GB2255360A - Method for the production of oil - Google Patents

Abstract

A method for conformance control of a reservoir where high formation temperatures are encountered comprises the steps of (a) injecting at ambient temperature into the formation a high temperature selective gel-forming composition comprising an aqueous solution of a polymer or copolymer of acrylamide and an effective amount of a cross-linking agent comprising a di- or higher valent metal cation and a chelating an ion, (b) allowing the solution to flow through a zone or zones of relatively high permeability in the formation under increasing temperature conditions, and (c) allowing the solution to gel in situ at a location where the temperature is sufficient to promote gelation.

Description

METHOD FOR THE PRODUCTION OF OIL This invention relates to an improved method for the recovery of crude oil from a petroleum reservoir.

It is rare to obtain in practice a waterflood sweep efficiency approaching theoretical perfection. Usually significant mobile oil still remains in the reservoir when the producing watercut exceeds the economic limit (usually at a WOR of about 50:1 for individual wells). There are several reasons for this. Geological layering leads to poor vertical sweep conformance. Depositional history can also lead to non-uniform areal sweep, which can be a problem unless counteracted by offset injector/producer alignment. Vertical and areal sweep efficiencies can both be adversely affected by natural or induced fractures. Other factors include water underrunning because of gravity segregation and coning, particularly of bottom aquifers.

During water flooding of petroleum reservoirs, the injection fluid tends to find the easiest (ie, the most permeable) route from an injection well to a production well. These high permeability routes can be the result of reservoir fracturing, prolonged waterflooding or reservoir geology (eg, faults, or the nature of the rock matrix.

In the case of deep wells, where the producing formation is at a relatively high temperature, eg 50 to 200"C, cold water injection produces a temperature gradient across the formation. The injection of cold water has a cooling effect in the vicinity of the injection well and for some distance beyond it.

To enhance reservoir conformance control, ie, to mobilise oil which may be present in less permeable areas, some form of blocking agent can be injected. This creates an obstruction in the high permeability channels which causes the lower permeability paths to appear more favourable to a subsequent flood. Blocking agents fall into three main categories: polymers, foams and emulsions, (water-in-oil (w/o) and oil-in-water (o/w)).

Typical of the polymer system is polyacrylamide which is generally used in conjunction with a cross-linking agent to thicken the gel after it has been deposited in the formation.

United States Patent No. 4,569,393 discloses a water permeability correction process to improve the sweep efficiency of waterflooding which involves the sequential injection of a solution containing a sequestered polyvalent metal cation, such as aluminium citrate, and a polymer solution containing a gelable polymer, such as polyacrylamide, followed by the injection of carbon dioixde to decrease the pH of the polymer which activates the delayed in situ gelation thereof.

Suitable acrylamide-based polymers are stated to be disclosed in USP 3,749,172. Particularly preferred are partially hydrolysed polyacrylamides possessing pendant carboxylic groups through which cross-linking can take place. USP 3,749,172 states that all the polymers useful in the practice of its invention are characterised by high molecular weight. With this proviso, the molecular weight is not critical so-long as the polymer has water-dispersible properties. It is preferred that the polymers have a molecular weight of at least 100,000. The upper limit is unimportant. Thus, polymers having molecular weights as high as 20,000,000 or higher can be used.

The polymers are used in the form of aqueous solutions, the polymer concentration generally being of the order of 500-10,000 ppm, more usually about 1,000-2,500 ppm.

USP 4974677 discloses a process for selectively plugging zones of high permeability within an oil-bearing subterranean formation.

The process comprises the steps of injecting into the formation a high temperature selective gel forming composition comprising an aqueous solution of high molecular weight polyacrylamide having 30 percent or less hydrolysis and an effective amount of a crosslinking agent which is a mixture of a phenolic component and an aldehyde and gelling the solution in-situ by thermally reacting said gel-forming composition at a temperature effective to promote gelation. The resultant gel is stated to be exceptionally thermally stable and, therefore, can be used as an effective profile control agent in all thermal enhanced oil recovery operations, including steam flooding.

According to the present invention there is provided a method for conformance control of a reservoir where high formation temperatures are encountered, which method comprises the steps of (a) injecting at ambient temperature into the formation a high temperature selective gel-forming composition comprising an aqueous solution of a polymer or copolymer of acrylamide and an effective amount of a cross-linking agent comprising a di- or higher valent metal cation and a chelating anion, (b) allowing the solution to flow through a zone or zones of relatively high permeability in the formation under increasing temperature conditions, and (c) allowing the solution to gel in situ at a location where the temperature is sufficient to promote gelation.

Examples of polymeric materials for use in making the temperature selective gel-forming compositions required in the practice of the present invention include polyacrylamides and polyacrylamide based copolymers, eg acrylamide - acrylate copolymers. Polyacrylamide based copolymers may be prepared by partially hydrolysing polyacrylamide or by copolymerising acrylamide with an acrylate. Preferably the acrylate content does not exceed 30%. Most preferably it is less than 20%.

The molecular weight of polyacrylamide or polyacrylamide copolymer should be at least 10,000 to be effective, preferably in the range 0.5 million to 20 million and most preferably in the range 1 to about 15 million. High molecular weight materials should be avoided as they may cause difficulties in polymer solubilisation, pumping and injection of the gel-forming composition due to inherently high viscosity.

The preferred polymer is a medium molecular weight copolymer of acrylamide and an alkali metal salt of acrylic acid, most preferably containing 5 to 15% by weight anionic content (acrylate).

The optimum anionic (acrylate) content is believed to be in the range 10 to 14% for deployment in cold sea water. Lower acrylate contents can lead to somewhat higher adsorption losses and to less effective porous medium blocking, while acrylate contents above 20% can lead to the chemical system becoming unstable over time in sea water.

The crosslinking agents are di- or higher valent metal cations which are effective for gelling the selected polymer when the aqueous admixture is within the temperature gelation range.

Suitable metal ions include Fe2+, Fe3+, A13+, Ti4+, Zn2+, Sn4+, Ca2+, Mg2+, and Cr3+. The preferred ion is Al3+.

The metal ions are employed in the form of complexes with chelating anions. Suitable chelating anions include acetate, nitrilotriacetate, tartrate, tripolyphosphate, metaphosphate, gluconate, phosphate, and, preferably, citrate. The chelating agent retards the onset and rate of gelation of the polymer. The molar ratio of metal cation to chelating anion is suitably in the range of 1:2 to 6:1, preferably 1:1 to 4:1.

Conveniently, the polymer can be dispersed in a given amount of injection water and to it then added the desired amounts of a solution or dispersion of the chelated polyvalent metal cation cross-linking agent. The amount of cross-linking agent used depends largely on the amounts of polymer solution. Lesser amounts of polymer require lesser amounts of cross-linking agent, and vice-versa. Further, it has been found that for a given concentration of polymer that increasing the amount of cross-linking agent generally substantially increases the rate of cross-linking.

After admixing with the aqueous solution of the cross-linking agent, the polymer concentration of fluid injected into the reservoir is suitably in the range 500 to 10,000 ppm, preferably in the range 500 to 2,500 ppm.

Suitably, the concentration of the cation of the cross-linking agent injected into the reservoir is in the range 100 to 1,500 ppm, preferably 250 to 750 ppm.

Some polymer adsorption will probably take place in the formation in the cooler zone before gelation occurs and if the cross-linking agent is present in the composition from the beginning of chemical injection then a proportion of cross-linking agent will be lost because of this adsorption effect.

This loss can be reduced by injecting first of all a small slug of the polymer alone (or any similar polymer). This will irreversibly satisfy most of the adsorption/retention capacity of the formation for polymer-type molecules.

It has been found that this decreases the adsorption of the cross-linking agent from a subsequently injected slug of polymer plus cross-linking agent. This is believed to occur because the cross-linking agent and the polymer are associated in solution even before the final gelation occurs and the adsorption sites for the polymer and associated cross-linking agent are already filled by pre-adsorbed polymer.

The injected composition gives little viscosity increase before gelling and moves freely into the more permeable zones of the formation, known as thief zones.

The present invention takes advantage of the temperature gradient in a reservoir in the following way. First a low molecular weight polymer and a crosslinker are selected which essentially do not give blocking in the porous matrix in the cold, but do give blocking if heated. These are both co-injected into the waterflood at low concentrations for a short period only. Most of the dissolved chemicals enter the thief zones. They have to follow the major established water paths wherever they go. Injectivity is not significantly changed because viscosity is not significantly affected. The chemicals quickly catch up with the cold front, and begin to block the thief zones. The follow up water begins to be diverted out of the thief zones and into the neighbouring zones from upstream of the blockages. Of course, some chemicals will be injected into the lower permeability zones, but not much.

Generally, even these chemicals are lost by adsorption and dispersion long before they get to the hot zone. The net result is that more water passes through the lower permeability zones, and more oil and less water are delivered to the producers.

The invention is illustrated with reference to the following Example.

Example 1 This example illustrates gelation using a temperature gradient. A sand packed slimtube of non-ferrous metal was used.

This was to avoid possible difficulties from ferric ion crosslinking, given the high surface to volume ratio of the slimtube.

The tube was 63m long with an internal diameter of 6mm and the long experiment duration (up to four months). It had a permeability up to 5 Darcies and was pressure tapped along its length. The reservoir temperature gradient was qualitatively represented by having the upstream end of the slimtube in a temperature controlled laboratory (at 250C) and the downstream end in a thermostat. The slimtube used had 5 cool sections, each 10 metres long, then a one metre warming section, and then 4 hot sections at 70"C, each 3 metres long. The hot sections could be held at any selected temperature between 25"C and reservoir temperature.In this experiment the hot sections were held at 70"C. A relevant reservoir flow rate of 0.2 metre/day in the thief zone was used during waterflooding to residual oil saturation and during the gel test.

The polymer used was a medium molecular weight copolymer of acrylamide and sodium acrylate containing 14% of by weight of the anionic polymer sold under the trade name Alcoflood 935(S) from Allied Colloids (Bradford, UK). The cross-linking agent used was aluminium citrate sold by Magnablend (Dallas, Texas) in the form of a solution containing 3% by weight aluminium.

The formation water used was a moderate salinity brine (30,000 TDS). Oil was well head crude from Kuparuk (North Slope, Alaska) and Forties (North Sea) reservoirs. Waterflood and gel flood used either Dorset (UK) seawater or a synthetic equivalent from BDH (Poole, Dorset). Tracers used included the chloride level (it was seen that during waterflooding seawater always completely displaced all formation water still present after oil flooding) and lithium as lithium chloride. Formaldehyde (500 ppm) was used as biocide in the gel flood.

The blocking behaviour from continuous injection of the acrylamide/acrylate copolymer (700 ppm) and aluminium citrate (250 ppm Al) in the formation water is seen in the following Table.

Table Resistance Factor @ 42 days

Section Temperature OC Resistance Factor 1 25 2 2 25 2 3 25 2 4 25 2 5 25 2 6 2 7 70 2 8 70 (54*) 185 9 70 8 10 70 2 * 40 days Essentially the chemicals moved through the cool zones without blocking. When they reached some distance into the hot zone, they blocked it. Here resistance factor is the pressure drop across the section divided by the pressure drop seen previously to water flow through the same section at the end of the waterflood stage.

The injected solution gives little viscosity increase before gelling. On heating to temperatures in excess of 50"C the bulk solution becomes modestly visco-elastic, but still flows easily.

However, porous material is very strongly blocked by this gel formation, as the above example illustrates.

Example 2 This example illustrates the reduction in the adsorption loss of an aluminium cross-linking agent by using a pre-slug of polymer alone.

A sand packed slimtube similar to that described in Example 1, but of shorter length, 10m, was used.

Since most adsorption loss is expected during transit of the cool zones, the whole slimtube was maintained at 25"C.

As in Example 1, a solution in formation water of Alcoflood 935(5) (700 ppm) and aluminium citrate (250 ppm Al) was passed through the slimtube. The effluent was analysed for aluminium and the amount compared with the amount injected in order to evaluate the adsorption loss on the sand. The analysis was carried out by atomic absorption of the effluent.

The adsorption loss of aluminium was found to be 20 microgram Al per gram of sand.

The experiment was repeated with the difference that in this case a pre-slug of a similar but slightly lower molecular weight copolymer of acrylamide and sodium acrylate, Alcoflood 915(S) (1,000 ppm) in formation water in the absence of aluminium citrate was employed.

When the solution in formation water of Alcoflood 935(S), (700 ppm) and aluminium citrate (250 ppm) was subsequently passed through the slimtube, the adsorption loss of aluminium was reduced to 6 microgram Al per gram of sand.

Claims (10)

Claims

1. A method for conformance control of a reservoir where high formation temperatures are encountered, which method comprises the steps of (a) injecting at ambient temperature into the formation a high temperature selective gel-forming composition comprising an aqueous solution of a polymer or copolymer of acrylamide and an effective amount of a cross-linking agent comprising a di- or higher valent metal cation and a chelating anion, (b) allowing the solution to flow through a zone or zones of relatively high permeability in the formation under increasing temperature conditions, and (c) allowing the solution to gel in situ at a location where the temperature is sufficient to promote gelation.

2. A method according to claim 1 wherein the molecular weight of the polymer or copolymer is in the range 500,000 to 20,000,000.

3. A method according to either of the preceding claims wherein the polymer is a copolymer of acrylamide and an acrylate.

4. A method according to claim 3 wherein the copolymer contains 5 to 20% by weight acrylate.

5. A method according to any of the preceding claims wherein the cation of the cross-linking agent is aluminium.

6. A method according to any of the preceding claims wherein the chelating anion is citrate.

7. A method according to any of the preceding claims wherein the concentration of the polymer in the gel forming composition is in the range 500 to 10,000 ppm.

8. A method according to any of the preceding claims wherein the concentration of the cation of the cross-linking agent is in the range 100 to 1,500 ppm.

9. A method according to any of the preceding claims wherein a slug of an aqueous solution of a polymer or copolymer of acrylamide in the absence of a cross-linking agent is injected into the formation prior to the injection of the composition containing both the polymer or copolymer and the cross-linking agent.

10. A method according to claim 1 as hereinbefore described with reference to the examples.