US 3256935 A Abstract available in Claims available in Description (OCR text may contain errors) P 995011 XR 3 2569935 June 21, 1966 'w, NABQR ETAL 3,256,935 METHOD AND SYSTEM FOR PETROLEUM RECOVERY Filed March 21. 1963 a Sheets-Sheet 1 Q 13. -v-r' r? .|2 I2 I l2. 7 v a I u I f f FIG. I; GEORGE w. mason MOHAMED MORTADA 1 INVENTORS, E Y v V v WWI-6W I E ATTORNEY. r Mm June 21, 1966 'QwNABQR ETAL 3,256,935 METHOD AND SYSTEM FOR PETROLEUM RECOVERY i Filed March 21, 196: a sheets-sheet 2' 1i F *7 I 16 a :6 d I6 I f r 7 7 l6 l6 l6 l7 6 lg lg I5 I77 GEORGE w. mason MOHAMED MORTADA INVENTORS BYM 8W ATTORNEY June 21, 1966 s. w. NABOR ETAL 3,256,935 METHOD AND SYSTEM FOR PETROLEUM RECOVERY Filed March 21, 1963 u s sums-sheet s v use w.- mean oumcu MORTAOA INVENTGRQ- I MLQM ATTQRNEY United States Patent ice This invention relates to a method, and a system, for petroleum recovery from subterranean reservoirs. More particularly, it relates to the primary recovery of,petroleum from anisotropically permeable reservoirs. Many methods and systems for recovering petroleum from subterranean reservoirs are known and used. Generally, the rates and extents of petroleum recovery from isotropically permeable reservoirs by such methods and systems are satisfactory. One reason for this is the fact that the uniform flow of fluids in isotropically permeable reservoirs produces circular drainage patterns about each petroleum recovering or producing well. For this reason, isotropically permeable reservoirs are produced by means of a plurality of. wells disposed in regular, uni . form, geometric well patterns extending over the reservoir. In the geometric well patterns, the wells are cated in the reservoir in straight parallel lines or rows. The rows of wells are equally or uniformly spaced from one another. equally or uniformly spaced from one another. The magnitude or distance of the spacing between rows of wells, and between wells in a row,.will vary with the geometric well pattern used. Usually, the primary factor to be considered in determining the actual magnitude of the spacing between rows and wells in any geometric well pattern is the circular area to be drained by each well. Thus, as an example, each well may be placed on from about each 3 to about each 40 acres. There are many advantages in using geometric patterns of wells to produce isotropically permeable reservoirs. One advantage is that the wells may be readily located to cover the reservoir. Another advantage is that less wells are needed for optimum petroleum recovery. The primary basis of these advantages is that each well can continue producing petroleum at full production rates untilits circular drainage pattern expands sufliciently to meet or interfere with the drainage pattern of one or more of its neighboring production wells. Once interference between drainage patterns occurs, the production rates of'the concerned production wells decrease. However, with the regular, uniform, geometric well patterns, the areas of the reservoir not withinthe circular drainage patterns at the onset of interference are very small compared to the areas within the circular drainage patterns. This results in substantial and efficient recoveries of the available petroleum in the reservoir. It would be desirable 'to use the regular, uniform, geometric well patterns to recover petroleum from uniformly anisotropically permeable reservoirs. These are reservoirs in which the permeability varies according to direction. In these reservoirs, the fluids will flow through the formation in greater amounts in certain directions. As a result, the drainage pattern of each petroleum producing well will be noncircular. For this reason, the Further, the wells in each of the rows are Patented June 21, race initial interference to a reservoir with meability. It is therefore an object of the present invention to provide a method, and a system, for petroleum recovery from oil-bearing reservoirs. Another object of this invention is to recover petroleum from anisotropically permeable reservoirs. Another object of this invention is to use regular, uniform, geometric well patterns in recovering petroleum from anisotropically permeable reservoirs. Another object of this invention is to use the direcisotropic pertional permeability of an oil-bearing reservoir to increase the recovery of petroleum. Another object of this invention is to use regular, uniform, geometric well patterns for producing petroleum from anisotropically permeable reservoirs without an early decrease in production rates due to the permature onset of interference between the drainage patterns from adjacent wells. Another object is to use a regular, uniform, geometric well pattern for recovering petroleum from anisotropically permeable reservoirs wherein the production rates can be maintained for a longer period of time before being decreased by interference between the drainage patterns about the wells. These and other objects will become more apparent when read in conjunction with the following description and the attached drawings of one illustrative embodiment of the present invention, where in the drawings: FIGURE 1, FIGURE 2, and FIGURE 3 show several different regular, uniform, geometric well patterns; regular, uniform, geometric well patterns have failed to I producethe advantageous results in anisotropically per- FIGURE 4 shows the geometric well pattern of FIG- URE l in an isotropically permeable reservoir at the onset of interference between the well drainage patterns; FIGURE 5 shows the geometric well pattern of FIG- URE 4 at the onset of interference between well drainage patterns in areservoir having anisotropic permeability; and FIGURE 6 shows a geometric well pattern inaccordance with the present invention at the onset of interference of well drainage patterns in the anisotropically permeable reservoir of FIGURE 5. The method and system of this invention are provided by means of a plurality of petroleum producing wells arranged in a novel, regular, uniform, geometric well pattern. In the geometric well pattern the wells are arranged in several equally spaced parallel rows aligned with the direction of greatest permeability. The wells are disposed at a uniform spacing ineach row. The ratio of the distance between adjacent wells in each row to the perpendicular distance between adjacent rows is proportional to the square root of the ratio of permeability in the direction of greatest permeability to the least permeability. Referring now to the drawings, in FIGURES 1 through 5, there are shown known systems of wells for recovering petroleum from isotropically permeable reservoirs by means of wells'located in regular, uniform, geometric well patterns which are conventional. In FIGURE 1 there is shown a geometric well pattern which may be described as a rectangular or square pattern. The reservoir 11 is penetrated by a plurality of wells 12 which shown in FIGURE 4. 16 are arranged at an equal spacing d between adjacent wells 16 in each of the parallel rows 17. The rows 17' are arranged at an equal spacing a. Spacing a is taken perpendicularly between the rows 17. The spacing a in the pattern in FIGURE 2 is one-half the spacing d. Further, each alternate row is shifted parallel to the remaining rows by one-half the spacing d. The ratio of d to a is 2 in the staggered line geometric well pattern in FIG- URE 2. Another conventional staggered line geometric well pattern is shown in FIGURE 3. This geometric pattern may be described as the hexagonal pat-tern. The hexagonal pattern provides a system of wells in which the wells are accurately staggered at a uniform distance from one another. Thus, any three adjacent wells form the ap'exes of an equilateral triangle. tween the wells is the same throughout the geometric well pattern. In the hexagonal pattern, shown in FIGURE 3, a plurality of wells 21 penetrate the reservoir 11. The wells 21 are arranged at an equal spacing d between adjacent wells 21 in each of the parallel rows 22. The rows 22 are arranged at an equal spacing a. Spacing a is taken perpendicularly between the rows 22. Each alternate row is shifted parallel to the remaining rows 22 by onehalf of the spacing d. Since each adjacent three wells 21 form the apexes of an equilateral triangle in the hexagonal pattern, the ratio of d to a is 2 /3. Thus, it will be apparent that in any regular, uniform, geometric well pattern the wells penetrating the reservoir are located in straight parallel rows. Further, the adjacent wells in each row are equally spaced. Also, the rows of wells are uniformly spaced from one another. The ratio of d to a will be determined by the particular spacing of wells and rows of wells in regular, uniform, geometric well patterns which are conventional. As a result, only the exact magnitudes of the spacings d and a will be contingent upon the area to be drained by each well in any conventional geometric well pattern. As an example of the above, the system of wells provided by the rectangular geometric well pattern shown in FIGURE 1 often is used in conventional methods of recovering petroleumfrom an isotropically permeable reservoir. The results are shown in FIGURE 4. The wells 12 are provided with suitable means for recovering petroleum from reservoir 11. Such means are well known and need not be described herein. As the petroleum in the reservoir 11 is produced at equal rates through wells 12, circular drainage patterns 31 are established as a result of the isotropic permeability of the reservoir 11. -The circular drainage patterns 31 are illustrated by the shaded areas. The drainage patterns 31 progressively expand at relatively constant rates of petroleum recovery. The constant rates of petroleum recovery will be maintained until the adjacent drainage pat-terns 31 meet as Onset of interference between drainage patterns 31 then begins. The production rates of petroleum through wells 12 decreases as a result of such interference. However, the petroleum remaining in the reservoir 11, as represented by the unshaded areas, is very small. Thus, one advantage of using a system of wells located in a geometric well pattern in isotropically permeable reservoirs is that substantially all of the area in the reservoir is drained of at least some petroleum before the onset of interference between the circular drainage patterns 31 occurs. Attempts to use a system of wells of a conventional, geometric pattern in methods of recovering petroleum from reservoirs exhibiting anisotropic permeability produce less favorable results than, the results obtained in isotropically permeable reservoirs. For example, the system of wells of the square geometric well pattern of FIG- URE 4 in conventional methods of petroleum recovery in an anisotropically permeable reservoir produces results illustrated in FIGURE 5. FIGURE shows a reservoir 41 which. for example, has a permeability in the direction The distance or spacing be-. of greatest permeability three times as great as the permeability in the direction of least permeability. The direction of greatest permeability is indicated by chainline 42. The reservoir 41 may be considered to be of uniform thickness and porosity. Also, the permeability may be considered to be uniformly anisotropic with each principal axis of permeability maintaining the same direction and magnitude at every point in the reservoir 41. Two of the principal axes of permeability lie in the bedding plane at right angles to one another. The third axis of permeability is normal to the bedding plane. Usually, the force of gravity may be neglected so that the anisotropic permeability comprises the permeability oriented in directions along the bedding plane. As the petroleum is produced at equal rates through wells 43, noncircular drainage patterns 44 are established as a result of the anisotropically permeable character of the reservoir 41. The noncircular drainage patterns are illustrated by shaded areas. The noncircular drainage patterns 44 progressively expand with the production of petroleum from the reservoir 41 until the adjacent patterns meet, as shown in FIGURE 5. Onset of interference between the noncircular drainage patterns 44 then begins. As a result of such interference, the rates of petroleum recovered through wells 43 decrease. Further, the interference between drainage patterns 44 is premature. The drainage patterns 44 meet along one direction, but are spaced apart by a substantial distance in a normal direction. This can be readily seen from FIGURE 5 where the shaded areas in the reservoir 41 from which petroleum has been recovered are relatively much smaller than the areas available for production when compared under the same conditions of the onset of interference to a reservoir having isotropic permeability, as seen in FIG- URE 4. Referring now to FIGURE 6, an illustrative embodiment of a method, and a system, of the present invention will be given. 'The reservoirs 41 of FIGURES 5 and 6 are considered to be the same, with the same anisotropic permeabilities and directions. One regular, uniform, geometric well pattern will be described for use, in accordance with this invention, in the reservoir 41 as a method for recovering petroleum. Similarly, a system of wells for recovering petroleum will also be described. In the practice of the method of this invention, the direction and magnitude of greatest permeability and the magnitude of least permeability must be known. In some instances, this information is available. However, where this information is not available, determination of the direction and magnitude of greatest permeability and the magnitude of least permeability will be the first step in the method of this invention. The permeability magnitudes and their directions may be determined by any suitable means. For example, cores taken from various portions of the reservoir are analyzed to determine the directions and ratios of the greatest and least permeabilities. Another means to obtain this information is by fluid injection in one well and measuring the pressure or fluid flow increase in the surrounding wells. Other means to obtain this information will be apparent to those skilled in the art. The direction of greatest permeability is shown by chain-line 42. As an example, the permeability in the direction of greatest permeability has a relative value of 3. The least permeability has a relative value of 1. Another step is to provide a plurality of petroleum recovering wells 47 penetrating the reservoir 41 in a novel, geometric well pattern. The wells 47 may be drilled or selected from wells in existence, or provided by a combination of both. In a compilation of such wells 47, any wells not needed in the novel, geometric well pattern in this invention may be shut-in. The wells 47 are arranged in several equally spaced parallel rows 45. These rows 45 are aligned substantially with the direction of greatest permeability as illustrated by chain-line. 42. The wells 47 are disposed at a uniform spacing between adjacent wells '47 in each row 45. Further, as part of this step, the spacing d between wells 47 and the spacing a between rows 45 is proportioned in the following particular manner. Particularly, the spacing of the wells in a row and between rows provides a ratio of d to a proportional to the square root of the ratio of the permeability in the direction of greatest permeability to the least permeability. A new geometric well pattern can be developed with the desired proportion of dto a by selecting the desired area to be produced by each well. I If desired, any conventional, regular, uniform well pattern can be-adapted to provide the desired proportion of d to a. Any conventional geometric well pattern is adapted to reservoir 41 by placing the wells at new spacing ratio of d to a, which new spacing ratio is substantially equal to product of the ratio of d to a of the original geometric pattern and the square root of the ratio of the permeability in the direction of greatest permeability to the least permeability. For example, the square pattern of FIGURE 5 wherein the ratio of d to a is unity can be adapted to provide a greatly improved petroleum recov-- cry from reservoir 41. This result is obtained by placing wells 47 in rows 45 with the ratio of d to a in the novel, geometric well pattern substantially equal to the' product of the ratio of d to a of the square patterriand the square root of the ratio of 3 to 1. Therefore, in accordance with the present invention, the conventional square, geometric well pattern becomes a rectangular geometric well pattern wherein the ratio of the distance between wells 47 in each row 45 to the distance, taken perpendicularly between adjacent rows 45 is substantially equal to the square root of 3. It will be apparent that any regular, uniform, geometric pattern can be used in this invention. If the rows of wells 43 in the square pattern of FIG- URE 5 were aligned with the direction of greatest permeability, premature onset of interference between the drainage patterns about the wells would occur because the drainage pattern 44 would meet along a direction in alignment with chain-line 42 before any substantial area of the reservoir 41 was produced. For this reason, by this invention, the-ratio of d to a of any known or designed geometric pattern desired to be used in reservoir 41 is adapted to provide drainage patterns which do not produce premature onset of interference with all the resultant undesired results. The system of wells 47 located in this manner-in reser- With 41 permits conventional methods of recovering petroleum to increase the duration of the petroleum recovery rates before a decrease therein occurs. Also, it enables such methods to recover petroleum from larger areas of the reservoir than otherwise. Further, the area drained by each well can be as great, if not greater, than in the conventional, regular geometric pattern. The wells 47 are provided with suitable means for recovering petroleum from reservoir 41. As'the petroleum is produced at equal rates through wells 47, noncircular drainage patterns 46 develop about these wells. The noncircular drainage patterns 46 steadily increase in size until they meet, as seen in FIGURE 6, as illustrated by the shaded areas. Interference between patterns 46 then begins. However, the improvement to petroleum recovery is evident upon comparing the drain: age patterns 44 of FIGURE 5 with the drainage patterns 46 of FIGURE 6. The areas of the reservoir 41 covered by drainage patterns 46 at the onset of interference are very large compared to the areas left for petroleum recovery. Further, the areas covered by the drainage patterns 46 are much larger than the areas covered by the drainage patterns 44 of FIGURE 5 under the same reservoir conditions and method of petroleum production. Thus, by the present invention we have provided a method for recovering petroleum from anisotropically permeable reservoirs using a system of wells in regular, uniform, geometric well patterns. The method, and the system, do not suffer from the undesired results produced by using methods and systems of the prior art, as illustrated by FIGURE 5. The results are obtained in the present invention by equidistant spacing between wells in each row and by equidistant perpendicular spacing between adjacent rows. Thus, any regular, uniform, geometric well pattern can be used. The present invention is also of utility where the spacings between wells, and between rows, may vary. However, the results obtained will be somewhat less than would be obtained in an exactingly uniform spacing be tion entitled An Approximate Method for Determining Areal Sweep Efficiency and Flow Capacity in Formations with Anisotropic Permeability in the December 1961 issue of Society of Petroleum Engineers at pages 277-286. From the foregoing description, an illustrative embodiment of a method, and a system, has been set forth satistying the objects of this invention. Various changes can be made to the method, and the system, by persons skilled in the art without departing from the intent of the invention. It is intended that such changes be recognized as being within the scope of the appended claims. What is claimed is: f l. A method for petroleum recovery from an oil-bean ing subterranean reservoir exhibiting anistropic permeability comprising the steps of: (at) determining the direction and magnitude of greatest perrneability and the permeability in the direction of least permeability, (b) providing a plurality of petroleum recovering wells penetrating the reservoir disposed in a geometric pattern where in such pattern the wells are arranged in several equally spaced parallel rows aligned with the direction of greatest permeability with the wells disposed at a uniform spacing in each row, and theratio of the distance between adjacent wells in a row to the perpendicular distance between adjacent rows of wells is proportional to the square tion of least permeability, and v (b) providing a plurality of petroleum recovering jwells penetrating the reservoir disposed in a geometric pattern where in such pattern the wells are arranged in several equally spaced parallel rows aligned with the direction of greatest permeability with the wells disposed at a uniform spacing in each row, and the ratio of the distance between adjacent wells in a row to the perpendicular distance between adjacent rows of wells is substantially equal to the product. of the distance between adjacent wells in each row and the perpendicular distance between adjacent rows of wells of a conventional geometric pattern to be adapted for use in the reservoir, and the square root of the ratioot the permeability in est permeability and the permeability in the dircc the direction of greatest permeability to the least permeability. 3. The method of claim 2 where in the conventional geometric pattern to be adapted for use in the reservoir the ratio of the distance between adjacent wells in a row to the perpendicular distance between adjacent rows of wells is unity. 4. The method of claim 2 where in the conventional geometric pattern tobe adapted for use in the reservoir the ratio of the distance between adjacent wells in a row to the perpendicular distance between adjacent rows of wells is 2.' ' 5. The method of claim 2 wherein the conventional geometric pattern to be adapted for use in the reservoir the ratio of the distance between adjacent wells inta row to the perpendicular distance between adjacent rows of 'wells is 2V3: References Cited by the Examiner CHARLES E. O'CONNELL, Primary Examiner. C. H. GOLD, ZALENSKI, Assistant Examiners. Referenced by
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