WO2001012951A1

WO2001012951A1 - Recovery of oil

Info

Publication number: WO2001012951A1
Application number: PCT/GB2000/003169
Authority: WO
Inventors: Robert William Turnbull
Original assignee: CMA INTERNATIONAL Ltd
Current assignee: CMA INTERNATIONAL Ltd
Priority date: 1999-08-18
Filing date: 2000-08-16
Publication date: 2001-02-22
Anticipated expiration: 2002-02-18
Also published as: AU6585100A; GB9919495D0

Abstract

A method of improving recovery of oil from a subterranean oil bearing rock formation. Cellular organisms produced as a by-product of other industrial processes such as wastewater treatment or fermentation are injected into the formation along with water. The organisms may be pre-treated to obtain the desired size range for the formation concerned or to release beneficial surface-active or other components from within the organism into the bulk water stream. Under specific conditions the organisms may be digested or oxidised to generate gaseous products or to form synthetic oil within the formation. These processes, either singly or in combination, can be used to modify the injection water flow in the reservoir and to cause the disassociation of residual oil within the pore spaces of the formation thus enhancing the recovery of such oil.

Description

Title of invention: RECOVERY OF OIL

Background

In the process of oil production, water is frequently used as an aid to recover oil from the rock formation. The water may originate from subterranean, surface or sea water sources. If water is produced with the oil this water may be separated (termed as produced water) and re-injected into the producing formation. The water in used to displace or "sweep" the oil towards the producing well area. It also provides a convenient mechanism for maintaining the pressure within the reservoir to minimise gas expansion or the separation (break out) of gas from the oil. The water does not displace the oil completely, but leaves an element of the original oil in place behind (termed the irreducible oil saturation). The water may not form a uniform displacement or "flood" front within the reservoir and may bypass sections of the reservoir by preferentially flowing through more permeable sections. The amount of residual oil depends on the reservoir and oil properties, and the degree of control over the flooding process.

In order to overcome these problems, and improve the total recovery of the original oil in place, a variety of techniques have been developed over the years. These processes are often termed tertiary, improved or enhanced oil recovery processes. There are several basic techniques that are commonly employed. The flood front may be controlled by selectively plugging the larger flow channels within the reservoir rock thus providing a more even sweep of the oil, with less chance of by passing areas. Modifying the viscosity of the injection water may also control the efficiency of displacement. The interfacial tensions between the rock, oil and water may be changed by the addition of surface active materials to allow an easier release and flow of oil. The oil may also be removed by dissolving another material into the virgin oil thus reducing the viscosity of the oil. It is also possible to reduce the viscosity of the oil by generating or injecting a gas or other fluid that is miscible in the oil at the reservoir conditions. Gas may also be used as an injection fluid to move the oil. In several fields gas injection is alternated with water injection to enhance recovery (water alternating gas process). It is desired to build-up an accumulation or "bank" of oil with the displacement fluid sweeping this bank through the depleted formation to gather more oil.

Microbial organisms have been used for a long period of time to produce the effects noted in the preceding paragraph. The techniques used to date rely on growing either natural organisms within the formation by supplying nutrients, or by first seeding the formation with microbial organisms specifically chosen to produce the desired recovery mechanism. Such methods are covered and exemplified by patents such as US 05163510, US 0490576, US 05297625, US 04799545, US 04522261 and US 04558739. The basic drawback with most of these in situ techniques is the cost and complexity of maintaining living organisms in the reservoir formation.

Summary of the invention

Many industrial processes exist the where a by-product of the process is cellular organisms. These organisms are particularly common from the water treatment and fermentation industries. Such organisms have a variety of uses such as fertilisers and fuel. To utilise the cellular materials in these manners is often a costly exercise in that has a negative effect on the operating costs of the originating industry. The raw materials may therefor be obtained on a zero delivered payment or paid to remove basis. A wide range of organisms is available.

These organic organisms can, by judicious selection and treatment, be utilised in the recovery of oil. Several mechanisms of utilisation are available. These mechanisms may be used either singly or in sequential order within a given injection well to generate the desired recovery conditions. According to the present invention a process for the recovery of oil from a subterranean formation comprises the injection of cellular organisms, produced as a by-product from other industrial processes, in an aqueous stream.

In the simplest form of the invention the organisms may be either agglomerated or disintegrated to provide the desired size range to effect plugging of high permeability flow paths within the reservoir. The organisms may be dispersed by means such as mechanical mixing, milling, ultrasonics or chemicals. Alternatively, the organisms may be agglomerated or flocculated by chemical or other conventional agglomeration means. The degree off dispersion or agglomeration can be adjusted to suit the rock characteristics and the volume of flow desired through the rock matrix. The rate of plugging may also be controlled by the concentration of treated organic material, which is injected with the carrier fluid. It has also been demonstrated in core studies that degrading the organisms can effectively reverse such plugging by chemical or other common means. In another form of the invention the organisms may be mechanically or chemically altered in size or dispersed in a manner to allow transit through the reservoir and movement of residual oil. Where this is desirable hydraulic fracturing may be utilised to increase the effective injection area of the formation from any given wellbore.

Laboratory work with simulated reservoir rocks has demonstrated that the organisms can be accumulated on or in the rock matrix, or conveyed through the matrix with minimal retention.

The cell contents of the organisms can contain valuable components such as lipids, enzymes, surfactants and polymers that may be utilised to enhance the oil recovery process. By releasing these contents through disruption of the cell membrane by mechanical, electrical, chemical or other common methods these may be made available to the injection water stream. The amount of such beneficial materials depends on the industrial source of the organism. It is therefore desirable in some cases to blend organisms from diverse processes to maximise the amount of the desired chemical components contained within the organism or that are associated with the separated cellular material.

In addition to the mechanisms described above the cellular material is a source of carbon and hydrogen (organics). A variety of industrial processes and plants exist for utilising the organic content of such materials in order to produce gaseous products at a range of pressures and temperatures.

A common example of such a process is that of raw sewage sludge digestion to produce methane for fuel under anaerobic conditions. Wet air oxidation processes or processes using pure oxygen or oxygenates such as permanganates are available for the destruction of a wide range of organic substrates for effluent reduction purposes. Free radical oxidation processes, by the use of peroxides or ozone for example, are also common organic destruction techniques. In many of these processes the restriction on wider industrial utilisation is the energy required either heating and/or pressurising the reacting components. In an oil production environment the pressure and temperature of the formation can effectively provide a large component of the energy required. Additional heat is also frequently available from combusting non-saleable gas, or as recovered energy from power generation equipment such as gas turbine generators.

Waste heat may be recovered by conventional heat exchangers or by direct injection of exhaust gasses into the reservoir as in the water alternating gas processes described earlier. The contributions of Carbon Dioxide, Nitrogen and water vapour present in such exhaust gas streams can contribute to the enhancement process depending on the specific physical conditions and residual oils within the reservoir.

These processes may therefore be used in the subterranean environment to utilise the organic content of the materials that are injected to generate products useful in improving the recovery of the oil. The processes may be conducted on a continuous or batch basis. Large amounts of organic material may be placed in situ prior to effecting the degradation or oxidation process by means of creating hydraulic fractures in the reservoir formation and filling the fractures with the injected material. The degree to which these processes may be utilised depends on the specific reservoir conditions and the economics of the process.

Micro-organisms are a source of synthetic crude oil (syncrude) when subjected to conditions of temperature and pressure. Oil and gas fields originate from the pressure and temperature alteration of accumulated marine and terrestrial animal and plant matter. Several processes are available and described in the literature for the conversion of biological and other organic waste materials into synthetic crude oils. A number of the processes use oils or poly-aromatic solvents as an integral element of the conversion process. In the normal industrial environment the energy requirements and the difficulty in separating and recovering the solvent oil have limited development of these processes. In most industrial applications the processes are conducted at moderate temperature and low pressure with partially dried organic material, as this has found to be the most economic compromise. High- pressure reactor systems with added solvents are more applicable where the organic material has a high moisture content or is in a liquid form.

In the oilfield situation, suitable oils of the required aromatic nature are naturally present, as is a naturally sustainable level of temperature and pressure. These features are beneficial to the conversion of micro-organisms to syncrude. Typically a conversion of a material such as sewage sludge to syncrude would be of the order of 20%to 60% of the original organic matter. In the oilfield situation either direct heating of the injection fluid, or the recovery of waste heat may be readily available to boost the temperature of the near wellbore reaction zone if so desired. The utilisation of produced waters that already have an element of hydrocarbons present, as a part of the injection stream is advantageous for enhancing the contact of oil and organic material. Laboratory investigations have shown an affinity between biological cellular materials and reservoir hydrocarbon oils. Another method of syncrude generation is the direct volatilisation of organic materials by pyrolysis. This process is feasible, but not preferred due to the potential for the residual carbonised deposits to damage the permeability of the injection zone. In this scenario a batch wise operation is preferred using a waste exhaust gas as an energy source for organics pyrolysis. Condensation of the syncrude occurs deeper within the reservoir thus increasing the oil saturation and enhancing recovery of the residual oil. The advent of increased controls on combustion gas emissions to air may change the economics of such processes and the coking feature can be mitigated by using an alternating process, for example pyrolysis, followed by an oxidative de-coking process.

Preferred embodiment

A preferred embodiment of the invention relates to the use of sewage sludge either primary, secondary or mixed sludge as the source of cellular material. These sludges may be first cleaned to remove surplus grit, sand or other detritus by conventional separation technologies if the level of these materials is detrimental to the injection process.

Sewage sludges provide a relatively consistent and continuous high volume source of feedstock where the composition and content are well understood. These materials have been the subjects of much research and development in the areas of digestion and oxidative processes, which may be adapted for use in the downhole environment to produce the products desirable for enhancing the recovery of oil. A transportation infrastructure is generally available for the bulk handling of such materials either by road, sea or pipeline.

Conventional oilfield surface facilities and well designs can be readily utilised or easily adapted for the injection process. Examples

Experiments have been conducted using a variety of organic sewage derived materials and cores representative of oil reservoirs to investigate the flow and residual oil recovery characteristics of the materials. Conventional core testing equipment and practices were utilised for all experiments.

Typical results from some of the laboratory works are presented in the following text and summary table. All cores reported were .5 inches diameter and 3 inches long with a flat (sawn) face.

Trial 1 illustrates the performance of a raw sewage and utilised septic tank fluid as a source material. After acclimatising the core with water to establish clean performance, injection of the sludge gave rise to a consistent trend in injectivity decline. For a given permeability and pore throat diameter the decline is correlated to the concentration, type and size of solids present in the injected material.

Trial 2 used the same core as for trial 1 after the injection of a 10% hypochlorite solution to restore the permeability by degrading the residual organic solids. In this case the material comprised of a homogenised secondary sludge with a size commensurate with the septic tank material. The degree of impairment is increased due to the increased presence of wood fibres and grit compared to the septic tank material.

Trial 3 illustrates the effect on impairment of controlling the size of the material injected by homogenisation and the removal of grit by centrifugation in a core with a microfissure.

Trial 4 used an unfractured core with a primary sewage sludge that had been homogenised, but not degritted. The core was first water flooded with clean water, oil flooded until the residual water saturation was achieved and then flooded with water until the residual oil saturation was attained.

Subsequent to this procedure injection of a homogenised primary sewage sludge was commenced. This test was conducted at ambient conditions to simulate the worst conditions for plugging and residual oil recovery. The test therefor indicates the effect of solids and naturally occurring surface- active materials only and no effects from syncrude or gas production.

nd: not determined as oil droplets also present.

Claims

WO 01/12951 _ PCT/GBOO/03169Claims

1. A process for the recovery of oil from a subterranean formation comprising the injection of cellular organisms, produced as a by-product from other industrial processes, in an aqueous stream.

2. A process in accordance with claim 1 , wherein the injected material comprises of treated sewage preferably in the form of pre or post digested sludge.

3. A process in accordance with claim 1 or claim 2, wherein hydraulic fractures are induced in the reservoir formation to either create an increased area for injection of organic materials into the rock matrix or to provide a storage area foΛhe build-up of an organic filter cake.

4. A process in accordance with any of claims 1 to 3, wherein the cellular wall of the organisms is disrupted by mechanical, chemical or other means to allow the release of cell contents into the carrier aqueous stream.

5. A process in accordance with any of claims 1 to 4, wherein the organic material is pre-treated by mechanical, chemical or other means to control the degree of penetration within the oil bearing formation.

6. A process in accordance with any of claims 1 to 5, wherein the cellular material deployed in the reservoir is anaerobicaiy digested to release methane.

7. A process in accordance with any of claims 1 to 6, wherein the cellular organisms are oxidised by incorporating air, oxygen or oxidising materials into the injection system.

8. A process in accordance with any of claims 1 to 7, wherein the organic material is degraded under conditions of pressure and temperature to form a synthetic oil.

9. The process in accordance with any of claims 1 to 8, wherein the aqueous stream may be wholly or partly comprised of water produced from the reservoir containing hydrocarbons.

10. A process where after the placement of organic materials in accordance with any of claims 1 to 9, subsequent injection of hot gases of diverse origins are used to degrade the organic material.