacking formation 10. Hydrochloric acid, and acids similar thereto, can be used to breakdown the solid gel on contact. Hydrochloric acid of a concentration of about 10 percent to about 28 percent preferably about 15 percent, by volume of solution, will generally be sufficient for this purpose. Although hydrochloric acid has been mentioned, other similar mineral acids and strong organic acids may be employed depending upon their availability, as is known to those skilled in the art. 
     In one example of the practice of this invention, a slurry is formed with 1,000 gallons of water. This slurry comprises about 40 pounds of base gel such as hydroxypropyl guar gum which forms a hydrate in the water. To this mixture is added about 600 pounds of hydroxypropyl guar gum which ahs been chemically treated to provide delayed hydration and thickening properties. Approximately 20 pounds of a buffer or catalyst suitable to obtain the desired pH and reaction time is added to this mixture. Cross-linking agents, such as borates and chromates, are then added in an amount of about 20 pounds. Forty-two pounds of sodium thiosulfate, an oxygen scavenger, is then added to the mixture. This gel mixture is pumped into the formation 10 near the productive interval. After solidification of the mixture and the elapse of the desired suspension time, the solidified gel plug 24 and formation gel 22 are removed by contacting them with 15 volume percent of hydrochloric acid in an amount sufficient to solubilize the gel compositions. 
     In another example of the practice of this invention, a mixture is made as above. Additional components are placed into the mixture. About 420 pounds of crushed oyster shells are next added to the mixture. Titanium, in an amount of about 4 pounds, is added to the mixture. Approximately 170 pounds of potassium chloride is subsequently added to the mixture. Four pounds of sodium persulfate is added to the mixture and serves as a gel breaker. Upon solidifaction, the gel plug 24 and formation gel 22 are capable of withstanding greater pressures. By chemical dehydration, the gel stem is liquefied by the sodium persulfate gel breaker. 
     As is understood by those skilled in the art, the composition of a gel stem will depend upon many variables including formation conditions. The above examples are mentioned as two possible variations among many others. 
     Although the present invention has been described with preferred embodiments, it is to be understood that modifications and variations may be restored to without departing from the spirit and scope of this invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the appended claims.