NZ621445B2 - Method for determining fracture spacing and well fracturing using the method - Google Patents

Abstract

method for determining the fracture spacing for a first set of fractures of a wellbore (102) is disclosed. A first fracture (110) dimension is chosen from the smaller of the length or height of a first fracture and an expected second fracture (120) dimension is chosen from the smaller of the expected length or expected height of a second fracture to be formed. An approximate position of the second fracture is determined from a percentage of the average of the first fracture dimension and the second fracture dimension. An approximate position of a third fracture (130) is determined so that ratio of the distances from the first fracture and the second fracture are about equal to a ratio of the first fracture dimension and the second fracture dimension. The well may then be fractured at the approximate position of the second fracture and may be fractured at the approximate position of the third fracture. ted length or expected height of a second fracture to be formed. An approximate position of the second fracture is determined from a percentage of the average of the first fracture dimension and the second fracture dimension. An approximate position of a third fracture (130) is determined so that ratio of the distances from the first fracture and the second fracture are about equal to a ratio of the first fracture dimension and the second fracture dimension. The well may then be fractured at the approximate position of the second fracture and may be fractured at the approximate position of the third fracture.

Description

SUMMARY The present invention provides a method for determining fracture g for a first set of fractures of a wellbore. The method sing providing a first fracture ion, DH, chosen from the smallest of the length or height of a first fracture. An expected second fracture dimension, DH, is chosen from the smallest of the expected length or expected height of a second fracture to be formed. An approximate position of the second fracture to be formed is determined, the approximate on being a distance, D14, along the wellbore from the first fracture, where D1.2 is a percentage of the average ofDH and DF2. An approximate position of a third fracture which is formed between the first fracture and the second fracture, the approximate position of the third fracture being a distance, D1_3, along the re from the first fracture and an imate distance D2-3 along the wellbore from the second fracture, so that the ratio of D1- 31D2-3 is about equal to the ratio of D121 :ng. The approximate on of the second fracture is used as input in a first numerical simulation to ate a desired second fracture position. The wellbore is fractured to form the second fracture at about the desired second fracture position.

The approximate on of the third fracture is used as input in a second numerical simulation to ate a desired third fracture position. The wellbore is fractured to form the third fracture, which can create x fracture networks, at about the desired third fracture position.

Also disclosed herein is a fractured wellbore. The fractured wellbore comprises a first fracture having a fracture dimension, D121, being the smallest of the length or height of the first fracture; and a second fracture having an expected second fracture dimension, DFZ, being the smallest of the expected length or expected height of a second fracture. The distance between the first fracture and the second fracture is a percentage of the arithmetical average of DH and ng.

A third fracture is positioned between the first fracture and the second fracture. The third fracture is a distance, D1_3, along the wellbore from the first fracture and a distance, D2_3, along the wellbore BRIEF PTION OF THE DRAWINGS illustrates a flow diagram of a method for determining fracturing intervals in a fracture process, according to an ment of the present disclosure. illustrates a schematic side view of a wellbore g fracture intervals, according to an embodiment of the present disclosure.

While the disclosure is susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and will be described in detail herein. However, it should be understood that the disclosure is not intended to be limited to the particular forms disclosed. Rather, the intention is to cover all modifications, equivalents and atives falling within the spirit and scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION The present disclosure sets forth a method of determining improved fracture g that allows stress induced by the net pressure of fractures to reduce in-situ stress anisotropy and thereby improve complex fracture networks at a low bility formation. Regardless of the net pressure value of each fracture, the method can generally determine an improved fracture space. 2012/052668 illustrates a method for determining re intervals for a well, according to with reference to an ment ofthe present disclosure. The method will also be described which illustrates a schematic view of well 100 comprising a wellbore 102 that has been fractured using the methods of the present disclosure. The wellbore 102 can be curved or can be at any angle relative to the surface, such as a al wellbore, a horizontal re or a wellbore formed at any other angle ve to the surface. In an embodiment, the wellbore is an approximately horizontal wellbore.

As shown at block 2 of the method comprises providing a dimension, DFI, of chosen to be a first fracture. For reasons that will be described in greater detail below, D121 can be either the length or height ofthe fracture, whichever is smallest. As illustrated in DH is shown as the height dimension of fracture 110. In an ment, the first fracture is formed, and then the size of D121 can be estimated based on, for example, microseismic measurements or fracture dimensions. Alternatively, DF1 can be any other suitable technique for measuring provided based on the proposed dimensions set forth in the fracturing schedule, or in any other suitable manner. Fracture 110 can be formed by any suitable technique.

As shown at block 4 of the method comprises providing an expected dimension, Dpz, of a second fracture 120. D122 can be chosen to be either the length or height of the second fracture, whichever is smallest. As illustrated in Dpz is shown as the height dimension of fracture 120. Alternatively, the same parameter, either length or height, as was used for DH can also be used for DFZ, regardless of which of the length or height is st for the second fracture.

PCT/U52012/052668 For es of determining the approximate position ofthe second fracture 120, a value for DH can be predicted in any suitable manner. For example, D122 can be provided based on the proposed dimensions set forth in the fracturing schedule.

As shown in it can be assumed for purposes ofthe calculations performed herein that 1/2 of the height of each of the fractures, including D121, Dpz, and the other fractures shown in are formed on either side of the wellbore 102. One of ry skill in the art would readily understand that in actuality the fracture is not likely to be so rically formed.

Before forming the second fracture 120, a desired interval, D14, between first fiacture 110 and second fracture 120 can be determined, as shown at block 6 of D14 can be estimated based on a percentage ofthe arithmetical average of DF1 and Dpz. For example, the estimated distance between the first fracture and the second fracture can be about 0.3*(DF1 + DF2)/2 to about 0.8*(Dp1 + DF2)/2, such as about 0.35*(D1:1 + DF2)/2 to about 0.7*(D1:1 + Dp2)/2.

In an embodiment, the estimated distance between the first fracture and the second re is about F1 + Dp2)/2.

As will be discussed below, the basis for estimating a distance between the first and second fractures is based on two analytical solutions and a numerical simulation. The two analytical solutions are the 2D fracture model (semi—infinite model) and the penny-shape fracture model, both h are generally well known in the art. From the analytical models, we can obtain the following estimate for a desired fracture space.

From the 2D re model (semi—infinite model), AM"_ /_V__ iii) _V* 2(3~2v)h1+ 2(3—2v)h2— 2 2 2(3—2v) (Eq'l) PCTfU52012/052668 Where: L1 is the distance along the wellbore from the fracturing point of the first fracture to a point at which the maximum stress contrast induced by the net pressure of the first fracture occurs; L2 is the distance along the wellbore from the fracturing point of the second re to a point at which the maximum stress contrast induced by the net pressure of the second fracture occurs; h is the fracture height of the first fracture; 112 is the fracture height of the second fracture; and v is the Poisson’s ratio of a formation; From the shape fracture model, La. /_l1+vl it fl1+05:(h1+h2) 1+0 L1+L2 — (qu) 2 (5—1))+ 2 (5‘0) 2 (5—D) Where: L1, L2, h], hz and v are the same as described above for Eq. 1; From Eq. 1 and 2, it is observed that the optimal re spacing can be calculated using the etical average height of the first and second fractures, or (hl + h2)/ 2 multiplied 1+ U with a certain factor such as 2 ————1-/—~—— for the semi—infinite fracture model and for 2(3 —2v) (5—0) the shape fracture model. In addition, it is proved by the 3D ical ellipsoidal crack solution that the stress induced by the net pressure of general bi-wing fractures can exist between the stress value determined by the penny-shape fracture model and the stress value determined PCT/U82012/052668 by the semi—infinite fracture model. Also, we have 0 S 2 6% S 07071 and 0.4472 S (i + U) S 0.5774 with 0 S U S 0.5 . However, since the Poisson’s ratios — U formations exist n 0.2 and 0.4, 0.3922 S 2 V 2—(3V—2) S 0.6030 and— V 0.5 S \i é + U) S 0.5517 . Therefore, the estimated fracture space, as determined using the above— u models, exists between about 35% and about 70% ofthe arithmetical average of the first and second fracture heights (assuming fracture height is the smallest dimension chosen from length or height of the fracture). A more detailed description of the derivation ofFormulae 1 and 2 is found in the conference ing publication by Hyunil Jo, Ph.D., Baker Hughes, SPE, entitled, f‘Optiniizing Fracture Spacing to Induce Complex Fractures in a Hydraulically red Horizontal Wellbore," SPE America’s Unconventional Resources Conference, Pittsburg, Pennsylvania (June 5-7, 2012), publication No. SPE—154930 (hereinafier referred to as 54930-PP") which is hereby incorporated by reference in its entirety.

The above analytical models assume that the first and second res are straight lines, or that they are parallel to each other. The numerical simulation, on the other hand, was developed by using the Boundary Element Method ("BEM") in order to consider curved fractures’ effect on the stress contrast induced by net re. The BEM simulation has the ability to consider the effect of stress ction between the first fracture which has propagated and the second fracture which is propagating.

WO 39689 PCT/U82012/052668 The results of the BEM simulation show that the second fracture is generally curved, and net pressure. While even if its curvature depends on s factors such as re spacing the exact reasons why the second fracture is curved are not clear, it might be caused by the shear fractures while stress distribution change d by the ction between the first and second the second fracture propagates. Simulations show that the amount of curvature appears to be dependent on net pressure and fracture g (e.g., the amount ofspace between the first and second fracture can affect the curvature of the second fracture). For e, as discussed in when the fracture greater detail in SPE-154930—PP, the fracture may have an attractive shape that value, the second fracture may have a space is within a certain value. r, beyond repulsive shape. For example, a second fracture spaced 200 feet from the first fracture may have the largest repulsive shape, which decreases as the spacing decreases. At a certain spacing, such but instead be parallel in as a 70 feet, the second fracture may no longer have a ive shape, regards to the first fracture. At a spacing of less than 60 feet, the second fracture may have an attractive shape. The shear stress distribution change induced by the interaction between the first and second fractures while the second fracture propagates may cause the shape of the fracture to be attractive, repulsive, or parallel.

The curvature ofthe second fracture can affect the stress contrast compared to a situation in which a parallel fracture is formed. It appears from the numerical simulation that the repulsive shape fractures can enhance the stress st induced by the fracture interaction (i.e. fractures vitiate the stress can reduce more in—situ stress anisotropy), while attractive shape contrast (i.e., can reduce less u stress anisotropy). The results of these numerical simulations appear to suggest that an increased stress contrast induced by the fracture interaction PCT/USZOlZ/052668 can be achieved at a fracture space between the first and second fractures of about 60 % of the average height ofthe first and second fiactures. This number can generally be used to provide l approximation of fracture position that can be used as input for performing numerical simulations to calculate a desired position for the second fracture.

As shown at block 10 of the estimated position calculated for the second re can be used to determine a desired second re on by employing numerical modeling methods. For example, simulations may be run to investigate a stress st value induced by net pressure for a re position calculated based on 60 % ofthe average height of the first and second fractures, as well as at other possible fracture positions in the l proximity ofthe estimated position, such as at 40%, 45%, 50%, 55%, 65% and 70% ofthe e height of the first and second fractures. The resulting stress contrast values can then be compared to determine the desired position at which the fracture should be formed. The wellbore can be fiactured at about the desired second fracture position, as shown at block 12 of A third fracture 130, which can create complex re networks, can be positioned between the first re 110 and the second fracture 120. As illustrated in the position ofthe third fracture 130 is a distance, D13, along the wellbore from the first fracture, and a distance D2_3 along the wellbore from the second fracture. In an embodiment, an approximate position ofthe third fracture can be determined by setting the ratio of D1-3:D2_3 to be approximately equal to the ratio of DF1IDF2, as shown at block 8 of For example, the ratio ofD1_3:D2_3 can be in the range of +/— 5% of the average value ofthe two fracture heights ofDF1 and DH, such as set forth in the relationship [DF1+/— (O.05)(DF1 + DF2)/2]:[DF2 +/—(O.05)(Dp1+ DF2)/2]- PCT/U82012/052668 For purposes of determining the approximate position of the third fracture 130, a predicted value for DH can be employed, similarly as was the case when determining the position of the second fracture. Alternatively, the value of Dpz that is used for determining the position of the third fracture can be obtained using other suitable techniques, such as by estimating the actual size based on microseismic measurements after the second fracture is formed, as is well known in the art.

As shown at block 14 of the estimated position calculated for the third fracture can be used to determine a desired third fracture position by employing numerical ng methods. For e, tions may be run to investigate a stress contrast value induced by net re for various fracture ons at or near the approximated third fracture position. The resulting stress contrast values for the various fracture positions can then be compared to determine the desired position at which the fracture should be formed. The wellbore can be fractured at about the desired third fracture position, as shown at block 16 of Additional fractures can be formed using the techniques described . In general, the process discussed above for estimating and determining a desired position for res 120 and 130 can be repeated to form any number of additional fractures. For example, illustrates a fourth fracture 140 and a fifth fracture 150 having fracture intervals determined by the methods of the t disclosure. The fifth fracture can be formed to create complex re networks. In an embodiment, the process of g the fourth fracture 140 and fifth fracture 150 can be performed if the space between the first and second fractures, D", is greater than the value of DH. 2012/052668 It has been found that improved complex fracture networks result in the space n the second and fourth fractures ifthe space between the first and second res, D14, is greater than the value of DFI. This is because when this condition is met, the stress shadow effect caused by first re almost disappears at the space n the second and fourth fractures. The stress shadow effect between fractures is generally controlled by the smallest areal fracture dimension (i.e., fracture height or fracture length), which is often fracture height. Thus, in cases where fracture height is the smallest of the fracture height or fracture length, for example, then the methods ofthe present invention can provide improved results if the space between the first and second fractures is greater than the height ofthe first fracture.

Before forming the fourth fracture 140, a desired al, D24, between second fracture 120 and fourth fracture 140 can be determined. D24 is estimated using a percentage of the average value ofDpz and DF4, where, DF4, is chosen from the smallest ofthe expected length or expected height of the fourth fracture 140.

For example, the estimated distance between the second fracture and the fourth fracture can be about 0.3*(D1:2 + Dp4)/2 to about 0.8*(Dp2 + DF4)/2, such as about 0.35*(DF2 + Dp4)/2 to about 0.7*(DF2 + DF4)/2. In an embodiment, the estimated distance between the second fracture and the fourth fracture is about 0.6*(DF2 + Dp4)/2. The estimated distance can be confirmed or ed based on numerical modeling methods, which are well lmown in the art.

The fifth re 150, which can create complex fiacture networks, can be positioned between the second fracture 120 and the fourth re 140. As illustrated in the position ofthe fifth fracture 150 is a distance, D25, along the wellbore from the second fracture, and a distance D4.5 along the wellbore from the fourth re. In an embodiment, the distances D25 PCT/U52012/052668 and D4_5 are chosen so that the ratio of D2_5:D4_5 is imately equal to the ratio of DF2:DF4.

For example, the ratio of D2_5:D4_5 can be in the range of +/— 5% of the average value of the two fracture heights of DFZ and DF4, such as set forth in the relationship [Drz +/— (0.05)(DF2 + DF4)/2] : [DF4 +/-(0.05)(DF2+ DF4)/2] .

For purposes of determining the position of the fifth fracture 150, a value for D124 can be predicted as was the case when determining the on of the fourth fracture. Alternatively, the value of D124 that is used for determining the position of the fifth fracture can be obtained using other suitable techniques, such as by estimating the size of DF4 based on microseismic measurements after the fourth fracture is formed, as is well known in the art.

As mentioned above, the s of g the fourth fracture 140 and fifth fracture 150 can be performed if the space between the first and second fractures, D14, is greater than the value of D". If, on the other hand, D1_2, is less than or equal to the value of D", a second set of fractures can be formed a ce greater than D122 from the fracture 120, instead of g fractures 140 and 150 as described above. The second set of fractures (not shown) can be formed by repeating the process discussed above for forming fractures 110, 120 and 130.

The present sure will be further described with t to the following examples, which are not meant to limit the invention, but rather to further illustrate the various embodiments.

EXAMPLES The following example is provided for illustrative purposes only, and is not to be taken as limiting the claims of this disclosure.

Referring to and assuming that D121, D122 and BM are height dimensions having the following values: PCT/U82012/052668 DF1=80 ft; Dp2=190 ft; DF4=90 ft; and Setting the space between the first and second fractures to 60% of the arithmetical average fracture height of the first and second fractures: The calculated interval, D1-2 = (80+190)/2*0.6 = 81ft.

The 3rd fracture is ated to be positioned a distance D13 = 80/(80+190)*81=24 ft from the first fracture and D23 = 190/(80+l90)*81 = 57 ft from the second fracture.

Because the space between the first and second fractures (81ft) is longer than DF1(80ft), a similar calculation process can be performed to determine als for the fourth and fifth fractures. Thus, the space between the second and fourth fiactures, D24, can be calculated as (190+90)/2*0.6 = 84ft.

The fifth fracture can be calculated as D2_5 = 190/(190+90)*84 = 57ft from the second fracture and D4_5 = 90/(190+90)*84 = 27ft from the fourth fracture. gh various embodiments have been shown and bed, the present sure is not so limited and will be understood to include all such modifications and variations as would be apparent to one skilled in the art.

Claims (14)

WHAT IS CLAIlVflED IS:

1. A method for determining fracture spacing for a first set of res of a wellbore, the method comprising: providing a first fracture ion, DP], chosen from the smallest of the length or height of a first fracture; providing an expected second fracture ion, DF2, chosen from the smallest of the expected length or expected height of a second fracture to be formed; determining an approximate position of the second fracture to be , the approximate position being a ce, D14, along the wellbore from the first fracture, where D1_2 is a percentage of the average of DF1 and D122; determining an approximate position of a third fracture to be formed between the first fracture and the second fracture, the imate position of the third fracture being a distance, D13, along the wellbore from the first fracture and an approximate distance D2_3 along the wellbore from the second fracture, so that the ratio of 2_3 is about equal to the ratio of DF1:DF2; using the approximate position of the second fracture as input in a first numerical simulation to calculate a desired second fracture position; fracturing the wellbore to form the second fracture at about the desired second fracture position; using the approximate position of the third fracture as input in a second numerical simulation to calculate a desired third fracture position; and fracturing the wellbore to form the third re at about the desired third re position.

2. The method of claim 1, further comprising fracturing to form the first fracture prior to providing the first fracture dimension, D121, wherein DF1 is estimated based on microseismic measurements of the first fracture.

3. The method of claim 1, further comprising forming the second fracture after determining D1_2.

4. The method of claim 1, wherein the distance between the first fracture and the second fracture ranges from about 0.3*(Dp1 + DF2)/2 to about O.8*(Dp1 + DF2)/2.

5. The method of claim 1, wherein the ce between the first fracture and the second fracture is about 0.6*(D1:1 + Dp2)/2.

6. The method of claim 1, wherein the ce between the first fracture and the second fracture is greater than D131.

7. The method of claim 6, further comprising determining a ce between a fourth fracture and the second fracture, the fourth fracture having a fourth fracture dimension, DF4, chosen from the smallest of the length or height of the fourth fracture, wherein the distance between the fourth fracture and the second fracture is at least 0.3*(Dp2 + DF4)/2 to about 0.8*(DF2 + DF4)/2.

8. The method of claim 7, wherein the ce between the fourth fracture and the second fracture is about 0.6*(DF2 + DF4)/2.

9. The method of claim 7, further comprising calculating a position of a fifth fracture of the fifth fracture to be formed between the second fracture and the fourth re, the position being a distance, D2-5, along the wellbore from the second fracture and a distance D4_5 along the ratio wellbore from the fourth fracture, so that the ratio of D2_5:D4_5 is approximately equal to Of .

10. The method of claim 1, wherein the first simulation takes into account a curved effect of the second fracture on the stress contrast induced by the net pressure of the first second fracture.

11. The method of claim 1, wherein the approximate position of the third fracture is ined after fracturing the wellbore at about the desired second re position.

12. The method of claim 1, wherein the wellbore is a horizontal portion of a well.

13. The method of claim 1, wherein if the distance between the first fracture and the second fracture is less than or equal to D“, a second set of fractures is formed a distance greater than DFZ from the second fracture.

14. The method of claim 13, n forming the second set of fractures comprises repeating the method of claim 1.