A METHOD FOR INCREASING RECOVERY OF HYDROCARBONS

Abstract: It has been aimed at disclosing a method which, in relation to prior art, ensures increased recovery of hydrocarbons from natural reservoirs (16, 18, 20) and where knowledge of reservoir conditions such as e.g. pressure and temperature has been provided. Based on these known reservoir conditions, hydrocarbon components in the form of light hydrocarbons which will appear in liquid phase at said reservoir conditions are provided and being injected in large amounts into the reservoir, e.g. within an area in an oil zone, beneath producing wells (22). By means of these light hydrocarbon components, the respective reservoir area is washed out, in that the components dissolve and attack themselves to and entrain washed out residual oil and/or other remaining heavier hydrocarbons for recovery. As the reservoir is gradually drained of hydrocarbons, the hydrocarbon liquid injection point/points is/are moved closer to the production well(s) (22), preferably concurrent with water being replenished.

Description: A METHOD FOR INCREASING RECOVERY OF HYDROCARBONS This invention relates to a method for increasing recovery of hydrocarbons from natural reservoirs. It has been aimed at increasing the hydrocarbon recovery substantially in relation to known recovery methods. The invention consists in increasing the degree of hydrocarbon recovery in an arbitrary reservoir consisting of substantially liquid phase hydrocarbons. The degree of recovery is the ratio of the amount of hydrocarbons recovered to the total amount of hydrocarbons existing in the reservoir. A reservoir is generally defined as a porous and permeable rock which, according to its nature, contains liquids and/or gases such as natural gas, oil, water, etc. The reservoir is that volume in the subsurface (the formation) in which hydrocarbons have gathered and from which one produces or brings up hydrocarbons to the surface for further utilization. The hydrocarbons appear in the natural reservoirs in pores in the rock itself. The actual formation may consist of various rock types, but to a certain degree it will always be permeable, i.e. to a larger or smaller extent it permits a flow of hydrocarbons in gaseous or liquid phase. When producing a reservoir, this involves a drainage of hydrocarbons. This drainage takes place by drilling wells down into the reservoir. Using a lower pressure within the well (s) than the ambient pressure in the surrounding reservoir, a pressure gradient will develop and cause hydrocarbons in liquid and/or gaseous phase to flow toward the well (s) and into it (them) . During production from a reservoir, one will in many cases utilize various methods for maintaining the pressure within the reservoir will often be utilized. Injection (pumping down) of water or gas, e.g. through wells in surrounding parts of the reservoir, is an example of traditional technology in order to maintain the pressure in the reservoir at a desired level. Otherwise, the reservoir pressure would normally decrease in accordance with the recovery of hydrocarbons, and this will restrict the recovery. In practice, the degree of recovery may vary from less than 20% and up to 70%, depending on the physical properties of the reservoir and the hydrocarbons. A number of methods exist for increasing the degree of recovery. The method of choice depends on which method that can be defended based on economical criteria, but the use of one or more of these methods will increase the degree of recovery from a reservoir relative to what otherwise would be achievable. Amongst the presently used methods for improved hydrocarbon recovery, methods such as combined gas/water injection, injection of foam, polymer gels and surface-active agents are used. Other methods used are e.g. the microbial method and so-called "gas flooding". Likewise, reference is made to Ruth, 1992-1995, A 5 Norwegian Research Program on Improved Oil Recovery Program Summary, Norwegian Petroleum Directorate - Stavanger 1996. The costs associated with the various known hydrocarbon recovery- improving techniques are often high, and the concurrent effect of the individually known technique is o limited. Within economically profitable criteria the interesting question is always how much additional hydrocarbons may be recovered from the reservoir by employing a known technique and how much of the hydrocarbons which, nevertheless, will remain unused. By using the techniques s available today, and within an economically sustainable framework, the present answers are that normally between 30 and 60% of the oil will remain in the subsurface. As mentioned, the known techniques are substantially based on maintaining the reservoir pressure at the highest possible o level by injecting water and/or gas into the reservoir or adjacent areas. Also, it may become necessary to use chemicals in one form or another or other mechanisms in order to drive more oil out of the reservoir. Serious disadvantages of the individually known techniques 5 relating to significantly improving the degree of hydrocarbon recovery are that water injection leaves behind a large amount of trapped hydrocarbons within the pores of a permeable rock after the water injected therein has been driven past the oil accumulations. This is due to the fact o that water does not mix with oil under natural conditions, and that the heavier oil provides surface tensions causing it not to penetrate the small rock pores as easily as water does . Upon gas injection, the gas will flow too easily and rapidly through the reservoir, and it carries only small amounts of residual oil up to the surface. This is also due to the fact that the gas does not dissolve or bind the heavier hydrocarbon components. The terms heavy and light hydrocarbons, respectively, denotes how volatile the individual components are. Upon injection of chemicals, a limited effect is also achieved, especially as seen in relation to the costs of using the subject amounts of chemicals, and the problems chemicals create at a subsequent stage when they, possibly, are returned to the production plant. Likewise, combinations of two or more of the known techniques have been tried, and this will, obviously, somewhat improve the degree of recovery but, nevertheless, marginally within an economically sustainable totality. Thus, known methods for increasing the degree of hydrocarbon recovery in a given reservoir, provide deficiencies, disadvantages and restrictions of use, and it is the object of the present invention to relieve or, substantially, reduce these deficiencies, disadvantages and restrictions of use and, thus, disclose a new method for increasing the degree of hydrocarbon recovery within a given reservoir in a cost efficient way. The method according to the invention is to be utilized in new producible oil fields and constitute the basis for a total optimalization of the field and enable an economical return and resource utilization hitherto not possible. Likewise, the method is usable and utilizable in old fields for increasing oil production, life time and total economical yield as compared to the previous . In a method of the kind as defined in the preamble of claim 1, said objects are realized through proceeding in a way as defined in the characterizing clause of claim 1. A natural hydrocarbon reservoir contains hydrocarbons of various components within the so-called methane group. The lightest hydrocarbon components are gaseous, the somewhat heavier components appear in liquid phase, while the heaviest components appear in the form of asphalt or tar. The distribution of the various components vary greatly from discovery to discovery. It may relate to everything from the more or less pure gas reservoir to those reservoirs merely containing heavy oil components. Gas, gas condensate and oil reservoirs differ in that they contain different compositions of the various hydrocarbon components such as methane, ethane, propane, butane, pentane, hexane, heptane, octane and so forth, down to the very heaviest components on the methane scale. Also, hydrocarbon components possess the property that they can appear in liquid or gaseous form, depending on temperature and pressure and which mixture proportion exists between the various components. Upon certain pressure and temperature conditions within a reservoir, the moderately heavy hydrocarbon components on the methane scale will be in liquid balance together with the heavier and heaviest components. These inherent properties make these components well suited to bind to and to dissolve the heavier hydrocarbon components. Generally, it is such that the lighter components of hydrocarbons dissolve heavier components effectively. A known phase diagram, see figure 7, shows which phase the various components exist at different pressures and temperatures. Components appearing adjacent to one another in the phase diagram, mix easily, and lighter components may be used to effectively dissolve and mix with heavier components. These properties may be utilized to dissolve and clean out the heavier fractions of hydrocarbons in a reservoir which is not producible by means of conventional technique. Figure 1 illustrates the water displacement of residual fractions of the heavier hydrocarbon components in the reservoir by using conventional technique. Figure Id shows that the residual volume opposes further volume reduction due to strong surface tensions. Thus, the residual oil becomes confined and can no longer pass through the small pores in the formation. Additionally, the formations may have a reduced water imbibition property, which causes the oil to more easily attach to the formation grains. Figure 2 illustrates displacement of residual components when using a hydrocarbon liquid containing a larger proportion of lighter hydrocarbon components than the residual oil. Figure 2b shows that the hydrocarbon liquid dissolves the residual oil, and in figure 2c, residual oil attached to the formation grains will gradually become dissolved. Finally, in figure 2d, the remaining hydrocarbon liquid will become a mixture of dissolved residual oil in the injected liquid. When a sufficient amount of dissolving hydrocarbon liquid has been injected and the residual oil is dissolved in this liquid, the water displacement may start. As the property of the mixture between the residual oil and the hydrocarbon liquid injected has been changed in relation to a case where only residual oil had remained, the water displacement will more readily take place. Figures 3c and 3d illustrate this course . By using the lighter components in this way to dissolve and mix with the heavier components, one will gradually obtain a liquid exhibiting lower surface tensions and a lower viscosity. Accordingly and gradually, all the heavier components have been washed out and, finally, a liquid remains which is easily displaceable by means of water. At the end, water is injected in such a way that it forces out the light components. Thereby and practically speaking, one has achieved a complete drainage of hydrocarbons from the reservoir. For this purpose, one uses an adapted mixture of the lighter components of hydrocarbons being liquid at the ambient reservoir conditions for the actual cleaning task. The actual liquid used will be characterized in that it consists of hydrocarbon components included in the methane group; it has a smaller fraction of the heaviest hydrocarbon components than the original reservoir content (the crude oil) ; it has a critical temperature which is higher than the reservoir temperature, and it should be relatively free of asphalt and wax. As an example, it will in many cases be possible to select a liquid consisting of components ranging from butane to heptane. Ordinarily, the liquid will consist of hydrocarbon components included in a primary group and in a secondary group. Here, the primary group is defined to comprise hydrocarbon components having a carbon number from C-2 (ethane) to C-16. These components contribute mostly to the desired dissolution of the residual oil. The secondary group is comprised of both lighter components than ethane, i.e. having a carbon number C-l, but also those components having a carbon number larger than C-16, i.e. C-17 plus. The secondary group is considered to have a lesser dissolving effect, but it is, nevertheless, to a certain degree necessary in order to give the liquid the other desired properties such as e.g. specific gravity, surface tension and critical pressure. It is not possible to make a specific instruction in that the liquid used must be optimized for each individual reservoir and thereupon based on tests, laboratory experiments and reservoir analyses. This liquid is circulated throughout the reservoir, thereby washing out the heavier hydrocarbon components. A liquid possessing the appropriate properties and existing in the necessary amounts for the purpose, may be provided in various ways. The production plant may have a processing plant in which the desired components are separated into a separate product flow, or they can be transported in from a neighbouring field. The hydrocarbon liquid may be provided through processing in such way that one reduces the amount of the heaviest gas components, or by reducing the amount of the lightest gas components, or both, by other means changing the portion of hydrocarbon components such that the desired property of the liquid is obtained. The desired hydrocarbon mixture substitute a liquid which remains in liquid phase at reservoir conditions, but the overriding principle is, thus, the employment of the lighter components of hydrocarbons which, in liquid phase in the reservoir, dissolve and wash out the heavier components. Finally, one may use water in order to displace the last fractions of reservoir hydrocarbons which, then, substantially consist of the lighter components which are easily displaced by of water. The remaining amounts of reservoir hydrocarbons are significantly smaller than those remaining using present technology, in that a theoretic degree of recovery exceeding 90% might be achieved. Injection of water must be adapted such that the liquid to be displaced by the water, has a sufficiently low viscosity and surface tension to be effectively displaced by the water within the reservoir. These parameters are determined through laboratory experiments, reservoir analyses and a good reservoir control. In this way, the reservoir, may be washed gradually until it for all practical purposes is empty of oil, through a controlled and optimized process where adapted light hydrocarbon components are circulated down, in one or more steps . According to the invention, injection wells are drilled, as known per se, to one or more specific areas of the reservoir, and where the specified liquid for washing loose remaining hydrocarbon fractions, e.g. oil, on the pore walls, is injected through the injection wells. As the reservoir in this way is gradually drained of hydrocarbons, replenishing water is according to the invention pumped to a water zone positioned e.g. at a lowermost or at another level within the reservoir, and, then, the water only has to displace the injected light liquid in the water zone. The ever present liquid in the reservoir, and which is used in the dissolution and washing out process, is gradually recovered as the reservoir is drained of hydrocarbons and replenished by water. For the liquid used in the method according to the invention to have the intended dissolving/washing out effect, it must be present in sufficient amounts and in a mixture that ensures that it appears in liquid phase at reservoir conditions, i.e. at the ambient pressure and the temperature within the reservoir area(s) into which the subject liquid is injected. Likewise, the liquid must have an optimal dissolving effect on the remaining hydrocarbons within the pores of the reservoir. In other words, the dissolving property of the liquid is to generate said washing out effect, and the washing out effect will i.a. result from the composition of the liquid, the special constituents and the viscosity thereof. Ordinarily, it will be appropriate to use moderately volatile hydrocarbons in the methane series from butane to heptane as injection liquid. Often, wells for injection of the liquid will already be available and may e.g. be water injection wells or old boreholes which merely need to be completed for injection of liquid in the zone of interest. This necessitates the preparation of reservoir drainage strategies based on reservoir- and well analyses and simulations founded on the technique described. It is desirable to have a processing plant positioned at installation of interest which is adapted to this draining strategy and the demands made to the processing of the particular well flow. Injection liquid and injection arrangements must also be provided. Non-restricting practical examples constituting various aspects of the method according to the invention, are explained in the following and with reference to the attached drawings, wherein: Figures 5 and 6 schematically show vertical sections through i.a. a formation enclosing a reservoir having an overlying gas cap, an intermediate producing oil zone, and an underlying water zone. Above the seabed, sea water follows and, above the surface thereof, an offshore installation is located, and where: Figure 5 represents a principle drawing, illustrating improved oil production in which a liquid possessing the previously described properties for injection into the lowermost layer of the oil zone in order to dissolve remaining amounts of oil, is used; Figure 6 shows an example of how one may replenish water and recomplete for injection of liquid into a new layer of the oil zone as oil is produced from the reservoir. In figures 5 and 6, reference number 10 denotes a drilling/ production platform where platform deck and rig are positioned above sea level. The seabed is denoted as 12. Obviously, the method may also be used in onshore oil fields. The underlying formation 14 consists of one or more porous, permeable rock types, wherein, in the present case, an upper gas cap 16, an underlying, more voluminous oil zone 18 being confined at its lowermost position by an underlying water zone 20, have developed. Between the platform 10 and an oil well 22 leading to an oil producing layer in the oil zone 18 extends an oil production riser string 24, and between the platform 10 and a gas well 26 within the gas cap 16 extends a riser string 28 for injecting gas. A third pipe string 30 from the platform 10 leads to a borehole 32 constituting an injection well for liquid in accordance with the present invention. A fourth pipe string 34 from the platform 10 leads to a borehole 36 constituting an injection well for water, which leads to the water zone 20. It appears from figure 5 that the liquid injection well 32 leads to a layer or horizontal area within the oil zone 18 spaced vertically below the layer or horizontal area to which the oil well 22 leads. Figure 6 substantially shows the same as figure 1, but in this situation, and relative to figure 5, some oil has been produced and, thus, a corresponding amount of oil has been washed out of the reservoir. Therefore, in figure 6, water has been replenished in an underlying zone 38, simultaneous with the injection well 32 being completed at a higher level 40 within the oil zone 18. Upon injection of the liquid, which at reservoir conditions exists in liquid phase, as a washing and dissolving liquid for desired recovery of residual oil, which oil cannot be produced by means of known methods, the underlying water zone 38,20 will advantageously influence the course of events, wherein the liquid is prevented from penetrating downwardly, and wherein the water zone 38,20 simultaneously ensures that the hydrocarbons will flow upwardly towards the producing ell(s) 22 in the oil zone 18. Concurrent with the flow of the injected fluid towards the production wells, the liquid will drive heavier oil fractions in front of itself and/or dissolve the same and, thereby, entraining this oil. An entraining effect is established and maintained, wherein the injected liquid acts as an entraining fluid for i.a. residual oil which has escaped normal recovery methods. Simultaneously, or at a subsequent point of time, water is injected into the water zone 38,20 through pipe string 34,36, causing the water surface 42, figure 6, to rise and push the liquid in front of itself towards the production wells 22. As production in this manner is carried out from the reservoir, one will, at any time, maintain the production by plugging, recompleting and drilling new wells (production- and injection wells) . According to the needs and relative to a timely optimal reservoir control and drainage, one may extend/shorten existing boreholes; i.e. mainly in the same way as in present ordinary production. In particular and upon completing recovery of oil and other hydrocarbons in older reservoirs, it may, in accordance with the present invention, be appropriate to mix the current hydrocarbon liquid into the water injection flow, and dissolve and wash out residual oil with a mixture of this liquid and water. This is especially advantageous in older reservoirs wherein large fractions of the reservoir already have been filled with water, but wherein and nevertheless, large amounts of residual oil are present within the volume now constituting the water zone. In such cases, it may be of interest to inject the liquid or a mixture of the liquid and water into the water zone 38,20 and re-permeate by washing the water zone to recover residual oil . Figure 4 shows how the hydrocarbon liquid is used for "sweeping" residual oil within reservoirs which already have been filled with water and abandoned, fig. 4a. The injected hydrocarbon liquid engages with the residual oil thereby becoming dissolved, fig. 4b. Finally, fig. 4c, the liquid will have dissolved and become bound to this residual oil, which will become entrained in the water flow up to the platform. Such a platform will be provided with separators, and both the liquid and residual oil in such a water stream can be separated from the water in hydrocyclones or other processing equipment. The liquid dissolving and binding itself to residual oil or other hydrocarbons, being recovered to the platform, may be effectively treated through processing and separated for recovery of the residual oil and, possibly, to bring back the liquid to its pristine state, so that it, may be reinjected.

Claims: C l a i m s 1. A method for increasing recovery of hydrocarbons from reservoir by injecting hydrocarbons, c h a r a c t e r i z e d i n that the hydrocarbons injected exist in liquid phase within the reservoir and are suited to dissolve remaining hydrocarbons in the reservoir, and that the recovery, preferably, is terminated through filling the reservoir with water which displaces the majority of the injected and dissolved hydrocarbons . 2. A method as defined in claim 1, c h a r a c t e r i z e d i n that the hydrocarbon components for washing out the reservoir are recovered and regained as the reservoir gradually is drained of hydrocarbons and replenished with water. 3. A method as defined in claim 1 and 2, and especially in final recovery of oil and other heavier hydrocarbons in older reservoirs, in particular in old reservoirs where large fractions of the reservoir have been filled with water, c h a r a c t e r i z e d i n that a mixture of the hydrocarbon liquid and water or only the hydrocarbon liquid is injected into the reservoir. 4. A method as defined in claim 1 or 2 , c h a r a c t e r i z e d i n that the hydrocarbon liquid is injected into an oil zone (18) , below the production well (s) (22). 5. A method as defined in claim 1 or 2 , c h a r a c t e r i z e d i n that the hydrocarbon liquid is injected into an oil zone (18) , above the production well (s) (22) . 6. A method as defined in claim 1 or 3 , c h a r a c t e r i z e d i n that the hydrocarbon liquid or a mixture of the hydrocarbon liquid and water is injected into the water zone (38,20) in the reservoir (16,18,20) . 7. Use of moderately volatile hydrocarbons in the methane series as injection liquid during recovery of hydrocarbons from natural reservoirs.

Inventors: LUNDE HELGE, LUNDE, HELGE
Applicants: LUNDE HELGE, LUNDE, HELGE
Application year: 2000
Main classification: E21B43
IPC codes: E21B43