IE48744B1 - Process for treating xanthomonas fermentation broth for use in displacement of oil from partially depleted reservoirs - Google Patents

Abstract

A process for preparing an oil recovery mobility control solution having activity enhancement of greater than 15% and a filter ratio of less than 3 through a Millipore filter with a pore size of 1.2 microns comprises heating an aqueous solution of Xanthomonas biopolymer at an equivalent xanthan concentration of 0.05 to 2.0% and a salt content of less than 0.2% for a period of from about 2 to 60 minutes at a temperature of about 60-98 DEG C, and when the equivalent xanthan concentration exceeds 3000 ppm diluting the solution to an equivalent xanthan concentration of from about 100 to 3000 ppm. Also disclosed is such a process which comprises the steps of: (a) diluting a whole Xanthomonas fermentation broth to an equivalent xanthan concentration of 0.15-1.5% with water having a salt content of less than 0.2%; (b) heating said broth for a period of from about 2 to 60 minutes at a temperature of about 77-98 DEG C; and (c) filtering said broth to yield a filtrate with activity enhancement of greater than 15% and a filter ratio of less than 3 through a Millipore filter with a pore size of 1.2 microns. e

Description

The hydrophilic colloids produced by Xanthomonas species are polysaccharides which contain mannose, glucose, glucuronic acid, O-acetyl radicals and acetal-linked pyruvic acid. These gums and their derivatives have found wide food and industrial applications. Of special interest is the increasing focus on the use of Xanthomonas gums in displacement of oil from partially depleted reservoirs.

Typically, oil is recovered from underground reservoirs via a series of sequential operations. A new well will generally produce a limited amount of oil as a result of release of internal pressure in the well. As this pressure becomes depleted, it is necessary to pump further quantities of oil by mechanical means. These measures recover only about 25% or less of the total oil stored in the reservoir.

A great deal of oil is still trapped within the pores of the formation. Further enhancement of recovery can then be effected by secondary methods. In one method of recovery, a waterflood is carried out by pumping water into a well or series of wells, displacing part of the trapped oil from the porous rock and collecting the displaced oil from surrounding wells. However, waterflooding still leaves about 55-60% of the available oil trapped in the formation. The explanation for this is that the water has a very low viscosity compared to the crude oil and tends to follow the path of least resistance, fingering through the oil and leaving large pockets untouched. In addition, surface forces in the formation tend to bind the oil and prevent its displacement.

A number of processes have been developed in recent years to recover further quantities of oil from these - 3 reservoirs by the use of mobility control solutions which enhance oil displacement by increasing the viscosity or permeability of the displacing fluid. Of interest are those enhanced recovery processes employing polymer flooding with a polysaccharide or polyacrylamide to increase the viscosity of the displacing fluid. Variations of this process include the use of surfactants and co-surfactants to release the oil from the rock formation. Polyacrylamides have been found to suffer such deficiencies as viscosity loss in brines and severe shear sensitivity. Since, as was well documented in the prior art, xanthan gum is relatively insensitive to salts (does not precipitate or lose viscosity under normal conditions), is shear stable, thermostable and viscosity stable over a wide pH range, xanthan gum is a good displacing agent. Moreover, the gum is poorly adsorbed on the elements of the porous rock formations and it gives viscosities useful in enhanced oil recovery (5 to 90 centipoise units at 1.32 sec.shear rate) at low concentrations (100 to 3000 ppm). The use of solutions of xanthan gum or derivatives of xanthan gum for oil recovery is described in U.S. Patents 3,243,000; 3,198,268; 3,532,166; 3,305,016; 3,251,417; 3,319,606; 3,319,715; 3,373,810; 3,434,542; 3,729,460 and 4,119,546. It is suggested in U.S. Patent 3,305,016 that aqueous solutions containing heteropolysaccharide in sufficient quantity to increase the viscoeity be employed as the thickening agent in preparing viscous waterflooding solutions. The polysaccharide may be prepared, separated, purified and then added. Alternatively, according to this reference, the entire culture after adding a bactericide (e.g., formaldehyde) to kill the bacteria, may be added to the floodwater.

It has been found that various heat treatments result in enhanced viscosities or filterability of whole and diluted Xanthoroonas fermentation broths. U.S. Patent 3,501,578 provides that a heat step is carried out prior to «, 48744 - 4 the precipitation of xanthan. Viscosity increases of 1.5 to 3.5 fold are obtained in the heat-treated broth. U.S.

Patent 3,773,752 describes a process for heating diluted fermentation broth after addition of an alkali metal salt until coagulation occurs and filtering the hot solution preferably after the addition of a coagulating agent such as alum. The process of U.S. Patent 3,801,502 calls for the addition of an alcohol, phenol, ketone or non-ionic surfactant during the heating process. Xn the process of 10 U.S. Patent 3,355,447, the heat-treated fermentation broth is diluted, filtered and the xanthan removed by alcohol precipitation.

This invention is concerned with an improved process for preparing an oil recovery mobility control solution having activity enhancement of greater than 15% and a filter ratio of less than 3 through a filter with a pore size of 1.2 microns which process comprises heating a fermentation broth of Xanthomonas biopolymer substantially free of insoluble matter having a particle size greater than 3 microns, at an equivalent xanthan concentration of 0.05 to 2.0% and a salt content of less than 0.2% for a period of from 2 to 60 minutes at a temperature of 60-98°C., and when the equivalent xanthan concentration exceeds 3000 ppm diluting the solution to an equivalent xanthan concentration of from about 100 to 3000 ppm.

Heating carried out for a period of from 5 to 20 minutes at a temperature of 80-98°C. is particularly preferred.

Final dilution of the heat-treated solution to use concentration, where required, is preferably effected with water having a salt content of at least 0.6%.

Whereas previously described heat treatments were individually concerned with enhancing either viscosity or filterability of whole Xanthomonas fermentation broths, the present invention is concerned with an integrated process for preparing mobility control solutions characterized by - 5 each of these desirable properties, i.e. enhanced viscosity and improved filterability, as well as enhanced injeetability and good thermal stability. Methods are described for treating Xanthomonas biopolymer solutions substantially free of insolubles having a particle size greater than about 3 microns, for which the process of the invention provides a product suitable for direct injection without filtration, although for use in oil fields of low permeability, filtration may be resorted to.

For purposes of describing the process of the present invention, the following terminology is used. As a measure of xanthan activity, solution viscosity in centipoises is determined at 6 RPM and 25°C. using a Brookfield viscometer with UL adaptor, corresponding to a shear rate of 7.3 sec'T For a given solution, the degree of dilution (with 500 ppm salt solution, NaCl: CaClj = 10:1) necessary to yield a viscosity of 10 cp. is determined. With untreated Xanthomonas polymer this viscosity is observed at a polymer concentration of 0.05% (500 ppm). The dilution factor observed with a given solution, multiplied by 0.05%, yields the equivalent xanthan concentration of that solution (also termed the active Xanthomonas polymer concentration).

The term activity enhancement is defined as the ratio of the dilution required to give a viscosity of 10 cp at 6 RPM on the Brookfield viscometer after heat treatment to the dilution required to give a viscosity of 10 ops at 6 RPM on the Brookfield viscometer before heat treatment, multiplied by 100.

Injeetability (not used interchangeably herein with the term filterability) is an important property of mobility control solutions. It is correlated with a test, as described later in detail, a procedure that measures flow rate through a Millipore (registered trade mark) filter (0.45 to 3.0 micron pore size) as a function of volume under a constant pressure of 40 psig. The filter ratio (F.R.) is defined as the ratio of the time required to collect the - 6 fourth 250 ml. of mobility control solution to the time to collect the first 250 ml. of mobility control solution. A filter ratio of 1.0 indicates that the solution has no plugging tendencies. An acceptable mobility control solution generally has a filter ratio of 1 to 3 (0.45 to 3 micron filter), and preferably below 1.7. The desirable filter ratio and filter pore size for a particular mobility control solution are dependent on the permeability of the subterranean stratum of the oil field for which oil displacement is planned.

Xanthan mobility control solutions may be subjected to subterranean termperatures of 80°C. or higher. The thermal stability of these solutions is affected by their salt concentrations as well as other factors. Thermal stability is measured as the viscosity ratio of the diluted broth after 7 days storage at 80°C. to that before storage (10 cps).

Studies of heat treatment of whole Xantbomonas fermentation broths show that the temperature required for achieving enhanced viscosity of whole broth is considerably higher than that for diluted broth.

The whole broth is far more stable and resistant to xanthan polymer reconfiguration by heating than is diluted broth. This can be explained by the presence of a higher salt concentration (ionic strength) in the whole broth, as well as by reduced molecular mobility.

Further studies demonstrate that although enhancement of xanthan activity is achieved by heating, heat-treated whole broth does not retain its injectability (measured as filter ratio). The injectability decreases as the heating temperature and holding time are increased.

Further investigations indicated that neither chemical (surfactants, phenols, etc.) nor physical (shear rate) treatments were effective in improving the injectability of heat-treated whole Xanthomonas fermentation broths.

Studies leading to the present invention show that - 7 Xanthomonas polymer reconfiguration is the main reason for the injectability change on heat treatment. Xanthomonas polymer configuration is dependent upon the treatment temperature, time and salt concentration. In turn, the configuration determines the viscosity, injectability and thermal stability of the Xanthomonas solution.

The novelty and advance over the prior art of the present invention reside in the findings that (a) significant enhancement of xanthan activity is obtained by the moderate (60-98°C.) heat treatment for a brief period of time, from about 2 to 60 minutes, of a Xanthomonas fermentation broth diluted with deionized water or water of low salinity, (b) that the moderate heat treatment of diluted whole Xanthomonas fermentation broth causes minimum Xanthomonas cell deterioratiion and so does not materially affect the injectability of the mobility control solution and (c) final dilution of the mobility control solution to use xanthan concentration with water of high salinity favours thermal stability.

In accordance with the present invention a whole Xanthomonas fermentation broth substantially free of insoluble matter having a particle size greater than about 3 microns is treated to provide mobility control solutions with favourable filter ratios such as described in U.S. 4,119,546.

Whole Xanthomonas fermentation broth substantially free of insoluble matter having a particle size greater than about 3 microns is diluted to a xanthan concentration of 0.05 to 2% with deionized water or with field water having a salt content below 0.2%. The diluted broth is then heated with agitation at a temperature of 60-98°C. for 2 to 60 minutes, preferably 5-20 minutes. The heat-treated broth is then if necessary diluted to use level (100 to 3000 ppm xanthan), preferably with water having a salt content of at least 0.6%. The diluent may also contain other additives such as preservatives, surfactants and scale inhibitors. - 8 Thus, the integrated process of the present invention offers a method for preparing mobility control solutions for use in oil recovery having the following practical and economic advantages: 1. Increased xanthan activity. 2. Improved thermal stability (when salt water is used for use dilution). 3. Elimination of need for Xanthomonas cell filtration with retention of good injectability.

Mobility control solutions prepared from the whole fermentation broths by the process of the present invention have filter ratios suitable for use in most oil fields.

Where subterranean strata are highly impervious, mobility control solutions with low filter ratios (1-3) through finer filters (0.45-0.65 micron pore size) must be used. Under such circumstances, mobility control solutions free of Xanthomonas cells and other insoluble matter must be employed.

For this limited alternative process, the whole Xanthomonas fermentation broth is diluted with water having a salt content below 0.2% to a xanthan concentration of 0.05 to 2.0%, preferably 0.14%-1.5%. The pH is optionally adjusted to 6.5 with an alkali metal base. The solution is stirred (preferably using low shear mixing) until the xanthan is uniformly dispersed (approximately 1 hour). Low shear mixing gives a higher solution viscosity after heat treatment than does high shear mixing.

The diluted broth is heated to a temperature of 6098°c., preferably 77-98°C., and filtered. A filter aid, e.g., diatomaceous earth (Dicalite Superaid trade mark), at a level of about 4 times the xanthan concentration per liter of diluted broth is added with stirring at 77-98°C., and the broth filtered through a pressure leaf filter heated during the run to 77-98°C.

Total time at the elevated temperature, including filtration - 9 time, should be from about 2 to 60 minutes. Filtration can also be done at ambient temperature after holding at the elevated temperature for the time period selected.

Final dilution to use xanthan concentration (100-3000 ppm) can be made with water, preferably having a salt content of at least about 0.6%. The filtration step may optionally be conducted after final dilution if desired.

Typical sparkling filtrates with doubled viscosities have filter ratios of 1.5 to 2 through a Millipore (registered trade mark) filter with a pore size of 0.45 micron. In addition, 1000 ml. of filtrate is collected within 20 minutes. Thus, this alternative process provides mobility control solutions that have enhanced viscosities and that are injectable into the strata of highly impervious oil fields.

Injectability Test Prepare 1050 ml. of 500 ppm xanthan solution in 500 ppm salt solution (10:1 - NaClsCaClj) as follows: In a Waring type blender equipped with a rheostat, measure sufficient broth (based on xanthan content) to provide 0.525g. xanthan solids. Dilute 1 to 6 with salt solution. Shear this mixture at 50 volts for 2 minutes. Dilute in the blender to 1050 ml with salt solution and shear at 50 volts for 1 minute. Use an experimental set-up that allows measurement of the flow rate through a Millipore (registered trade mark) filter disc (47 mm, 0.45-30 microns pore size) as a function of volume under a constant pressure of 40 psig. Use a reservoir that will accommodate at least 1000 ml. filtrate.

Charge the reservoir with 1050 ml of xanthan solution (500 ppm). Set the pressure at 40 psig (2.76 x 10® dyn cm ). Open the valve and start recording filtrate volume vs. time (seconds).

Filter ratio = time to collect the 4th 250 ml of solution time to collect the 1st 250 ml of solution - 10 The following examples are provided for illustrative purposes and should not be deemed to limit the invention, the scope of which is defined by the appended claims.

EXAMPLE 1 Xanthomonas fermentation broth substantially free of insoluble matter having a particle size greater than about 3 microns may be prepared in the following way: Cells of Xanthomonas campestris NRRL B-1459a from a YM agar slant are transferred to 300 ml of YM broth contained in a 2.8-liter Fernbach flask and shaken on a rotary shaker for about 31 hours at 28°C. A 25 ml aliquot is transferred to a 2.8-liter Fernbach flask containing 500 ml of a medium of the following composition: Ingredient Gram/liter Glucose-fructose (Isosweet 100, Corn Products) 10.1 Crude glucose (Cerelose) 25.7 nh4no3 1.0 MgSO4.7H2O 0.10 MnSO4.H2O 0.03 FeSO4.7H2O 0.01 Anhydrous citric acid 1.0 K2HP04 4.1 KH2PO4 0.69 The Cerelose and Isosweet 100 are dissolved in distilled water and autoclaved separately. The rest of the ingredients are combined, adjusted to pH 6.4 and autoclaved. The separately autoclaved materials are then combined.

After shaking at 28°C. for about 33 hours a 200 ml portion is transferred to a 4-liter mechanically agitated fermentor containing 2 liters of the following medium: Ingredient Cerelose (autoclave separately) Isosweet 100 (autoclave separately) NH4NO3 MgSO4«7H2O MnS04.H20 FeSO4.7H2O Anhydrous citric acid CaCl2-2H2O Na2HPO4 NaH2PO4 Grama/liter .7 .1 1.0 0.10 0.03 0.01 1.0 0.20 3.34 0.70 The sugars dissolved in 300 ml of water are autoclaved separately. The rest of the ingredients dissolved in 1700 ml of water are autoclaved, and the two solutions then combined. Aeration is at a rate to provide 1.5 millimoles of oxygen per liter per minute. The fermentation is conducted at 30°C. for 48 hours during which time the pH of the medium is maintained between 5.9 and 7.5 by the addition of a sodium phosphate buffer made up with tap water. Ethylenediaminetetraacetlc acid is also added to the sodium phosphate buffer to prevent the precipitation of calcium phosphate salts. At the end of the fermentation, the viscosity of the broth exceeds 7800 centipoiee units at .27 sec.-·'· shear rate and the xanthan concentration is above 1.5%.

EXAMPLE 2 Xanthomonas whole fermentation broths substantially free of insoluble matter having a particle size greater than about 3 microns were treated by the process of the invention. A portion of each was diluted with deionized water to a xanthan concentration of 0.6% and heated for 5 minutes at 80°c. The heat-treated solutions were then further diluted to a viscosity of about 10 cps. and compared . 48744 with similarly diluted samples which were not heat treated. The test results were as follows: Broth Heat 1.2 micron Activity Thermal No. Treatment Filter Ratio Enhancement (c) Stability 1 (a) - 1.06 - 0.77 1 (a) + 1.07 +70% 0.82 1 (b) - - - 0.22 2 (b) - 1.01 - - 2 (b) + 1.01 +65% - (a) Final dilution with 0.6% sodium chloride (b) Final dilution with 500 ppm sodium chloride (c) Activity enhancement = Ratio of dilution required to give 10 cps. at 6 RPM on the Brookfield viscometer after heat treatment to the dilution required to give 10 cps at 6 RPM on the Brookfield viscometer before heat treatment, multiplied by 100.

EXAMPLE 3 A series of Xanthomonas broths with xanthan 20 concentrations above about 3% were diluted with deionized water to a xanthan concentration of 0.75%, heated for 5 minutes at a temperature of 85°C.and diluted to lOcps. viscosity. The results are summarized as follows: Broth Number Heat Treatment 1.2 micron Filter Ratio Activity Enhancement 1 - 1.22 + 1.15 +56% 2 - 1.12 + 1.13 +67 3 - 1.12 + 1.22 +67 Broth Number 1.2 micron Heat Treatment Filter Ratio Activity Enhancement 4 - 1.12 1.07 +67 5 - 1.16 + 1.08 +67 6 - 1.27 + 1.16 +57 7 - 1.21 + 1.11 +67 8 - 1.22 + 1.09 +84 9 - 2.11 4- 1.31 +67 10 - 1.28 + 1.10 +67 11 - 1.37 + 1.05 +84 12 - 1.13 + 1.04 +78 Average - 1.28 1.13 +70 EXAMPLE 4 A sample of whole Xanthomonas fermentation broth having an initial 0. 65 micron FR on 2.59 was treated using the invention to improve its viscosity and mobility characteristics. A 1% solution of xanthan in 500 ppm salt solution (10:1 Na:Ca) was heated to 85°C. for 5 minutes, then cooled to room temperature, diluted to 0.14% using the same salt solution, 5600 ppm diatomaceous earth bodyaid was added, and the heat-treated diluted solution was filtered at room temperature on a precoated pressure leaf filter. The sparkling filtrate exhibited a 40% activity enhancement and an improved 1.71 filter ratio at 0.65 microns filter pore size. - 14 EXAMPLE 5 A sample of whole Xanthomonas fermentation broth which initially plugged a 0.45 micron filter, was treated to improve its viscosity and injectivity characteristics. The broth was diluted to IS with 500 ppm salt solution, heated to 85°C and then diluted to 0.14% by adding 500 ppm salt solution at 85°C., containing sufficient diatomaceous earth bodyaid to speed subsequent filtrations. The heattreated solution was then filtered on a precoated pressure leaf filter at about 85°C. and a flux of 13.1 gal/hr ft2 obtained. Total time at 85°C. was about 20 minutes. The sparkling filtrate exhibited an activity enhancement in excess of 18% and improved injectivity as demonstrated by filter ratios of 1.94 at 0.45 micron and 1.23 at 0.65 micron pore size.

Claims (6)

1. CLAIMS:1. A process for preparing an oil recovery mobility control solution having activity enhancement as hereinbefore defined of greater than 15% and a filter ratio as hereinbefore defined of less than 3 through a filter with a pore size of 1.2 microns which process comprises heating a fermentation broth of Xanthamonas biopolymer substantially free of insoluble matter having a particle size greater than about 3 microns, at an equivalent xanthan concentration of 0.05 to 2.0% and a salt content of less than 0.2% for a period of from 2 to 60 minutes at a temperature of 6098°C., and when the equivalent xanthan concentration exceeds 3000 ppm diluting the solution to an equivalent xanthan concentration of from 100 to 3000 ppm.

2. The process of claim 1 wherein said heating is carried out for a period of from 5 to 20 minutes at a temperature of 80-98°C.

3. The process of claim 1 or claim 2 wherein said dilution is effected with water having a salt content of at least 0.6%.

4. A process as claimed in claim 1 which comprises diluting whole Xanthomonas fermentation broth substantially free of insoluble matter having a particle size greater than about 3 microns, to an equivalent xanthan concentration of 0.14-1.

5. % with water having a salt content of less than 0.2%; heating said broth for a period of from 2 to 60 minutes at a temperature of 77-98°C; filtering said broth and diluting to an equivalent xanthan concentration of from 100 to 3000 ppm when the equivalent xanthan concentration exceeds 3000 ppm. - 16 5. The process carried out at a of claim 4 wherein the filtration is temperature of from 77 to 98°C.

6. The process of claim 4 wherein the dilution of said heat-treated solution is effected with water having a salt content of at least 0.6%.