WO2016167666A1

WO2016167666A1 - Improved oil recovery by pressure pulses

Info

Publication number: WO2016167666A1
Application number: PCT/NO2016/050068
Authority: WO
Inventors: Svein Hestevik
Original assignee: Resonator As
Priority date: 2015-04-15
Filing date: 2016-04-14
Publication date: 2016-10-20

Abstract

Method and device for improved recovery of oil from subterranean oil reservoirs, by application of pressure pulses to at least part of the oil producing reservoir. The method comprises positioning at least one reciprocating machine (ReM) disposed in a fluid filled well penetrating the reservoir and generating pressure pulses by activating said ReM. The method is typically conducted in a pressurized injection well with the at least one ReM arranged close to the surface of the well.

Description

IMPROVED OIL RECOVERY BY PRESSURE PULSES

The present invention concerns a method for improved oil recovery as defined by the preamble of claim 1. According to another aspect the present invention concerns a device for improved oil recovery as defined by the preamble of claim 15. Background

For optimal recovery of oil from subterranean formations or reservoirs, a number of challenges have to be handled.

When the parts of the oil that is readily and immediately recoverable have been produced, significant amounts of oil are still present in the reservoir but tend to stick to rock surfaces in the formation due to adhesion forces.

In general, the recovery factor for an oil reservoir is limited by the residual oil trapped in the pore system because after a successful water flood the amount of oil left is too small to make a continuous oil system. The continuous water flows easily through pores and around oil regions. Although the flowing water gives a small pressure gradient in the separated oil bodies, the capillary forces between oil and water prevent them from moving. The problem is that the pressure gradient, typically around 0.1 mbar/mm, gives too small pressure differences across the small oil bodies to overcome the capillary forces that keep them stationary.

The recovery factor for a hydrocarbon reservoir is one of the most important parameters for the oil industry. It is defined as the amount of hydrocarbons produced divided by the amount of hydrocarbons initially present in the reservoir. The amount of hydrocarbons is measured in different ways, as the total volume of hydrocarbons, the amount of gas and/or oil measured separately, or their energy equivalent; the energy produced by combustion. The standard volume measure of oil and gas at the surface is done at standard atmospheric conditions. For calculations of expected production and recovery factors, the oil and gas in the reservoir it is usually also calculated as their volume at atmospheric conditions. This may be quite different from their volume in the reservoir, especially for gas. However, for calculations of the flow of oil and gas in the reservoir, their actual volume in the reservoir must be used.

The volume division of hydrocarbons into oil and gas is still not straight forward, because some of the oil in the reservoir may turn into gas at surface conditions, while some of the gas may turn into oil. This depends also upon the surface processing equipment, for instance will use of only one oil- gas separator turn more of the oil into gas than using several separators in series. Usually three separators in series are used on production platforms, while due to space and weight limitations only one is used on drill rigs for testing exploratory wells.

There are three main reasons why the recovery factor may be small:

1 Decreasing reservoir pressure to the point where profitable production stops. This can be, and often is, countered by pumping down cheap fluid, usually sterilized seawater.

2 A part of the reservoir is not produced to the existing wells, for instance due to barriers to flow (low permeability areas). This can be countered by drilling more wells, both to produce from (partly) isolated areas, and to inject fluids to push oil towards the producing wells.

3 The pore system in the reservoir makes it virtually impossible to flush out all the oil in these pores with water, or other fluids that do not mix with oil on a molecular level. The main reason is that oil and water are immiscible; oil in water will be present as drops of various sizes. There is always some water left in the reservoir; at least about 13% of the pore space is filled with water. This is the initial water in the hydrocarbon reservoir. The reason for the presence of this water is that all porous spaces in rocks below seas, or below the water table level on land, were filled completely with water. The hydrocarbon is generated from organic matter deposited in clay, and this clay will not turn into reservoir rock. In time the oil and gas generated break up the rock and flow out of it and into water filled reservoir rocks and is trapped there. Most of the water is pushed out, but never all of it. This is equivalent to the impossibility of pushing all the oil out of reservoir rock by flushing it with water.

For water-wet rock this initial water fills all the small spaces, cracks and dead ends. This initial irreducible water is discontinuous, so it will not flow, however large the pressure gradient driving liquid flow through the reservoir. For water wet rock the oil is filling the larger spaces and flow readily through the pore system along any pressure gradient. But when sufficient water has invaded the pore system the oil that is left breaks up into drops that flow with the water until it hits an opening that is to small to let it pass, and is stuck there. At least up to 20% of the oil initially present may be left in the rock due to this, however long the water injection is continued. The producing wells will then end up producing only water.

For oil wet rock the initial water is broken up into drops in the oil that fills all the pore spaces, including the small spaces, cracks and dead ends. This initial irreducible water is discontinuous because the water drops are separated by pores too small to allow the water drops in, so it will not flow, however large the pressure gradient driving liquid flow through the reservoir. For this oil- wet rock the oil is continuous, sharing the larger pore spaces with water drops, but also filling the smaller spaces and is connected through all the small pores. The oil therefore flows readily through the pore system along any pressure gradient. But when sufficient water has invaded the pore system the oil that is left breaks up and fills only the cracks and dead ends, and some of the small pores, but is no longer connected throughout the pore system. At least up to 20% of the oil initially present may be left in the rock due to this, however long the water injection is continued. The producing wells will then end up producing only water.

Increasing water flow rate through the rock increases the pressure gradient and will increase slightly the oil produced, but this increase is very small. In addition it is impossible to generate really large pressure gradients throughout the reservoir rock. Actual pressure difference between injection and producing wells may be up to 50 bars, but with a realistic distance between these well with at least about 100 m, this give a maximum pressure gradient of 0.5 bars per meter. In lab experiments with rock core samples of about 20 cm in length, this gives a pressure difference of 0.1 bars across the sample, corresponding to a water column of one meter. At present there are a number of methods to increase oil production. The preferred method depends upon type of hydrocarbons present and availability of resources. Briefly, the most used methods are:

1. Increasing sweep efficiency by either drilling more wells or decreasing the viscosity of oil.

Decreasing viscosity of oil increases the flow rate for a given pressure gradient. Increasing flow rate is increasing total oil production because then it is profitable to keep on production more oil, as termination of production depends on flow rate, see section 1.1 above. Also, if oil viscosity is reduced as compared to that of water, there are fewer tendencies for the oil- water displacement front to become unstable. The water then tends to finger through the oil towards the production wells, resulting in increased water production. Decreasing oil viscosity is often done by heating the oil, either by pumping down, through injection wells, air that will ignite the oil, so the burning oil heats the rest of the oil, or by pumping down heat, usually as hot water steam. This is only economical for heavy, high viscosity oil. It is then a standard method, as energy is readily available in the oil produced, or even better, in any gas produced from the separators.

Decreasing oil viscosity relatively to that of water can be done by increasing the viscosity of injected water by adding polymers to it. Since most of the reservoir pore volume must be filled with water and polymers this is, however, quite expensive, especially for high temperature reservoirs. For these reservoirs the availability of stable polymers is quite restricted.

2. Decreasing the interface tension between oil and injected water. This makes it easier to push oil drops through narrow pores. The main problem doing this is that deforming an oil drop from a sphere to a stretched out string fitting in the narrow pore requires an increase in the oil drop surface. Since the surface energy is proportional to surface area, this demands a certain energy, and the only energy available is from the pressure gradient. Reducing interface tension reduces this surface energy. Adding surfactants to the injected water strongly reduces surface tension between oil and water, but a large amount of surfactants is needed, especially as the surfactants tend to become stuck to the rock surface in the pore system. Due to the cost this is seldom an economical method. The forces generated by interface energies, especially the surface energies at the interface between fluid and solids are often referred to as capillary forces. 3. Injection and displacement of oil by a fluid that is miscible with oil. All the oil in the reservoir will then dissolve in the injected fluid and be carried along with it. These fluids are, however, not water based and often more expensive than the oil produced. Since the injected fluid will be left in the reservoir when all the oil is produced this is then obviously not economical. But for some types of oil and for certain temperature and pressure ranges carbon dioxide can be used. Oil reservoirs not too far away from cheap sources of C0₂ may use this method profitably. Cheap sources of C0₂ may be factories where C0₂ is a waste product, or natural C0₂ wells. This may also permanently remove some C0₂ from the atmosphere, since the reservoir will be filled with C0₂ when all the oil has been produced (this will not be the case for C0₂ wells).

Another measure attempted has been to apply vibrations to the formation in order to allow remainder oil amounts to be recovered.

CA patent 1 096 298 (1975) teaches a method for applying vibrations to a formation in which fluid is allowed to flow through and around a number of coaxially arranged cylindrical elements that are localized in a wellbore or the like of the formation.

US patent No. 4 884 634 (1986) teaches a method in which oil recovery is attempted increased by generating vibrations close to the resonant frequency of the formation, the means suggested for achieving the vibrations being to inject a metallic fluid into the formation and apply electric energy to at least two electrodes positioned in separate wells penetrating the same formation. This method is intended to also increase the temperature in the formation which may contribute positively to the oil recovery.

From GB patent N. 2 257 184 is known a method for improved oil recovery from a subterranean formation in which elastic waves or vibrations are applied simultaneously with electric vibrations to thereby reduce adhesion forces between oil and water. It is emphasized that the electric stimulation will increase the temperature and also the pressure in the formation. The oil produced allegedly removes additional heat from the reservoir and thus allows more energy to be applied than what would else have been the case. US patent No. 5 460 223 concerns a method and an apparatus for improved oil recovery from a formation penetrated by at least two horizontally penetrating wells running at different vertical levels. The apparatus is intended to be arranged in the lowermost of the two horizontal wells to stimulate recovery of oil from the uppermost of said wells.

US patent 8 113 278 teaches a method for stimulating oil recovery by use of vibrations, in which in situ generated seismic energy is applied by the use of an hydraulic, seismic pressure wave source. The generator produces low frequency (sound) waves and comprises an accumulator, a pump, energy transferring section, rotor and stator as well as a pressure transferring valve. The waves are apparently generated by building and releasing pressure.

US 2002/ 00070017 describes yet another variant of vibration based stimulation of oil recovery from reservoir. The genuine feature seems to be application of a plurality of vibration generators operating at different or overlapping frequencies. The different generators can run simultaneously or in series.

US patent No. 6 499 536 (2002) describes a method and equipment for same purpose as the ones previously discussed, in which magnetic or magnetostrictive material is injected and brought to vibrate by se of alternating electric fields.

From U 2 168 006 is described a method for similar purpose in which a vibrator is connected at the well head and is mechanically connected to the piping system so as to generate vibrations I the range of 20 to 1000 Hz. A treatment of 2 days before start-up of production is indicated. It is well known in the art also to inject chemicals into reservoirs using pumps, such as piston pumps, providing a pressure fluctuation in the reservoir according to the rate of the pump action. Such a pump is described in US 2009/0200018.

The methods and apparatuses discussed above may function well for some kind of oil wells and for some time without causing problems, but they all have flaws or disadvantages such as need for use of particular, expensive materials (metallic/ magnetic etc.) or materials that should not be left behind in nature, the need for plural wells localized adjacent to one another, use of complicated or sensitive equipment with a significant number of valves or other movable parts, use of flowing materials which requires use of pumps and space demanding receptacles etc. Thus there is still a need for improvement in this area and particularly a need for simpler equipment which can be used in virtually all wells, not limited by orientation (vertical/ declined/ horizontal) or the presence of closely adjacent wells. There is a need for equipment that has low requirements with regard to maintenance and long service-life under extreme conditions.

Objectives It is an object of the present invention to provide a method and an apparatus which overcomes the deficiencies of the prior art.

More specifically it is an object of the invention to provide a method and a device for improved oil recovery from a subterranean formation or reservoir which is more versatile and less sensitive to the external environment than existing methods and apparatuses; in particular a method an a device which require little maintenance and experiences few operating interruptions.

The present invention

The above stated objectives are achieved by means of the method according to the present invention as defined by claim 1.

According to another aspect the present invention also concerns an apparatus as defined by claim 15.

Preferred embodiments of the inventions are disclosed by the dependent claims.

An explanation of the underlying mechanisms is given hereunder, without limiting the scope of the invention to a specific theory. By "reciprocating machine" as used herein is understood a reciprocating machine which is typically electrically powered. The machine may be powered solely by electricity, but may also include hydraulic components for conveying movement.

By "hermetically sealed" is understood a cover or barrier that is continuous, fluid-tight and not dependent upon packers to prevent leakage or fluid intrusion.

It should be noted that the terms "vibrations" and "pressure pulses" have been used interchangeably in the present description and it is not the intent to distinguish between these terms.

While injection wells typically are filled with pressurized water containing additives of different kinds, different media may be used to pressurize the wells, included gases under such a pressure that they for most practical purposes behave like liquids. Pressurized C0₂ is one example of such a fluid, under most conditions a gas, but which may also serve to transmit pressure waves in a well when subjected to sufficiently high pressure. Wherever the term "pressurized fluid" is used herein, this shall be considered to contemplate water as well as pressurized fluids like C0₂ when under such a pressure that it has the ability to transmitting pressure waves.

By inducing low to high frequency pressure variations of sufficiently large energy in the injected water flow, the peak pressure differences across the isolated oil drops are much higher than by any possible pressure gradient in a continuous flow, and will mobilize some of the residual oil towards the production wells. When hydrocarbons are produced, the pore pressure in the reservoir decreases, resulting in a decrease in the maximum possible rate of production.

Interfacial tension is the energy per area unit needed to form or increase a contact surface between two different substances that do not mix. The interfacial tension represents a small amount of energy per area unit, it is therefore still often given in the old physical unit dynes per square centimeters,

The reason why drops of oil surrounded by water are spherical is that, for any given volume of oil, the sphere has the least possible surface. Deforming a spherical drop into any other shape increases its surface and changes its surface energy. This additional energy has to be supplied from an external source to deform the drop. Deforming an oil drop is necessary in order to force it into a channel with a diameter that in any direction is smaller than the spherical diameter of the drop. Deforming oil drops by adding energy is therefore a possible means for improving oil recovery. One possible external energy source is a source of vibration of the fluid surrounding the drop. These vibrations will periodically deform the drop shape.

Another possibility to get an oil drop into a narrow channel is to divide into several drops. This will increase the total surface of the first, big drop. Dividing it into drops of equal sizes, the surface of each drop decreases with the square of the reduced diameter, but the volume is reduced with the third power of the diameter, and the number of drops therefore increases in inverse proportion to the single drop volume.

Interfacial tension is also found between liquids and solids, and is measured as the energy per unit area needed to increase the contact area between the fluid and the solid. If this energy per area unit is small, there is a tendency for the contact area to increase; the fluid is wetting the solid. If the fluid is an oil drop in water on the solid surface of rock the shape of the oil drop is determined by the minimum surface energy in the three contact areas, between oil and water, between oil and rock, and between water and rock.

From these general observations, mathematical models have been developed to estimate the required energy for conducting a process with use of vibrations to improve oil recovery for specific oil formations. The present invention, however, simply relates to some specific principles of conducting such a process, not mathematical conditions or limitations therefore. The true behavior of oil drops moving in the direction from an injection well to a production well, forced by surfactant containing flooding water and simulated by vibrations can only be determined empirically. Such effects have been investigated over a couple of decades and have generally been found to be positive and significant in the sense of contributing to an improved degree of oil recovery. Thus, while the general principle and the expected effects of providing vibrations to an oil formation is well known, the manner with which the vibrations are supplied as herein described, is not.

The reciprocating electric machine (ReM) used to generate the vibrations according to the present invention is typically positioned at the top of the well, near the surface in the case of an onshore well and near the seafloor in the case of an offshore well, and may preferably be positioned in direct connection with the well head. Positioning of at least one ReM at the top of the well does not exclude the possibility of positioning and activating additional ReMs in other locations, requiring use of run-in equipment to properly position such additional ReMs in the well in question. Generally ReMs can be used in any number and in any number of wells. The ReMs preferably have hermetically sealed outer housings and do not, with the exceptions specifically mentioned herein, comprise any external moving parts. They are therefore more resistant/ less sensitive to the external environment than most other equipment used for similar purposes, hereunder less influenced by impacts, dirt, high or varying temperatures, corrosive fluids, and the like.

The ReMs may be run into the wells by different means, such as with the use of e-lines, pipeline tractors (internal conduit vehicles), coiled tubings, or electrical umbilicals. During deployment each ReM may be accompanied by weight bars or the like to facilitate the run-in and also by equipment allowing the exact position of the ReM to be controlled, such as Casing Collar Locator (CCL) units or the like.

The present invention provides a means for allowing vibrations to be generated in injection fluids typically water with additions of surfactants or other additives, during production, in connection with injection wells, without interrupting or disturbing the injection- or recovery process, with comparatively inexpensive means which require little maintenance and which are well protected against the tough environment typically predominant in an oil formation under production.

In applying the vibrations to and through a non-compressive fluid, the vibrations are effectively spread through the fluid and into the formation, thus providing the desired breakdown of adhesion forces between the oil and the solids in the formation generally encountered with vibration stimulated oil production. Below the present invention is described in further detail in the form of some non-limiting embodiments given with reference to enclosed drawings, in which:

Figure 1 is a schematic illustration of an oil field containing a number of injections wells and a productions well.

Figure 2 is a side sectional view of the area between an injection well and the production well during oil recovery.

Figure 3A is a side sectional view of one of the injection wells of Figure 1 with an implementation of the present invention.

Figure 3B is a side sectional view of one of the injection wells of Figure 1 with another implementation of the present invention.

Figures 4A-D are schematic illustrations of embodiments of a reciprocal machine suitable for use with the present invention. Figures 5A is a schematic illustration of a different embodiment of a eM suitable for use with the present invention, and Figure 5B is an example of use of a particular variant thereof.

Figures 6 A, B, C are schematic illustrations of particular use of an embodiment of a ReM suitable for use with the present invention.

Figures 7A, B, C, are schematic illustrations of particular use of another embodiment of a ReM suitable for use with the present invention.

Figures 8A, 8B are schematic views of pressure pulse propagation obtained according to the present invention.

Figures 9A, 9B are schematic views of pressure variations inherent with use of the present invention.

Figure 1 shows schematically a top view of an oil field 11 provided with five injections wells 12_x - 12₅ and a production well 13. The direction of flow of injected fluid is indicated by stapled arrows. Two of the injection wells are provided with an apparatus or device 14 in the form of a ReM according to the present invention. Any number of injection wells could, however, be provided with such apparatus or device 14.

Figure 2 is a side sectional view of the formation between one injection well 12₂ and the production well 13 having a mainly vertical portion 13v and a deviating or mainly horizontal portion 13h. The production well 13 extends first vertically through a number of non-bearing layers of rock and deviates to a mainly horizontal part 13h in an oil-bearing stratum. A ReM 14 is arranged near the injection head of the injection well 12₂. Arrows with dotted lines indicate different zones in the formation, a mobile oil zone 21, an intermediate zone 22, and a pressurized, water saturated zone 23. The latter is fully saturated with aqueous injection fluid and is, during production, constantly subjected for pressure pulses from the ReM 14.

Arrows with full-drawn lines close to the injection well indicate how the aqueous injection fluid, injected through injection well 12₂, is spread vertically while moving generally from right to left in the drawing. Pressure pulses propagating from a device arranged at the well head of the injection well, in close contact with the aqueous injection fluid, will propagate all the way to the barrier where the aqueous fluid meets part of the formation still being oil moist rather than water moist, i.e. to the pressurized, water saturated zone 23 shown in Figure 2. The more vertical, full-drawn arrows in the moving oil zone 21 indicate how released oil from the formation flows to the horizontal part of the production well to be produced therefrom to the surface. The combined action of the static pressure of the injection fluid and the pressure pulses from the ReM, contributes to releasing oil drops from the formation and to the general movement from right to left in drawing 2 of the zones 21-23, also indicated as "frontal advance" in Figure 2.

Figure 3A shows schematically and simplified a side sectional view of an arbitrary injection well 12, at which the present invention is implemented. The main feature shown by this figure is the positioning of the one ReM 14, at or near the surface where it easily can be monitored, serviced, and if required replaced. The positioning of the ReM 14, immersed in the non-compressible column of aqueous injection fluid, ensures that the pressure fluctuations are effectively conveyed throughout the parts of the reservoir saturated by the injection fluid. The double arrows pointing to the right from the well indicate in a generalized manner how the vibrations propagate from the well. A person skilled in the art will understand that the vibrations have the form of pressure fluctuations that may spread out in all directions from the well, not just in one direction. The pressure fluctuations spread out through the rocks of the formation as well as through the liquids in the formation.

While Figure 3A is simplified and may give the impression that the pressure fluctuations generated by the ReM 14 are substantially uniform and horizontal, the situation will typically be more complex as elaborated below, in particular in the discussion of Figures 8A and 8B.

Figure 3B shows an embodiment slightly different from that of Figure 3A. One difference is that the device used for formation of pressure waves is represented by three different ReMs, 14i_₃, not just a single one. The issue of main importance in using plural ReMs is that they are tuned for different frequencies. It is difficult to make a ReM of a type suitable for environments of the kind relevant for the present invention, able to operate at varying frequencies; therefore intermittent use of two or more ReMs operating at fixed, but different, frequencies, is a viable and practical option to the idealized continuous sweep through a broad frequency range.

When the ReMs according to the present invention is arranged in a channel constituting a kind of shunt in relation to the well, for a certain extension, as shown in Figure 3B, the connection of the ReMs to the well may be said to be shunted, i.e. arranged in a channel shunted to the well. For some embodiments such shunted connection of the ReMs to the well is preferred.

Figure 4A shows a side view of an embodiment of a ReM useful with the present invention. It comprises a main body 41, cooling ribs 42 and stabilizing fins 43. The longitudinal axis L is indicated by a dotted line. Figure 4B shows generally the same as Figure 4A, but from a viewing angle 90 degrees different from that of Figure 4A, the difference visible only from the appearance of the stabilizing fins 43.

Figure 4C again shows the eM from the same angle as Figure 4A, the difference being that the outer body is partially removed, so that the internal elements thereof are partly shown, hereunder the reciprocating unit (motor) 44 and a chain-like structure 45 connected thereto and also suspended at the opposite end of the ReM, i.e. the end of the stabilizing fins. The chain-like structure 45 is immersed in a fluid, typically a mixture of liquid and gas. When the reciprocating unit 44 moves to the left, the chain 44 is pulled in the same direction and forces the fluid outwards and to the right. This causes a temporarily pressure build-up in the main-body of the machine. In all the embodiments of Figure 4A -4D the ReM will typically include at least one spring at ach end to cushion and stop the movement in the direction in question, temporarily storing the kinetic energy as potential energy and thereafter releasing the energy as kinetic energy in the opposite direction. This is a common feature of most ReMs. The springs may be mechanical springs, gas springs or a combination thereof.

If the outer body 46 of the ReM is at least partly comprised by e flexible material or "skin", this outer body 46 will temporarily move out to assume a slightly convex shape as a response to the pressure build-up, as shown by the dotted lines of Figure 4D. When the reciprocating unit 44 moves to the right, the chain like structure 45 is also pulled to the right by its suspension at the right side end. During this phase of operation there are two alternative scenarios. One is a repetition of the action taking place during the movement to the left, i.e. the fluid surrounding the chain-like structure is again forced outwards and in a movement opposite to the chain-like structure, again causing the "skin" material to shortly assume a convex shape. The other scenario is one in which fluid is allowed to flow through the chain-like structure when it moves to the right, due to an arrangement functioning like check-valves arranged at the links of the chain-like structure. In such a case there is no pressure build-up when the chain-like structure returns to its right-most position and the "skin" of the machine remains at rest. This latter variant may be preferable over the first one due to the fact that it allows more time for the skin to resume its original shape before next stroke is generated.

It is worth noticing that all embodiments of Figure 4A - 4D concerns use of an electric reciprocating machine which allows the use of a sealed, liquid tight, continuous outer holster, herein referred to as a hermetically sealed construction, not relying upon seals or packers to allow the internal mechanics and electronics being isolated from the surrounding environment. The sealed, liquid tight continuous holster is typically made in a flexible synthetic resin material which, after being subjected to a suitable heat treatment, constitutes a seamless holster.

Under both the scenarios described above there will be a repetitive, pulsating action of the outer material or skin of the eM. When the ReM is localized in a fluid, and in particular a mainly incompressible fluid, such a pulsating volume change of the ReM will generate a vibration in the surrounding fluid with the same frequency as the frequency of the movement of the reciprocating unit 44 (alternatively with a frequency equal to the double of the frequency of the reciprocating unit 44). In this manner a vibrating pulse may be generated from the ReM without use of expensive materials (metallic/ magnetic etc.) or materials that should not be left behind in nature, without the need for plural wells localized closely together, without, use of complicated or sensitive equipment with a significant number of valves or other movable parts, and without use of separate flowing materials (other than the injection fluid) which would have required use of pumps and space demanding receptacles or the like.

Figure 5A shows an embodiment, designated 14₂, of a ReM different from that of figures 4A-D. The outer, general shape may be identical, and is shown as identical. In the embodiment of Figure 5, the outer body of the ReM is provided with a rib structure 51 of slightly flexible material in which on side of the ribs are generally flat while the other side has a curved or tapered part close to the outer periphery of the ribs. If not suspended to any external structure, this embodiment of the ReM would "swim" in the direction to the right as shown in the drawing. When suspended, this embodiment of the ReM sets up pressure pulses mainly along the direction of the arrows shown in Figure 5, The advantage of the embodiment according to Figure 5 over the embodiment of Figs. 4A-D is that there is no need for a chain-like structure, with or without valve-like elements, which may be more subjected to wear and/ or need for replacement than the simpler rib- structure. However, this embodiment of the ReM may beneficially be suspended to a membrane like structure in order to provide a better momentum for generating pressure pulses in the fluid in which it is suspended.

Figure 5B shows a particular variant of the embodiment of the ReM shown in Figure 5A. Here, one of the ribs, 51L of the rib structure 51, is enlarged so as to typically cover the entire cross-section of a conduit into which the ReM is intended to be suspended by means of brackets 52. The conduit in question may typically be a shunted passage in relation to an injection well. With the brackets rigidly attached, the "swimming" action of the ReM in operation will generate pressure pulses in the conduit and in turn also in the injection well. Figures 6A-C show schematically how a ReM 14₂ of the type illustrated in Figure 5A may beneficially be suspended to a membrane 61, which may be manufactured in any flexible material able to endure the conditions in a well. In Figure 6A the ReM is in its neutral position and the membrane 61 is at rest and therefore constitutes a plane surface. As shown by Figure 6B, the ReM 142 will pull on the membrane 61 when moving to the right, thus causing the membrane to assume a convex right side, building up a slight pressure increase on the right hand side of the membrane and a corresponding pressure decrease to the left of the membrane. When the ReM changes direction and moves to the left, as shown in Figure 6C, the pressure buildup is to the left of the membrane and the pressure reduction is to the right hand side of the membrane 61. When the ReM moves back and forth at a set or variable frequency, pressure pulses or fluctuations are generated with the same frequency or frequencies. The ReM 14 indicated in Figure 3A may typically in practice be an assembly of a ReM and such a membrane as shown in Figures 6A-C. It should be understood that the membrane 61 is typically suspended across a conduit associated with an injection well, similar to the embodiment of Figure 5B, within a pressurized fluid in said well.

Figures 7A-C show schematically more or less the same as Figures 6A-C, but using another type of ReM. In the case illustrates by Figures 7A-C, a ReM 14₃ is used which comprises a hammer member 141 directly or indirectly connected to or being part of the reciprocating unit (motor) of the ReM. The hammer member 141 protrudes from the ReM 14₃ housing which is suspended from a rigid support 72 which in turn is secured to ground or to a fixed installation (not shown). A membrane 71 constituting an uninterrupted surface is used for this embodiment. The membrane is suspended with desired tension within a well fluid typically at an injection wall, as in the previous embodiment. In the step shown in Figure 7A, the hammer member 141 is moving outwards (to the left) and is about to hit the membrane which is in a neutral, even position. Figure 7B shows a situation after the hammer member 141 has made an impact to the membrane and created a pressure pulse in the well. The membrane is at this point displaced, exhibiting a convex left surface and a concave right surface. The hammer member 141, connected directly or indirectly to the reciprocating member of the ReM is now retracting and no longer in contact with the membrane which also has started moving back towards its neutral positon. Figure 7C shows the initial phase of the next stroke, quite similar to the situation shown in Figure 7A, but with the membrane slightly overcompensated as a result of the previous stroke, i.e. the membrane is shown slightly convex on its right side. It should be emphasized that the exact position of the membrane at the time of next hit by the hammer member 141 is not important. It is, however, of importance that the frequency of the reciprocating motion of the eM 14₃ is adapted to the resonant frequency of the membrane 71, or vice versa, ensuring that the membrane 71 is somewhere near its natural, unbiased position each time it receives an impact by the hammer member 141. If the hammer member 141 is not directly connected to the reciprocating unit, a mechanical spring, gas spring or a combination thereof may be arranged to retract the hammer member after each blow. Gas springs are preferred for some embodiments for enduring tougher conditions than mechanic springs.

It should be noted that while the embodiments according to Figures 4 to 6 includes ReMs with no external moving parts, the embodiment according to Figure 7 is different in this respect.

While Figure 3A only indicates that pressure fluctuations propagate outwards from the well, Figures 8A and 8B show more in detail how pressure pulses are spread from distinct perforations in a well casing. Even these figures are simplified in the sense that only two level of perforations are encountered, while in practice there may be many. The principle of interest, however, is well seen from these figures. In Figure 8A pressure pulses are generated from above at a fixed frequency, Fl, e.g. a frequency of 120 Hz. Such a low frequency is generally preferred over a higher one due to the fact that attenuation of the vibrations or pressure pulses as a function of distance from the origin of the pressure pulses is higher for high frequency vibrations than for low frequency vibrations. This is well known in the art. In a preferred embodiment, frequencies chosen for the present invention are in the range 1 to 1000 Hz and more preferred in the range 10 to 100 Hz.

The pressure fluctuations are transmitted from the well through the perforations, illustrated as PI and P2. The pressure pulses from these seemingly different sources collide and interfere with each other. When the frequency is constant there will be zones or directions in which the pressure maximum from PI meets a pressure maximum from P2. In these areas or directions the pulses add to each other and form an increased pressure maximum.

In other directions pressure maximum from PI will coincide with pressure minimum from D2. In these directions the pulses will neutralize one another and there will be no apparent pressure fluctuation. This is naturally not a desirable situation as there will be little or no effect of the generated pressure pulses in these directions. However, there is a simple measure that may be taken to counteract this undesired effect, namely to consistently change the frequency of the generated pressure pulses. Figure 8B shows a situation in which the generated frequency, F2; has been changed (reduced) by about 12.5 %. This moderate frequency change leads to a complete change of the directions in which the maximum and minimum amplitudes of the pressure pulses are found.

Thus, changing the frequency of the generated pressure pulses, intermittently or continuously within comparatively narrow boundaries, constitutes a preferred embodiment, since it will counteract the negative effects of interference ("quiet zones") between pulses leaving different perforations of a well casing, or more generally interference between different apparent vibrations sources. By "different apparent vibration sources" is understood real, different vibration sources, such as different ReMs arranged in a well, but also just seemingly different vibration sources, such as pressure pulses from one ReM leaving two or more perforations in a well casing.

While changing the frequency between two or three fixed values may be sufficient to reduce the problem, a consistent frequency sweep between an upper value and a lower value may be preferred, e.g. a sweep in the range from about 10 Hz to about 100 Hz.

However, as discussed above, continuous frequency sweeps are not viable with the most relevant equipment for performing the method according to the present invention. Instead, intermittent use of alternating frequencies may be applied using an assembly of two or more ReMs having different resonant frequencies, thereby alleviating the potential problems of "quiet zones" within the formation subjected to the treatment. Again, for simplicity the pressure pulses in Figures 7A and 7B are only drawn to the right hand side of the well, while in practice the pulses will propagate in all directions from the perforations in the well casing.

Figures 9A and B show the general effect of the pressure pulses to a pressurized formation. While the basic steady pressure in the formation may be e.g. 10 bars, illustrated by Figure 9A, the fluctuation of the pressure may typically be in the magnitude of +/- 0.5 bars as illustrated by Figure 9B. This is found to be sufficient to expel a surplus amount of oil that is well worth the effort thereby required. It is preferred from energy consideration that the pressure fluctuations generated according to the present invention, in some embodiments are less than +/- 1-5 bars.

Common for all embodiments is the feature that all moving parts are effectively shielded from the environment by the outer material of the ReM and thus not subjected to dirt, corrosive fluids or the like. A common feature for the kind of ReMs to be used herein the reciprocating unit 44 which is known per se. Typically it comprises a piston which is arranged to move back and forth in a mainly closed cylinder, the walls of which being provided with at least one coil of an electric conductor which again is connected to an AC power source. When alternating current is flowing through the coil, the piston moves back and forth by the ever changing forces acting on the permanent magnets as the current in the coil changes direction. Different features may be included to retard the movement of the piston towards the end of each stroke to avoid overburdening of the components of the machine. Since such machines are generally known, it is not described in further detail here.

Specifically preferred embodiment

It is a preferred embodiment of the present invention to positioning the reciprocating machine (ReM) close to the surface of the well, and in particular in direct connection with a wellhead.

While other principles of operation may be employed, it is preferred a principle selected among electric operation, hydraulic operation and electro-hydraulic operation. For the time being, electric operation is the most preferred principle of operation. This also allows use of ReMs with no externally moving parts, which is also preferred. The ReM should be sealed so as to be fluid-tight under well conditions by any manner known in the business. Specifically preferred is the use of an ReM which has an hermetically sealed outer housing.

The term "reciprocating machine" is not intended to cover machines having functionality of adding material to a well, such as pumps.

The method of the invention is typically conducted in a pressurized well, more typical pressurized injection well, and most typically during ongoing recovery or production from the we

Claims

1. Method for improved recovery of oil from subterranean oil reservoirs by application of pressure pulses to at least part of the oil producing reservoir, characterized in positioning at least one reciprocating machine (hereinafter ReM) disposed in a fluid filled well penetrating the reservoir and generating pressure pulses in the fluid by activating said ReM.

2. Method as claimed in claim 1, wherein positioning said ReM close to the surface of the well.

3. Method as claimed in claim 1 or 2, wherein positioning said ReM in direct connection with the wellhead.

4. Method as claimed in any one of the preceding claims, wherein using a ReM which is operated in a manner selected among electric operation, hydraulic operation and electro-hydraulic operation.

5. Method as claimed in any one of the preceding claims, wherein using a ReM which is sealed so as to be fluid-tight under well conditions.

6. Method as claimed in any one of the preceding claims, wherein using a ReM which has hermetically sealed outer housing.

7. Method as claimed in any one of the preceding claims, wherein generating the pressure pulses in a pressurized well.

8. Method as claimed in any one of the preceding claims, wherein generating the pressure pulses in a pressurized injection well.

9. Method as claimed in any ones of the preceding claims, wherein generating pressure pulses simultaneously with the oil production.

10. Method as claimed in any one of the preceding claims, wherein using a ReM which has no external moving parts.

11. Method as claimed in any one of the preceding claims, wherein deploying the ReM by means of equipment selected among the group consisting of an e-line, an internal conduit vehicle, a coiled tubing, an electrical umbilical, and a wire-line.

12. Method as claimed in any one of the preceding claims, wherein applying pressure pulses at different frequencies to reduce or eliminate undesired effects of interference between different apparent vibration sources.

13. Method as claimed in claim 12, wherein applying pressure pulses at different frequencies involves intermittent use of at least two eMs having different resonant frequencies.

14. Method as claimed in claim 12 or 13, wherein two or more frequencies are applied intermittently, selected within a range from 1 to 1000 Hz, more preferably within a range from 10 to 100 Hz.

15. Device for improved recovery of oil from subterranean oil reservoirs by application of pressure pulses to at least part of the oil producing reservoir, comprising a vibration generator, power supply to said vibration generator and deployment means for the vibration generator, characterized in that the vibration generator is a reciprocating machine (ReM).

16. Device as claimed in any one of the preceding claims, wherein the ReM is suspended in a flexible sheet material acting like a membrane.

17. Device as claimed in any one of claims 15-16, wherein the device is designed to generate a pressure fluctuation of at least +/- 0.5 bars.