Method for sealing off porous subterranean formations and for improving conformance of in-situ combustion

Filed Sept. 1, 1961

Aug*. 3, 1965 θ. willman 3 198 249

METHOD FOR SEALING OFF POROUS SUBTERRANEAN FORMATIONS AND . _σ IMPROVING CONFORMANCE OF IN-SITU COMBUSTION

Filed Sept. 1, 1961 _{2 Sheets}__{Sheet χ}

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FIG.

BERTRAM T. WILLMAN INVENTOR.

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Aug. 3, 1965 B. τ. WILLMAN 3,198,249

METHOD FOR SEALING OFF POROUS SUBTERRANEAN FORMATIONS AND FOR IMPROVING CONFORMANCE OF IN-SITU COMBUSTION

Filed Sept. 1, 1961 _{2 Sheets}__{Sheet 2}

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FIG. 2

BERTRAM T. WILLMAN INVENTOR.

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AGENT

United States Patent Office „ ., ^{3198 249} ________ Patented Aug. 3, 1965

3,198,249 ^FOR SEALING OFF POROUS SUBTERRANEAN FORMATIONS AND FOR IMPROVING CONFORMANCE OF IN-SITU COMBUSTION Bertram T. Willman, Tulsa, Okla., assignor, by mesne assignments, to Esso Production Research Company, Houston, Tex., a corporation of Delaware

Filed Sept. 1,1961, Ser. No.135,580

Claims. (Ci. 166—4)

This invention relates to a method for sealing the pores of a permeable earth formation, to prevent or substantially reduce the flow of fluids therethrough, by precipitating dissolved solids therein. The method is useful in preventing water or gas from flowing into a well bore. It is also useful to prevent sand in-fill. A particularly useful application of the method of the present invention is in the control of the bum-out pattern during in-situ combustion for the secondary recovery of oil.

In the secondary recovery of oil the method of insitu combustion is well known and widely practiced utilizing an existing well communicating with the oil reservoir as the input well and employing one or more existing, adjacent wells as the producing well. It involves the initiation of combustion along the bore of the input well. , j^sn'^don’ ^a'^r ,^or oxygen is continuously injected into the formation, with the result that a burning wave or front gradually advances toward the producing well or weds. The principal fuel which supports the combustion is a carbonized deposit formed in-situ by the cracking of a portion of the petroleum immediately ahead of the burning front. Upon continued combustion heated gaseous products are. developed which under the existing pressure are forced into the air reservoir ahead of the burning front. The heat imparted in this manner to the oil m the sands, volatilizes a portion of the oil and reduces the viscosity of the remaining heavier constituents of the oil. The vaporized portions of the oil are entrained by the stream of gaseous products of combustion and move therewith toward the producing well while the fluidized heavier portions of the oil are mechanically forced under pressure of the gaseous products of combustion toward the producing well. As the vaporized portions of the oil move into cooler regions of the oilcontaimng sands, they are partially condensed and release the latent heat of condensation at that point which together with the sensible heat in the gaseous products of combustion serves to increase the temperature in regions of the sands more remote from the input well. Thus,, the entire oil reservoir is progressively heated and 50 the oil.in vaporous and/or liquid state is forced into· the producing well bore where it is removed by ordinary pumping means. Vaporization of a portion of the oil and, in addition, formation of steam from the connate water adds -to the total volume of gases facilitating re- 55 moval of the oil from the reservoir.

A. more detailed review of the in-situ combustion process is found in the Petroleum Engineer of July 1958 pages B-29 through B-32, B-36, B-41 and B-42.

The in-situ combustion process is extremely efficient 60 from the standpoint of oil recovery from those portions of the reservoir which actually burn out. A critical· problem arises, however, in the effort to maintain a uniform, continuous propagation of the burning front across

This conformance θ5 . - ---------- First, the inj’ected air and resulting combustion products are less dense than reservoir fluids and thus, gravity segregation results producing a tendency to “overburn” or contact preferentially the top portion of the oil-bearing formation. Second, the products of combustion tend to channel through the entire oil-bearing formation, problem arises for two reasons.

the oil-bearing formation and thereby cause irregularities to occur in the burning front. Because of the adverse viscosity ratio between reservoir liquids and gas, this channelling effect is severe, causing !ow sweep efficiencies. While overall recovery is poor because efficient recovery occurs in only a small portion of the reservoir, this poor conformance does have the advantage of releasing heat rapidly over much of the reservoir lowering viscosities of the oil and thereby increasing production rates.. Ideally therefore, one would like to start the process using the expected poor conformance to rapidly stimulate production. The burned-out portions of the irregular pattern should desirably be plugged to prevent bypassing, of air and combustion gases around unaffected areas in the oil-bearing formation. The injected air would then be forced to burn up previously unburned portions and thereby improve conformance.

Accordingly, this -invention provides an effective method to solve the problem of poor conformance in the in-situ combustion process. The method of this invention comprises: interrupting the in-situ combustion process in a reservoir which has been partially exploited thereby and then plugging off the channels which have resulted from poor conformance by injecting a solution containing a high concentration of a dissolved material. The burned-out formation behind the burning front will be very hot. The temperatures therein will commonly be on the order of 800 to 1,000’ F. for some distance behind the burning front. As the solution containing the dissolved material reaches this hot rock, evaporation will occur. As the solvent evaporates, the pore structure will be plugged, or at least partially plugged, with the dissolved material which is left -after evaporation of the solvent. This technique is selective in that it plugs .).&gt; only the desired portion of the formation, since any solution entering non-burned portions of the pay zone will not stop flow in those regions because the plugging action is automatically controlled by temperature and evaporation.

The combustion process is then reinitiated. Air or oxygen, or dilute oxygen, is then -again injected and will be forced into the unburned portions of the reservoir because the burned-out zone has been -plugged by the solute. The result is that increased uniformity in the advance of the burning front can be accomplished. The process may be repeated as often as desirable; that is, tne burning interrupted, a slug of solution containing dissolved solids injected, air injection reinitiated as often as desired. The burned-out formation need not, and in fact cannot, be entirely filled with solute to seat it off. As the solvent evaporates in the reservoir, a plug of solute will be formed where temperatures are hot enough to cause evaporation of the solvent. This will prevent further flow in this layer.

It is contemplated that a great variety of mateirals are technically suitable for the solution and will plug the pore space when the solvent evaporates. Concerning the choice of a solvent, water is preferred because it is both entirely suitable and also the least expensive of common solvents. As for the solute, the material chosen will depend on (1) the porosity of the formation under consideration, (2) the reservoir pressure and burning temperature, and (3) the solubility of the solute in the solvent. Common salts such as sodium chloride and calcium chloride are suitable. Strong brine, for example, is suitable under some conditions. Efficient plugging of the pores depends on depositing an amount of material sufficiently great to fill the pore space from the volume of solvent which is evaporated in cooling the oil sand from 70 combustion temperature to approximately the saturation temperature at reservoir pressure. Organic solvents are also suitable but are, of course, expensive. Accordingly,

3,198,249 ⁴solvent bank which this causes can easily be tolerated, since automatic reconcentration takes place at the leading edge of the solution bank, due to solvent evaporation. The solvent drive injection should be as rapid as possible, without fracturing the formation, in order to minimize ⁵ heat dissipation in the reservoir. The drive is continued until the volume of solution plus solvent approaches the total pore volume of the burned-out region, as indicated by a rise in injection pressure requirements; or by a calculated estimate of the volume based on the quantity 1° of oxygen consumed during combustion, and the quantity of oil produced from the formation; or an estimate based on prior knowledge of burn-out rates from earlier operations.

It is unnecessary to complete the plugging step before ⁴³ resuming oxygen or air injection, since the initial volume of air injected will obviously serve as a supplemental drive behind the solvent bank. The practical merit of this alternative is great, since the overall time interval between combustion phases is further minimized. The combus²³ tion process will then be reinitiated and the flame front wil be forced to seek new paths since the plugged areas will prevent further extension of the channeling and overburn effects.

Referring to FIGURE 2, borehole 31 extends through ²³ numerous earth formations including stratum 32. As frequently happens, stratum 32 has an undesirable production capacity, either water or gas for example. Accordingly, it is desired to seal off stratum 32 from the well bore. This may be accomplished according to one ³³ embodiment of the method of this invention by first inserting packers 33 and 34, and heating that portion of the stratum which immediately surrounds the well bore. This heating may be accomplished by any suitable means, such as a well bore heater, by injecting hot combustion ³³ gases or it may be done by initiating an in-situ combustion. Cnee the necessary temperature is reached in area 35 immediately surrounding the well bore, the heating step is discontinued and the solution is introduced for plugging the formation. As before, upon contact of the solution with the hot .sand the solvent evaporates leaving a deposit of solute which plugs the pores of the formation, thereby sealing off the formation as desired.

Again referring to FIGURE 2, another application of the invention is illustrated by assuming that the well bore is uncased, and that stratum 32 is an unconsolidated sand which sloughs into the well bore, interfering with drilling operations or causing other obvious difficulties. This sand can be readily consolidated by heating the sand θ surrounding the bore, and injecting a sugar solution, whereby carbonization of the sugar will occur, as already discussed.

The best mode of carrying out this invention is illustrated by the following example:

Example

A dry Gifford Hills sand having a permeability of 15 darcies was packed in a one inch pipe 25 inches long, a portion of which was heated to 1,050° F. in an oven. The pipe inlet was unheated except for metal conduction ⁰⁰ to prevent coking or carbonizing the sugar solution as it entered the sand. Eight thermocouples were placed equidistant from the inlet end to the outlet end of the pipe to measure the temperature gradient. A sugar solution containing 2.2 pounds of sucrose per pound of water was ³³ injected into the sand at 350 pounds per square inch gauge. A 350 p.s.i.g. sand pore pressure was maintained by a back pressure regulator at the pipe outlet. The sand became plugged upon the injection of 61 cc. (.58 pore volume) of sugar solution. Permeability was not restored ‘° even with a 650 p.s.i.g. differential between core inlet and outlet.

After the pipe was cooled and cut open, it was observed that unconsolidated sugar-saturated sand extended from __r the inlet to a point approximately 6½ inches from the ¹⁰ inlet where the sugar had carbonized at 370° F. and conώ any combination of organic or inorganic solvent and organic or inorganic solute is useful, provided the solvent will evaporate at less than burning temperature and sufficient solute will deposit in the pore space to completely plug or reduce the permeability to the desired level.

The preferred solution is ordinary sugar or sucrose in water. In a typical operation it may be assumed that the burned oil sand is at about 900° F. and that the pressure of the system is about 200 p.s.i.g.; that the specific heat of the formation is 0.23 B.t.u. per pound; and that the sand density is 120 pounds per cubic foot. This system will have sufficient sensible heat available from the rock to evaporate water equal to about 14,200 B.t.u. per cubic foot of burned-out rock at an initial temperature of 900° F. This amount of heat will supply the latent heat of evaporation for about 17 pounds of water per cubic foot of rock cooled to 387° F. at 200 pounds per square inch gauge of pressure. Assuming an average sample of a sand is 23 percent porous, 0.23 cubic foot of plugging material per cubic foot of oil sand is needed after evaporation of the water to completely plug the desired amount of oil sand. Using sucrose as a plugging material, with a density of 1.59, 22.8 pounds of sugar are required per cubic foot of plugged rock. Since 17 pounds of water are evaporated, the solution injected must have about 1.3 pounds of sugar per pound of water. This is well below the solubility of sugar in water at reservoir conditions. For example, 2.38 pounds of sugar can be dissolved in one pound of water at 40° C.

The preferred nature of sugar as solute is due not only to its high solubility, but also to its well-known chemical property of undergoing thermal decomposition to form a hard, carbonized mass which is both impermeable and insoluble. Thus, when the sugar solution is brought in contact with the hot sands behind the burning front, the sugar is not merely deposited in the pore structure, but is also carbonized, so that it will not redissolve upon subsequent treatment of the formation.

Other sugars in addition to sucrose are suitable, such as dextrose (grape sugar), levulose (fruit sugar), lactose (milk sugar), and maltose (malt sugar), including mixtures of various sugars. Molasses is a convenient source of sugars for use in the method of the invention.

A better understanding of the invention may be obtained by reference to the drawings.

FIGURE 1 shows an in-situ combustion operation involving the use of an injection well and a producing well.

FIGURE 2 illustrates an embodiment of the invention whereby a water-producing or gas-producing formation may be sealed off from a well bore.

Turning now to FIGURE 1 in detail, an injection well 11 and a producing well 12 have been drilled into the oil sand 13 from the earth’s surface 14 and have been cased in the usual manner. The oil-bearing formation is beneath an oil impervious cap rock 15. The strata underlying the oil bearing zone are not shown. A gun perforator, an abrasive jet, or other conventional tool has been used to provide perforations 16 and 17. Conventional well heads 18 and 19 are provided with connections through which fluids may be introduced into the wells in the conventional manner. As the combustion process progresses, burnedout area 20 will be formed in the oil-bearing formation. Due to the channeling effects the burn-out has progressed in an irregular pattern characterized by undesirable overburn 21, as mentioned above. According to the method of this invention, the undesirable channeling effects and bverburn are corrected. The combustion process is interrupted and a bank of the solution, for example, sucrose in water, is injected through well head 18 and perforations 16 into the burned-out portion of the oil-bearing formation. The amount of solution injected need not be large, since it is necessary to form a plugged area only a few inches thick to effectively block off the overburned and channeled regions. Thus, the solution bank is followed by solvent alone as a driving medium, to avoid any appreciable loss of excess solution. Any dilution of the

3,198,249 solidated the sand into an extremely strong, hard, impermeable and insoluble mass that continued for another 6½ inches to the point where the temperature reached 800° F. The remaining 12 inches of sand were dry and unconsolidated. Very little temperature differential was evidenced anywhere between the center of the sand and its corresponding pipe wall. During the sugar injection 21 cc. of water containing no sugar were produced by collecting evaporated solvent.

The above example is included for illustrative purposes only and should not be construed to limit the invention. Many variations of this method would occur to one skilled in the art. For example, in actual operation the sugar solution or other solution would be injected in substantial quantities into the reservoir and then followed by a gaseous driving medium, or by solvent, for example water, .which would propel the solution through the formation. Appreciable dilution will doubtless occur with a solvent drive, but it can be tolerated since the solution bank will automatically be regenerated and concentrated as evaporation occurs at the front. For this reason initial concentration of a solution is probably not too critical over a range which can be specified for given reservoir conditions.

With a knowledge of reservoir temperature, the solution bank can even be positioned approximately where desired by injecting water, ahead of the slug of solution or injecting a dilute solution to cool a portion of the reservoir by evaporation and move the bank of solution further out before evaporation of the solvent occurs and the solute is deposited in the pores of the rock.

A convenient guide to use in determining when to inject the solution is available from exhaust gas analysis. When the oxygen concentration in the exhaust gases exceeds a predetermined value, for example 2 to 10 percent, bypassing is occurring and the bank of solution should be injected; however, it is not necessary to wait until bypassing actually occurs before injecting the solution.