US3155161A - Method of fracturing a formation traversed by a well - Google Patents

Description

SEARQH RUUM H. J. TADEMA 3,155,161

METHOD OF FRACTURING A FORMATION TRAVERSED BY A WELL Nov. 3, 1964 Filed NOV. 1, 1960 FIG. IB

FIG. IA

FIG.

FIG. 2

INVENTORI HARCO J. TADEMA MK HIS ATTORNEY United States Patent 3,155,161 METHOD OF FRACTURING A FORMATION TRAVERSED BY A WELL Harco J. Tadema, Delft, Netherlands, assignor to Shell Oil Company, New York, N.Y., a corporation of Delaware Filed Nov. 1, 1960, Ser. No. 66,488 Claims priority, application Netherlands, Nov. 13, 1959, 245,336 1 Claim. (Cl. 166-42) The invention relates to a method of fracturing a formation traversed by a well by means of a fracturing fluid.

In order to improve the production of oil and gas from subsurface formation traversed by a well it has been the practice to enlarge or create flow channels or fractures in the formations. Fractures are created or enlarged by the application of high pressures to fluids disposed in the wellbore adjacent the portion of the formation to be treated. When the pressure of the fluid exceeds the breakdown pressure of the formation the formation will fracture. In formations which tend to fracture along a horizontal plane such as loose-grained material having a low rock bonding strength the breakdown pressure equals the geostatic pressure plus any rock-bonding strength. In formations that tend to fracture along vertical planes the breakdown pressure is 60 to 70 percent of the geostatic pressure plus rock bonding strength.

Fracturing the formation in this way increases the surface area of the formation in contact with the well thereby promoting a ready exchange of media between the formation and the well this being important both in the production of liquid or gas from the formation and in various processes for specific treatments of the formation.

It is often desirable to produce the fracture at a certain elevation in a formation within the well. Thus, for example, in some oil-bearing formations the fracture should be located as low as possible in the formation to minimize the simultaneous production of gas and oil.

In one method of fracturing two packer elements are set at the required elevation in the formation thus forming in the well an isolated zone bounded by the packer elements and the wall of the formation. By injecting fracturing fluid under pressure by means of tubing extending into this zone the formation is fractured at the elevation thereof. While this is a satisfactory method it requires the installation and removal of two packing elements and a tubing string from the well. This increases the cost and expense of the fracturing operation.

Accordingly, an object of this invention is to provide a method of fracturing a formation traversed by a well at a desired place which can be carried out by the use of simple equipment.

A further object of this invention is to provide a novel method for fracturing a formation traversed by a well at a desired location by using fluids having different specific gravities and controlling the specific gravity of the fliud in the well so that the hydrostatic pressure of the fluid at the desired location equals the formation breakdown pressure.

According to the invention, the bottom of the column of fracturing fluid should be adjacent the area at which the bottom of the fracture in the formation is desired, and moreover a pressure gradient in the fracturing fluid should be created that the hydrostatic pressure of the fracture fluid exceeds the breakdown pressure of the part of the formation to be fractured. Measured in a downward direction, the pressure increase in the column of fracturing fluid per unit of length of the well is greater than the increase in the breakdown pressure in the formation per unit of length of the well. The difference be- 3,155,161 Patented Nov. 3, 1964 ice tween the hydrostatic pressure of the fluid at a certain level and the breakdown pressure of the formation at the same level therefore decreases towards the bottom of the column of fracturing fluid. When the hydrostatic pressure is increased (for example) by the introduction of fluid at the top of the column of fracturing fluid, or by exerting pressure on the top of the well) a local equilibrium between the hydrostatic pressure of the fluid and the breakdown pressure will be reached first at the bottom of the column of fracturing fluid. When the fluid pressure increases still further, the fracturing of the formation will be initiated at the bottom of the column of fracturing fluid and, depending on the state of stress prevailing in the formation, will extend in a horizontal plane or one or more vertical planes.

It is preferable to control the pressure gradient in the column of fracturing fluid by means of the specific gravity of the fracturing fluid. Also a column of driving fluid having a lower specific gravity may be introduced above the fracturing fluid to insure that the hydrostatic pressure of the fluid will not exceed the breakdown pressure of the formation except at the desired location of the fracture.

The desired level of the lowest part of the column of fracturing fluid relative to the formation may be adjusted or positioned by means of a packer element closing off the whole passageway of the well below the point at which it is desired to produce the fracture.

The properties of the fracturing fluid should also be such that when there is suflicient pressure a fracture can be produced in the formation. These properties, such as controllable viscosity and the ability to transport propping agents, are outside the scope of the invention.

The above and additional advantages of this invention will be more easily understood by those skilled in the art from the following detailed description of a preferred embodiment when taken in conjunction with the attached drawing in which:

FIGURE 1 is a diagrammatic representation of a formation traversed by a well illustrating one embodiment of this invention;

FIGURES 1A and 1B illustrate graphically the value of the breakdown pressure and the value of the hydrostatic pressure of the column of fracturing fluid in the well as a function of the depth of the well; and,

FIGURE 2 is a diagrammatic representation of a formation traversed by a well showing a second embodiment of this invention.

Referring to FIGURE 1, the wall of the well 10 is lined from the top of the well to the top of the formation 12 to be treated with a casing 13. The top of the casing 13 is closed and provided with a connection 14 via which fluid can be supplied from the storage vessel 16 by means of the pump 15 to the well 10. The well 16 is filled to above the top of the formation 12 to be treated with a fracturing fluid 17, while the remaining volume of the well is filled with a driving fluid 18.

FIGURE 1A is a graphical representation of the value of the hydrostatic pressure in the well 19 and the breakdown pressure in the formation 12 as a function of the depth of the well. To this end the pressure is plotted on the X-axis, and the height of the column of fracturing fluid relative to the bottom of the well along the Y-axis. In FIGURE 1, the bottom of the column of fracturing fluid coincides with the bottom of the well and the bottom of the formation 12.

As the formation 12 shows a tendency to horizontal fracturing, the formation breakdown pressure is proportional to the sum of the geostatic pressure and the rockbonding strength. Since the latter is usually negligible compared to the other stresses, the formation breakdown pressure may be regarded as being almost proportional to the geostatic pressure, and this pressure is in its turn almost proportional to the depth of the well. The breakdown pressure therefore decreases linearly from the lowest point in the well towards the top. The values of the breakdown pressure in the formation 12 are shown by the line 19. The value (A) of the breakdown pressure at the top of the formation 12 is determined by the weight of the overburden above the formation 12, plus any rockbonding strength at the top of the formation 12. Since the breakdown pressure is substantially proportional to the geostatic pressure, the slope of the line 19 depends solely on the specific gravity of the rock in the formation 12.

The value of the hydrostatic pressure in the fluid-filled well as a function of the depth of the well is shown in FIGURE 1A by line 20. This line is bent when the fracturing fluid 17 comes into contact with the driving fluid 18. If no pressure is exerted on the column of fluid by the pump 15, the extension of the line 20 will intersect the Y-axis at a height of the column of fluid corresponding to the depth of the well.

Since, in the case under discussion, the formation shows a tendency to horizontal fracturing, the fracturing fluid has a specific gravity greater than that of the rock in the formation 12, and hence, preceeding in a downward direction, the increase in the hydrostatic pressure of the fluid in the well per unit of length of the well will be greater than the increase in the breakdown pressure in the formation 12 per unit of length; as a result, the difference between the hydrostatic pressure and the breakdown pressure is smallest at the level of the bottom of the column of fracturing fluid 17.

The desired specific gravity for the fracturing fluid, which should usually be between 2 and 3, is obtained by adding such weighting materials as barytes, pyrites, red lead and lead glance (galenite). The whole fluid column in the well may, if necessary, consist of fracturing fluid, but it is preferable to endeavor to use a minimum quantity of weighting materials, thus only part of the well is filled with fracturing fluid. The remainder of the well is filled with a driving fluid having a lower specific gravity than the fracturing fluid, as reflected in the course of the upper part of line 20 in FIGURE 1A.

After the required quantities of fluid have been introduced into the Well, the pressure of the fracturing fluid is increased. This is preferably done by introducing fluid under pressure from the storage vessel 16 by means of the pump 15 through the line 14 to the top of the casing 13 in the Well It). This fluid should preferably have the same composition as the driving fluid. A sufficiently high hydrostatic pressure at the bottom of the column of fracturing fluid can be achieved filling the well with the heavy fracturing fluid instead of driving fluid. The resultant increase in pressure is propagated without loss throughout the well until at a certain moment the hydrostatic pressure of the fluid at the lowest part of the well is equal to the breakdown pressure of the formation in that part. This condition is shown gnaphically by the line 20'. In the higher parts of the well 10 the hydrostatic pressure is less than the breakdown pressure of the formation in these parts and thus no fractures can occur in these parts.

If the pressure in the well increases still further by the introduction of fluid at the top of the casing 13 corresponding to line 20", this procdues a fracture 21. As can be seen from FIGURE 1A, the hydrostatic pressure in parts of the well above the lowest region of the formation remains less than the breakdown pressure in the lowest Zone, thus no fracturing of the formation will occur in these parts.

Since fracturing fluid disappears into the formation after the fracture 21 has been produced, the hydrostatic pressure at the bottom of the fluid column in the well should be kept constant by introducing fluid to the top of the well. The pressure exerted by the fracturing fluid present in the resultant fracture will thus remain higher than the formation breakdown pressure thereby extending the fracture 21 into the formation.

It can be seen from the graphical representation in FIGURE 113 that the quantity of fracturing fluid in the well 10 has to be suflicient to prevent the interface between the fracturing fiuid and the driving fluid from falling below the bottom of the casing 13. If the interface falls below the casing 13, there is a chance that the hydrostatic pressure of the fluid at the top of the formation 12 may exceed the formation breakdown pressure, thus producing a fracture in the upper portion of the formation.

FIGURE 2 shows a second embodiment of the method according to the invention which permits the fracture to be produced at a place higher than the bottom of the formation 12. For this purpose a packer element 22. completely shutting off the passageway of the well, is set immediately below the location where the fracture is to be produced. Otherwise, the method is identical to that described in connection with FIGURE 1. In this case, the hydrostatic pressure of the fluid is adjusted until it exceeds the breakdown pressure of formation 12 adjacent packer 22.

It is to be noted that if the formation which is to be fractured tends to form vertical fractures, the formation breakdown pressure is between 60 and 70% of the geostatic pressure (plus any rock-bonding strength). In the case of a rock with a specific gravity equal to the specific gravity of the formation 12, the slope of the line 19 which shows the value of the breakdown pressure as a function of the height of the well will therefore be greater than given in the drawing. By using a fracturing fluid having a specific gravity greater than 60 to 70 percent of the specific gravity of the formation rock, and applying extra pressure to the top of the column, equilibrium between the hydrostatic pressure and the formation breakdown pressure is first reached at the bottom of the column of fracturing fluid. When the extra pressure is increased still further the hydrostatic pressure exceeds the formation breakdown pressure here and begins to fracture the formation at this point along one or more vertical planes. The top of the vertical fractures is determined by the region in the well in which the hydrostatic pressure and the formation breakdown pressure are in equilibrium. The location of this region can be accurately controlled by controlling the pressure applied to the column of fluid by the pump 15.

From the above description it is seen that a novel method has been provided for controlling the location of fractures created in a formation traversed by a well. The control is achieved by adjusting the specific gravity of the fracturing fluid adjacent the desired location of the fracture to develop a pressure gradient that exceeds the formation breakdown pressure gradient. The fracture is created by increasing the hydrostatic pressure of the fracturing fluid until it exceeds the formation breakdown pressure. Fractures in other formations traversed by the Well are prevented by controlling the specific gravity of the fluid to maintain the hydrostatic pressure in all cases less than the breakdown pressure of the formations.

While but a single embodiment of this invention has been described, many modifications and improvements are possible within its broad spirit and scope.

I claim as my invention:

A method of fracturing a formation traversed by a well by means of a fracturing fluid under pressure comprising:

partially filling the well with a column of fracturing fluid, the bottom of said column adjoining the area at which the bottom of the fracture is desired and the top of said column of fracturing fluid being nonconstrained, the specific gravity of said fracturing fluid being adjusted until the pressure gradient of the column of fracturing fluid exceeds the pressure gradi- References Cited in the file of this patent ent of the formation to be fractured; filling the remainder of the well with a driving fluid, the UNITFD STATES PATENTS specific gravity of said driving fluid being adjusted R6- 23,73 Farris Nov. 10, 1953 until the pressure gradient of the column of driving 5 2,596,845 Clark May 13, 1952 fluid is less than the pressure gradient of all forma- 2,838,117 Clark et a1 June 10, 1958 traversed by the W611; 2,851,109 Spearow Sept. 9, 1958 increasing the pressure on the drlvlng fluid at the sur- 2 906 340 Herzon Sept 29 1959 face until the pressure of the fracturing fluid exceeds the breakdown pressure of the formation to be frac- 10 tured.

2,950,247 McGuire et al Aug. 23, 1960

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