A horizontal fracture secondary diversion volume fracturing method and its application

Abstract

The present invention discloses a method for secondary diversion volume fracturing of horizontal seams and its application, the method comprising: injecting small-size ultra-low-density proppants to seal the seam ends to complete a primary diversion in the horizontal seam, then injecting small-size low-density proppants to fill the horizontal seams for secondary diversion, and then injecting fracturing fluid to fully extend the vertical seams. The present invention forms vertical seams on the basis of horizontal seams and effectively extends them, thereby increasing the overall transformation volume of the fractures, and completing a complex fracture network in which horizontal and vertical seams coexist through repeated operations. The fracture parameters and construction parameters are optimized and designed through reservoir numerical simulation and fracture extension simulation software, and the addition methods of fracturing fluids with different viscosities and proppants with different particle sizes are optimized, so as to finally form a complex fracture network with good conductivity, thereby improving the effects of post-fracturing production increase and stable production.

Description

Technical Field

[0001] The invention belongs to the field of fracturing, in particular to the technical field of fracturing transformation of shallow tight sandstone oil and gas reservoirs, and more specifically to a new method for secondary diversion volume fracturing of horizontal fractures in shallow tight sandstone oil and gas reservoirs. Background Art

[0002] After years of research, the fracturing transformation technology of medium-deep and deep oil and gas reservoirs has become increasingly mature, and a series of technologies and supporting technologies have been formed in the areas of pre-fracturing assessment, fracturing tool development, fracturing technology, fracturing fluid damage mechanism, new fracturing materials and fracture monitoring technology. However, there are also large areas of shallow oil and gas reservoirs distributed in China, such as the Weibei Oilfield in Ordos, the Qilicun Oilfield in Yanchang Oil Mine, the Changling Oilfield in Jilin and the Changyuan Oilfield in Daqing, most of which are buried less than 650m. These oil and gas reservoirs are as difficult to fracturing as medium-deep reservoirs due to their shallow burial, multiple thin layers, low reservoir temperature, low pressure coefficient, complex triaxial stress, poor diagenesis and loose cementation. Moreover, most wells have poor post-fracturing effects, especially the short stable production period of fracturing, making it difficult to achieve economic and effective development.

[0003] This is mainly because the reservoir is shallowly buried and the overlying vertical stress is the smallest among the three-dimensional stresses, which means that horizontal fractures are easily formed during fracturing. In vertical wells, there are often multiple horizontal fractures coexisting, which increases the filtration loss in the fractures, the width of the horizontal fractures is relatively small, and the proppant migration distance in the fractures is short. It is easy to settle and block near the fracture mouth, making sand addition difficult; in horizontal wells, there may be only one horizontal fracture extending in the small layer where the horizontal wellbore passes. Although the displacement of a single fracture is relatively large and the width of the horizontal fracture is relatively large, it is still much smaller than the height of the vertical fracture. The sand settling effect of the proppant in the horizontal fracture of the horizontal well is still obvious, and the proppant length of the horizontal fracture is still relatively short, making it difficult to effectively increase the fracture transformation volume.

[0004] In addition, after the formation of horizontal fractures or fracture systems dominated by horizontal fractures, because the vertical permeability is much lower than the horizontal permeability, only part of the sandstone thickness close to the fracture surface has seepage effect. After fracturing, the oil supply capacity from the vertical direction to the horizontal fracture is weak, and it is difficult to provide continuous oil flow, resulting in a stable production period after fracturing and a rapid decline.

[0005] Therefore, it is necessary to propose a new method for secondary diversion volume fracturing of horizontal fractures in shallow tight oil reservoirs to increase the reconstruction volume of fractures and solve the above-mentioned problems.

[0006] Chinese patent CN108457637A discloses a fracturing method for shallow sandstone oil and gas reservoirs. Based on the judgment of the fracture morphology, the orthogonal design method is adopted, and mature numerical simulation software for oil and gas reservoirs is applied to optimize the corresponding fracture length, conductivity and number of horizontal fractures. On this basis, the perforation section and perforation parameters are optimized according to the expansion law of different fracture morphologies. The purpose of this patent is to prevent the difficulty of adding sand or sand blockage in the formation of horizontal fractures or T-shaped fractures during the fracturing process, but it does not mention how to form a nearly circular balanced horizontal suture or how to form a vertical fracture.

[0007] Chinese patent CN108979611A discloses a method for fracturing and transformation of horizontal wells in horizontal fractures of oil and gas reservoirs. By determining the low vertical stress and target layer thickness, selecting the horizontal wellbore trajectory and optimizing it through drilling design software, the final fracturing method has been determined, and during the fracturing transformation implementation process, the artificial fractures formed are cut into several horizontal fracture planes parallel to the formation to increase the fracture transformation volume. This patent mainly optimizes the wellbore trajectory according to different reservoir thicknesses, but does not involve how to perform secondary diversion of a single horizontal fracture during the fracturing transformation process to form a complex fracture network. The fracture formed by this patent method is still a single horizontal fracture.

[0008] The document "Research on Optimization Technology of Horizontal Fracture Parameters" (Block Oil and Gas Field, 1998) assumes that the fractures formed by fracturing are horizontal fractures, and uses numerical simulation methods to establish a horizontal fracture reservoir parameter optimization model. On this basis, the effects of the effective radius of horizontal fractures and fracture conductivity on oil well production, oil well water content and recovery degree are studied, and the horizontal fracture parameters are optimized on this basis. However, this document only considers the optimization of fracture parameters of a single horizontal fracture, without considering the influence of multiple fractures formed in the vertical direction, and does not mention how to form a nearly circular balanced transformation of a horizontal fracture and how to form a vertical fracture.

[0009] The document "Study on Fracturing Design of Horizontal Fractures in Penglai Town Formation Gas Reservoirs in Xinchang Gas Field" (Natural Gas Industry, 1999) derived and established a mathematical model for horizontal fracture extension when injecting three-stage power-law fluid fracturing fluid, and developed horizontal fracture fracturing design software based on the support migration distribution model of Lowe et al. The controllable factors affecting the fracturing effect of horizontal fractures were calculated and analyzed through examples, including the influence of conductivity, support seam length, construction displacement, sand-liquid ratio and fracturing fluid viscosity coefficient on the fracturing effect. However, the document did not consider the fracturing construction process, perforation parameters and proppant type.

[0010] The document "A Fracturing Method Applicable to Low-Permeability Shallow Reservoirs" (Special Oil and Gas Reservoirs, 2004) introduces a polymer fracturing fluid system applicable to low-permeability shallow reservoirs that is both feasible and economical. The liquid can be completely dissolved in water without leaving any residue to damage the propped fractures. It can not only increase the viscosity of the liquid, but also has the characteristics of stabilizing clay. The document only introduces the fracturing fluid system applicable to shallow fracturing, and does not mention the optimization of parameters such as fracturing design and construction of the entire shallow fracturing. Summary of the invention

[0011] Horizontal fractures are often produced during the fracturing process of shallow sandstone oil and gas reservoirs. However, due to their narrow width, limited oil leakage area and limited transformation volume, the initial production after fracturing is low and the stable production period is short. Most shallow oil reservoirs are difficult to develop economically and effectively.

[0012] In order to overcome the problems existing in the prior art, the present invention provides a horizontal seam secondary diversion volume fracturing method, in which a near-circular balanced extension of the horizontal seam is achieved through a primary diversion on the horizontal plane, thereby expanding the fracture oil leakage area. A vertical seam is formed on the basis of the horizontal seam through a secondary diversion on the longitudinal plane and effectively extends, thereby expanding the transformation volume of the fracture. The method is simple and easy to implement, does not require complex tools and materials, and overall complicates the entire fracture system through the secondary diversion method, thereby achieving the purpose of volume fracturing and improving the post-fracturing effect.

[0013] One of the purposes of the present invention is to provide a volume fracturing method for secondary diversion of horizontal seams, comprising: injecting small-particle ultra-low-density proppants to seal the seam ends to complete a primary diversion in the horizontal seam, then injecting small-particle low-density proppants to fill the horizontal seams for secondary diversion, and then injecting fracturing fluid to fully extend the vertical seams.

[0014] In a preferred embodiment, the volume fracturing method comprises the following steps:

[0015] 1) Use medium viscosity fracturing fluid to create horizontal fractures;

[0016] 2) Small-size and ultra-low-density proppant plugging and one-step diversion;

[0017] 3) Filling horizontal fractures with small-size and low-density proppants and performing secondary diversion;

[0018] 4) Vertical joint extension construction;

[0019] 5) Stop the pump for 3 to 5 minutes;

[0020] 6) Extension construction of horizontal joints newly connected to vertical cracks;

[0021] 7) Construction of low-density and large-size proppant filling vertical seams;

[0022] 8) Replacement work.

[0023] In a preferred embodiment, in step 1), when performing horizontal well fracturing, the viscosity of the medium-viscosity fracturing fluid is 15mPa.s to 20mPa.s, and the liquid volume is 20% to 30% (preferably 80mPa.s) of the total amount in the fracturing section. 3 ~100m 3 ), the displacement takes the optimized maximum value and is increased to the optimized maximum value within 1min to 2min.

[0024] When fracturing a horizontal well, the construction parameters are optimized. Since the subsequent steps require the addition of ultra-low density proppant for a plugging and diversion, the amount of medium-viscosity fracturing fluid added should reach 70% to 80% of the designed fracture length.

[0025] In another preferred embodiment, in step 1), when performing vertical well fracturing, the viscosity of the medium viscosity fracturing fluid is 25mPa.s to 30mPa.s, and the liquid volume is 200m 3 ~250m 3 , the displacement takes the optimized maximum value and is increased to the optimized maximum value within 1min to 2min.

[0026] In a preferred embodiment, in step 2), the apparent density of the small-particle ultra-low-density proppant is 1.05 g / cm 3 ~1.15g / cm 3 , the particle size is 140 mesh ~ 210 mesh, and the viscosity of the fracturing fluid used is 3mPa.s ~ 5mPa.s.

[0027] In a further preferred embodiment, in step 2), a continuous sand adding mode is adopted, the sand-to-liquid ratio is 1% to 3% to 5% to 7%, and the amount of fracturing fluid used in each stage is 10m 3 ~12m 3 .

[0028] Among them, since the apparent density of the small-particle ultra-low-density proppant is basically equivalent to that of the fracturing fluid, the sedimentation of the proppant and early sand plugging are avoided. The viscosity of the fracturing fluid is 3mPa.s~5mPa.s. First, it does not affect the carrying of the small-particle ultra-low-density proppant, second, it reduces the damage to the reservoir, and third, it forms a viscous fingering effect with the medium-viscosity fracturing fluid in step 1) (if the medium-viscosity fracturing fluid is in front and the low-viscosity fracturing fluid is in the back, and the viscosity difference ratio of the medium-viscosity fracturing fluid to the low-viscosity fracturing fluid is 3~6, ​​a viscous fingering effect will be formed), and the small-particle ultra-low-density proppant is quickly moved to the end of the long axis or short axis of the crack to form a local plugging effect as soon as possible. Complete the first turn of the horizontal fracture, realize the nearly circular balanced extension of the horizontal fracture, and expand the oil leakage area. Among them, the specific construction parameters can be appropriately adjusted according to the wellhead pressure rise rate (the pressure rise rate is controlled at 0.5MPa / min~1MPa / min).

[0029] In a preferred embodiment, in step 3), the apparent density of the small-particle low-density proppant is 1.15-1.30 g / cm 3 , particle size 70 mesh ~ 100 mesh, the viscosity of the fracturing fluid used is 9mPa.s ~ 12mPa.s.

[0030] In a further preferred embodiment, in step 3), a continuous sand adding mode is adopted, the sand-to-liquid ratio is 3% to 6% to 9% to 12% to 15%, and the liquid amount used for each sand-to-liquid ratio is 80m 3 ~120m 3 .

[0031] Among them, the amount of liquid used can be appropriately reduced in the low sand ratio stage.

[0032] In a further preferred embodiment, in step 3), in the last 30% of the construction stage, the viscosity of the fracturing fluid is increased to 40mPa.s-50mPa.s, and the sand-liquid ratio is increased to 20%-25% or higher than 25%, so that the wellhead pressure is increased, and the rising rate of the wellhead pressure is controlled to be above 1MPa / min (for example, 1MPa / min or 1.2MPa / min). When the vertical crack is opened, the wellhead construction pressure begins to decrease significantly under the premise that other parameters remain unchanged. If the pressure decreases by more than 3MPa within 1min-2min, it is judged that the vertical crack is opened.

[0033] In a preferred embodiment, in step 4), the fracturing fluid of 40 mPa.s to 50 mPa.s is continued to be used for construction.

[0034] After the vertical cracks are formed in step 3), fracturing fluid with a viscosity of 40mPa.s to 50mPa.s is used for construction, and the construction displacement is increased as much as possible under the pressure limit condition to fully extend the vertical cracks. The amount of fracturing fluid is added according to the fracturing design plan.

[0035] In step 5) of the present invention, the pump is stopped for 3 to 5 minutes to observe the amplitude and speed of the wellhead pressure reduction. If the pressure reduction amplitude exceeds the difference between the minimum horizontal principal stress and the overlying stress, the pressure in the vertical fracture may meet the opening pressure of each connected horizontal fracture.

[0036] In a preferred embodiment, step 6) is to repeat steps 1) to 3).

[0037] In a preferred embodiment, in step 7), a fracturing fluid with a viscosity of 30mPa.s to 40mPa.s is used, and the amount of fluid is designed to be 110% to 120% of the vertical fracture volume (the height of the vertical fracture should vertically penetrate the span of the target layer, and the length should be at least equal to the radius of the horizontal fracture), and the displacement takes the optimized maximum displacement.

[0038] Among them, one is to effectively support the vertical cracks and efficiently connect the horizontal cracks; the other is to use high-viscosity fracturing fluid to push the small-particle proppant that may be left over from previously filling the horizontal cracks to the end of the vertical cracks to prevent the small-particle proppant from damaging the conductivity of the vertical cracks.

[0039] In a further preferred embodiment, in step 7), the particle size of the low-density large-particle proppant is 30 mesh to 50 mesh and the density is 3.0 g / cm 3 Within.

[0040] In a further preferred embodiment, in step 7), a continuous sand adding mode is adopted, the sand-to-liquid ratio is 15-18-21-24-27-30%, and the amount of liquid and proppant dosage in each sand-to-liquid ratio stage is added according to the fracturing design plan.

[0041] In a preferred embodiment, in step 8), the displacement fluid volume of the vertical well is calculated as 100% to 105% of the wellbore volume of the target layer, and the displacement fluid volume of the horizontal well is calculated as 110% to 115% of the wellbore volume of the target section.

[0042] In a further preferred embodiment, in step 8), the initial 30% to 40% of the fluid is replaced with a fracturing fluid having a viscosity of 30mPa.s to 40mPa.s to avoid the sanding effect in the wellbore, especially in the horizontal wellbore; thereafter, a fracturing fluid having a viscosity of 1mPa.s to 3mPa.s is used for displacement. The displacement takes the optimized maximum value.

[0043] In a preferred embodiment, the following steps are performed before step 1):

[0044] (1′) Evaluation of key reservoir parameters;

[0045] (2′) Calculation of geological engineering sweet spots and determination of segment cluster locations;

[0046] (3′) Optimization of fracturing fracture parameters;

[0047] (4′) Optimization of fracturing operation parameters;

[0048] (5′) Vertical well perforation operation or horizontal well bridge plug perforation combined tool operation.

[0049] In a preferred embodiment, in step (1′), reservoir physical properties, sensitivity, rock mechanics, three-dimensional geostress and high-angle natural fracture characteristics are mainly analyzed. Various test and analysis methods such as seismic, well logging, well logging, well testing and pilot well target layer and core within 50m above and below are adopted to simulate actual underground conditions.

[0050] In case of horizontal well fracturing, the distribution profile of various parameters of the horizontal wellbore (static parameters of core test are generally required) is generally obtained by referring to the correspondence between the logging parameters of the pilot well (dynamic parameters) and the core test results (static parameters), and then by analogy of the logging parameters.

[0051] In a preferred embodiment, in step (2'), the geological sweet spots and engineering sweet spots are calculated by conventional methods, and then the final geological engineering comprehensive sweet spot index is calculated by equal weight method. The vertical well is calculated in units of small layer thickness, and the horizontal wellbore is calculated in units of meters.

[0052] For the determination of segment cluster locations, vertical wells should still target the small layers divided by logging; for horizontal wells, the segment length should be longer than the radius of the horizontal fracture, and the interval between each fracture should be between 25m and 30m.

[0053] Regardless of whether it is a vertical well or a horizontal well, the maximum and minimum difference between the comprehensive sweet spot indicators of the layers or segments should be less than 20%.

[0054] In a preferred embodiment, in step (3'), a horizontal fracture production prediction model is used, while considering the possibility of communicating vertical fractures.

[0055] On the basis of step (1′), combined with the data of adjacent wells, a relatively detailed geological model should be established using the geological modeling software PETREL. Then, the model results should be imported into the commercial simulation software ECLIPSE commonly used for fracturing production prediction. According to the equivalent conductivity method (after enlarging the fracture support width by a certain multiple, the permeability of the proppant in the fracture is proportionally reduced so that their product, i.e., the conductivity of the fracture, remains unchanged), hydraulic propped fractures are set. Then, according to the orthogonal design method, the post-fracturing production dynamics under different conditions of horizontal and vertical fracture lengths, conductivity and fracture spacing are simulated. The fracture parameter system corresponding to the maximum relative post-fracturing production or the maximum economic net present value is taken as the optimal value.

[0056] In a preferred embodiment, in step (4′), commercial simulation software commonly used in fracturing design, such as FracPT, Gohfer, Meyer, Stimplan, etc., is used to simulate the changes in fracture length, fracture spacing and conductivity under different construction parameter conditions (displacement, viscosity and changes, fracturing fluid volume and proportion of different viscosities, proppant dosage and proportion of different particle sizes, sand-to-liquid ratio and specific sand addition program design, etc.).

[0057] When horizontal cracks and vertical cracks exist at the same time, they should be simulated separately, and then the sum of the two simulation parameters is taken. The fracturing operation parameters corresponding to the crack parameters that can achieve the optimization of step (3') are taken as the optimal values.

[0058] In a preferred embodiment, in step (5'), for vertical wells, perforation and layered fracturing tools can be directly performed in a conventional manner. For staged fracturing of horizontal wells, a bridge plug and a perforation tool string need to be lowered (the segment cluster position is determined according to the results of step (2') and step (3')). After the bridge plug is lowered to the predetermined position, it is sealed and released, and the perforating gun is gradually lifted up to the predetermined position for perforation. After all perforations are completed, the perforating gun is lifted up, and the fracturing process is reversed). However, the bridge plug is not lowered in the first stage, and the perforating gun is carried by a continuous tubing, or the lower toe end is delayed to open the sliding sleeve without using a continuous tubing.

[0059] In a preferred embodiment, the following steps are performed after step 8):

[0060] (1″) Construction of other layers or sections, repeating steps 1) to 8) until all sections are constructed;

[0061] (2″) After-pressure drilling and plugging (for horizontal wells), flowback, testing and production, etc., wherein vertical wells do not require drilling and plugging; refer to the conventional process and parameters for execution, which will not be repeated here.

[0062] The present invention, on the basis of judging the formation of horizontal seams, transports ultra-low density proppants with small particle size to the ends of the long axis and short axis of the horizontal seams according to the migration speed of the fracturing fluid in the long axis direction of the horizontal seams being greater than the migration speed of the fracturing fluid in the short axis direction, completes the primary turning in the horizontal seams by sealing the seam ends, realizes the nearly circular balanced extension of the horizontal fractures, and expands the oil leakage area of ​​the fractures in the horizontal direction; then injects fracturing fluid to continuously increase the net pressure in the seams, so that the fractures complete the secondary turning, form vertical seams on the basis of the horizontal seams and effectively extend, increase the overall transformation volume of the fractures, and complete a complex fracture network with coexistence of horizontal seams and vertical seams through repeated operations. The fracture parameters and construction parameters are optimized and designed through reservoir numerical simulation and fracture extension simulation software, and the addition methods of fracturing fluids with different viscosities and proppants with different particle sizes are optimized, so as to finally form a complex fracture network with good flow conductivity, and improve the effect of post-pressure increase and stable production.

[0063] The second object of the present invention is to provide an application of the horizontal seam secondary diversion volume fracturing method described in the first object of the present invention in the fracturing construction of shallow tight sandstone oil and gas reservoirs.

[0064] In summary, the ideas of the present invention are as follows:

[0065] 1) A turn in the horizontal seam to achieve a nearly circular and balanced extension of the horizontal fracture and expand the oil leakage area. Due to the anisotropy of horizontal stress, the horizontal fracture has directionality when extending, extending faster along the direction of the maximum horizontal principal stress and relatively slower along the direction of the minimum horizontal principal stress. That is, the projection of the horizontal fracture extension in the vertical direction is an ellipse. The greater the difference in horizontal stress in the two directions, the longer the major axis of the ellipse. If the difference in horizontal stress in the two directions is zero, the horizontal fracture extends in a circular shape under the condition of isotropic stress. The first goal of the present invention is to make every effort to achieve a nearly circular extension of the horizontal fracture. To this end, during the fracturing process, an ultra-low density small-particle proppant is added (small-particle temporary plugging agent is generally not recommended because the temporary plugging agent cannot effectively support the end of the horizontal fracture after it is completely dissolved). In order to facilitate the long-distance migration of the above-mentioned small-particle proppant, the average particle size is taken as 1 / 10 of the width of the horizontal fracture. Although there is also proppant migration in the short axis direction of the horizontal crack, since the long axis direction is the direction of the maximum horizontal principal stress and the direction with the largest horizontal permeability, the migration speed of the fracturing fluid in the long axis direction is greater than the migration speed in the short axis direction, and the greater the difference in the horizontal stresses in the two directions, the greater the migration speed of the fracturing fluid in the long axis direction. Therefore, the speed and proportion of the proppant with end plugging function in the long axis direction are also greater. At this time, the plugging function of the proppant in the long axis direction can be well exerted. And in the migration of fracturing fluid and proppant in the long axis direction, the proppant in the short axis direction will also be partially driven away to the long axis direction. The smaller the difference in the horizontal stresses in the two directions, the closer the ratio of the above-mentioned proppant migration and plugging in the long axis and short axis directions of the horizontal seam. In other words, the difference in the horizontal two-way stress can automatically adjust the distribution ratio of the proppant in the long axis and short axis directions of the horizontal seam, and achieve a single turn to achieve a balanced extension of the horizontal seam.

[0066] 2) Secondary diversion in horizontal fractures, forming vertical fractures on the basis of horizontal fractures and effectively extending them, thus increasing the transformation volume. After the proppant has effectively blocked the long and short axes of the above-mentioned horizontal fractures, the fracturing fluid is continuously injected to ensure a substantial increase in the net pressure of the horizontal fractures. Once the difference between the minimum horizontal principal stress and the overlying vertical stress is exceeded, secondary diversion of the horizontal fractures will occur. This secondary diversion mainly occurs in the vertical direction, which can connect more reservoirs in the vertical direction. Since the pressure in the fractures is higher as it is closer to the wellbore, once the horizontal fractures undergo a vertical diversion, the vertical fractures are easily formed in the wellbore. Of course, secondary vertical diversion of horizontal fractures may also occur in fractures near the wellbore, which can easily form complex fractures.

[0067] As the vertical fracture height extends, the net pressure in the fracture gradually decreases. At the end of the vertical fracture, when the net pressure is less than the minimum horizontal principal stress, the fracture will start from the fracture end and continue to extend horizontally. The mechanism of the secondary turning of the vertical fracture near the wellbore, the vertical extension and the communication with the horizontal fracture is similar to the above, except that the vertical fracture height extension here should be relatively small.

[0068] The above does not take into account the existence of high-angle natural fractures. If there are high-angle natural fractures, the pressure near the opening of the high-angle natural fracture should be calculated, and this value should be used to replace the difference between the minimum horizontal stress and the overburden stress mentioned above, and other methods remain unchanged.

[0069] For vertical wells and horizontal wells, the basic principles are the same, but the corresponding parameters need to be designed differently. For example, in a vertical well, multiple horizontal fractures may be opened and extended at the same time, while in a horizontal well, only one horizontal fracture may be opened and extended at the beginning. Therefore, the width of the fracture and the selection of the particle size of the ultra-low density proppant must be designed in a targeted manner.

[0070] The endpoints and any values ​​of the scope disclosed in the present invention are not limited to the precise scope or value, and these scopes or values ​​should be understood to include values ​​close to these scopes or values. For numerical ranges, the endpoint values ​​of each scope, the endpoint values ​​of each scope and the individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be regarded as specifically disclosed in this article. Hereinafter, in principle, each technical solution can be combined with each other to obtain a new technical solution, which should also be regarded as specifically disclosed in this article.

[0071] Compared with the prior art, the present invention has the following beneficial effects: the method for secondary diversion volume fracturing of horizontal seams in shallow tight oil and gas reservoirs proposed in the present invention provides a guiding scheme for forming a complex seam network system mainly composed of nearly circular horizontal seams and vertical seams. By carrying out fracturing construction according to this method, the oil leakage area can be expanded in the horizontal direction and the transformation volume can be increased in the vertical direction, providing a powerful means for achieving increased and stable production of shallow oil and gas reservoirs. BRIEF DESCRIPTION OF THE DRAWINGS

[0072] Figure 1 A schematic flow chart of the volume fracturing method of the present invention is shown. DETAILED DESCRIPTION

[0073] The present invention is described in detail below in conjunction with specific embodiments. It is necessary to point out that the following embodiments are only used to further illustrate the present invention and cannot be understood as limiting the scope of protection of the present invention. Some non-essential improvements and adjustments made to the present invention by those skilled in the art based on the contents of the present invention still fall within the scope of protection of the present invention.

[0074] It should also be noted that the various specific technical features described in the following specific embodiments can be combined in any suitable manner without contradiction. To avoid unnecessary repetition, the present invention will not further describe various possible combinations.

[0075] In addition, the various embodiments of the present invention may be arbitrarily combined as long as they do not violate the concept of the present invention. The technical solutions thus formed belong to part of the original disclosure of this specification and also fall within the protection scope of the present invention.

[0076] The raw materials used in the examples and comparative examples, unless otherwise specified, are disclosed in the prior art, for example, they can be directly purchased or prepared according to the preparation methods disclosed in the prior art.

[0077] [Example 1]

[0078] Well X is a tight sandstone oil well. The well is a vertical well with a reservoir depth of 386m in the middle. The reservoir lithology is brown-grey fine sandstone with a reservoir thickness of 8.8m and a permeability of 0.76×10 -3 μm 2 , porosity 12.2%, oil saturation 50.0%, Young's modulus 13292MPa, average Poisson's ratio 0.25, original formation pressure 2.06MPa, pressure coefficient 0.6, formation temperature 34.5℃, temperature gradient 2.7℃ / 100m. The specific implementation process of the well is as follows:

[0079] 1) Evaluation of key reservoir parameters

[0080] A detailed analysis and evaluation of reservoir properties, sensitivity, rock mechanics and three-dimensional geostress was conducted, and it was determined that the fractures formed in the well were horizontal fractures.

[0081] 2) Calculation of geological engineering sweet spots and determination of perforation locations

[0082] On the basis of step 1), the optimal perforation positions were determined to be 383m-384m and 388m-389m according to the calculation method of geological sweet spots and engineering sweet spots, and the perforation density was 16 holes / m.

[0083] 3) Optimization of fracturing parameters

[0084] The horizontal fracture production prediction model was adopted, combined with the data of adjacent wells, and the geological modeling software PETREL was used to establish a fine geological model of Well X. Then the model results were imported into the commercial simulation software ECLIPSE commonly used for fracturing production prediction. The hydraulic support fractures were set according to the equivalent conductivity method, and then the orthogonal design method was used to simulate the post-fracturing production under different conditions such as horizontal fracture length, vertical fracture length, different conductivity and fracture spacing. The fracture parameter system corresponding to the maximum post-fracturing production or the maximum economic net present value was taken as the optimal value. The optimization results are a horizontal fracture length of 62m, a vertical fracture length of 80m, and a conductivity of 18md.c.

[0085] 4) Optimization of fracturing construction parameters

[0086] The fracturing design simulation software Meyer was used to simulate the changes in fracture length and conductivity under different construction parameter conditions (displacement, viscosity and change, fracturing fluid volume and proportion of different viscosities, proppant dosage and proportion of different particle sizes, sand-to-liquid ratio and specific sand addition program design, etc.). The simulation was performed based on the coexistence of horizontal and vertical fractures, and then the sum of the two simulation parameters was taken. The construction parameters were optimized according to the fracture parameters optimized in step 3).

[0087] 5) Perforation operation

[0088] The perforating gun of type 102 and the perforating bullet of type 127 are used. After the perforating is completed, the oil pipe and the packer are lowered into the oil pipe according to the design of the fracturing string to carry out the fracturing construction.

[0089] 6) Medium viscosity fracturing fluid to create horizontal fractures

[0090] The construction parameters are optimized according to the parameters in step 4). Since the subsequent steps require the addition of ultra-low density proppant for a plugging and diversion, the amount of medium viscosity fracturing fluid added reaches 70% to 80% of the designed fracture length, and a total of 200m of medium viscosity fracturing fluid is added. 3 The viscosity of the fracturing fluid required for construction is 30mPa.s, and the displacement is the maximum value optimized in step 4), which is 4m 3 / min.

[0091] 7) Ultra-low density and small particle size proppant plugging and one-step diversion

[0092] The apparent density of the ultra-low density small-size proppant used in the construction is 1.15g / cm 3 The particle size is 140 mesh to 210 mesh. The construction adopts the continuous sand adding mode, the sand-liquid ratio is 1% to 3% to 5% to 7%, and the amount of fracturing fluid used in each stage is 10m 3 The viscosity of the fracturing fluid is 5mPa.s, which quickly moves the ultra-low density and small-size proppant to the end of the long-axis or short-axis fracture, forms a local plugging effect as soon as possible, completes the primary turning of the horizontal fracture, realizes the nearly circular balanced extension of the horizontal fracture, and expands the oil leakage area.

[0093] 8) Filling horizontal fractures with small-size and low-density proppants and performing secondary diversion

[0094] Based on step 7), select a particle size of 70-100 mesh and an apparent density of 1.25 g / cm 3 Low density proppant.

[0095] The viscosity of the carrying liquid is 12mPa.s. In the continuous sand adding mode, the sand-to-liquid ratio is generally 3%~6%~9%~12%~15%. The liquid volume for each sand-to-liquid ratio is 10m 3 , the amount of liquid used can be appropriately reduced in the low sand ratio stage.

[0096] In the last 30% of the construction phase, the viscosity of the fracturing fluid was increased to 40mPa.s, the sand-liquid ratio was increased to 25%, the wellhead pressure increased significantly, and the rising rate was 1.2MPa / min. After increasing to a certain level, the wellhead construction pressure began to decrease significantly under the premise that other parameters remained unchanged, and decreased by 3.6MPa within 1min to 2min, indicating that the vertical fracture was opened.

[0097] 9) Vertical seam extension construction

[0098] After the vertical cracks are formed in step 8), continue to use fracturing fluid with a viscosity of 50mPa.s, and try to increase the construction displacement under the pressure limit condition to fully extend the vertical cracks. The amount of fracturing fluid is added according to the fracturing design plan.

[0099] 10) Stop the pump appropriately

[0100] Based on step 9), the pump was stopped for 5 minutes, and the magnitude and speed of the wellhead pressure reduction were observed. The pressure decreased by 3.1 MPa.

[0101] 11) Extension construction of horizontal joints newly connected to vertical cracks

[0102] Repeat steps 6) to 8).

[0103] 12) Conventional low-density large-size proppant filling vertical joint construction

[0104] In this stage, the fracturing fluid with a viscosity of 40mPa.s was used, and the amount of fluid used was 120m 3 , the displacement is the maximum designed displacement of 4m 3 / min. The proppant particle size is 30-50 mesh and the density is 3.0g / cm 3 The proppant within the range is added in a continuous sand adding mode with a sand-to-liquid ratio of 15-18-21-24-27-30%. The amount of liquid and proppant dosage in each sand-to-liquid ratio stage is added according to the fracturing design plan.

[0105] 13) Replacement work

[0106] After the fracturing construction is completed, the fracturing fluid with a viscosity of 5mPa.s is used for displacement. The maximum displacement design is 4m 3 / min.

[0107] [Example 2]

[0108] Well X is a tight sandstone oil well. It is a vertical well with a reservoir depth of 326m in the middle. The reservoir lithology is brown-grey fine sandstone with a reservoir thickness of 6.5m and a permeability of 0.68×10 -3 μm 2 , porosity 10.1%, oil saturation 46.8%, Young's modulus 14376MPa, average Poisson's ratio 0.24, original formation pressure 1.97MPa, pressure coefficient 0.63, formation temperature 31.7℃, temperature gradient 2.7℃ / 100m. The specific implementation process of this well is as follows:

[0109] 1) Evaluation of key reservoir parameters

[0110] A detailed analysis and evaluation of reservoir properties, sensitivity, rock mechanics and three-dimensional geostress was conducted, and it was determined that the fractures formed in the well were horizontal fractures.

[0111] 2) Calculation of geological engineering sweet spots and determination of perforation locations

[0112] On the basis of step 1), the optimal perforation position is determined to be 325m-327m and the perforation density is 16 perforations / m according to the calculation method of geological sweet spots and engineering sweet spots.

[0113] 3) Optimization of fracturing parameters

[0114] The horizontal fracture production prediction model was adopted, combined with the data of adjacent wells, and the geological modeling software PETREL was used to establish a fine geological model of Well X. Then the model results were imported into the commercial simulation software ECLIPSE commonly used for fracturing production prediction. The hydraulic support fractures were set according to the equivalent conductivity method, and then the orthogonal design method was used to simulate the post-fracturing production under different conditions such as horizontal fracture length, vertical fracture length, different conductivity and fracture spacing. The fracture parameter system corresponding to the maximum post-fracturing production or the maximum economic net present value was taken as the optimal value. The optimization results are 57m for the horizontal fracture length, 78m for the vertical fracture length, and 16md.c for the conductivity.

[0115] 4) Optimization of fracturing construction parameters

[0116] The fracturing design simulation software Meyer was used to simulate the changes in fracture length and conductivity under different construction parameter conditions (displacement, viscosity and change, fracturing fluid volume and proportion of different viscosities, proppant dosage and proportion of different particle sizes, sand-to-liquid ratio and specific sand addition program design, etc.). The simulation was performed based on the coexistence of horizontal and vertical fractures, and then the sum of the two simulation parameters was taken. The construction parameters were optimized according to the fracture parameters optimized in step 3).

[0117] 5) Perforation operation

[0118] The perforating gun of type 102 and the perforating bullet of type 127 are used. After the perforating is completed, the oil pipe and the packer are lowered into the oil pipe according to the design of the fracturing string to carry out the fracturing construction.

[0119] 6) Medium viscosity fracturing fluid to create horizontal fractures

[0120] The construction parameters were optimized in step 4). Since the subsequent steps required the addition of ultra-low density proppant for plugging and diversion, the amount of medium viscosity fracturing fluid added reached 70% to 80% of the designed fracture length, and a total of 180 m 3 The viscosity of the fracturing fluid required for construction is 27mPa.s, and the displacement is the maximum value optimized in step 4), which is 4m 3 / min.

[0121] 7) Ultra-low density and small particle size proppant plugging and one-step diversion

[0122] The apparent density of the ultra-low density small-size proppant used in the construction is 1.15g / cm 3 The particle size is 140 mesh to 210 mesh. The construction adopts the continuous sand adding mode, the sand-liquid ratio is 1% to 3% to 5% to 7%, and the amount of fracturing fluid used in each stage is 15m 3 The viscosity of the fracturing fluid is 5mPa.s, which quickly moves the ultra-low density and small-size proppant to the end of the long-axis or short-axis fracture, forms a local plugging effect as soon as possible, completes the primary turning of the horizontal fracture, realizes the nearly circular balanced extension of the horizontal fracture, and expands the oil leakage area.

[0123] 8) Filling horizontal fractures with small-size and low-density proppants and performing secondary diversion

[0124] Based on step 7), select a particle size of 70-100 mesh and an apparent density of 1.25 g / cm 3 Low density proppant.

[0125] The viscosity of the carrying liquid is 15mPa.s. In the continuous sand adding mode, the sand-to-liquid ratio is generally 3%~6%~9%~12%~15%. The liquid volume for each sand-to-liquid ratio is 15m 3 , the amount of liquid used can be appropriately reduced in the low sand ratio stage.

[0126] In the last 30% of the construction phase, the viscosity of the fracturing fluid was increased to 50mPa.s, the sand-to-liquid ratio was increased to 25%, the wellhead pressure increased significantly, and the rising rate was 1.2MPa / min. After increasing to a certain level, the wellhead construction pressure began to decrease significantly under the premise that other parameters remained unchanged, and decreased by 4.2MPa within 1min to 2min, indicating that the vertical fracture was opened.

[0127] 9) Vertical seam extension construction

[0128] After the vertical cracks are formed in step 8), continue to use fracturing fluid with a viscosity of 50mPa.s, and try to increase the construction displacement under the pressure limit condition to fully extend the vertical cracks. The amount of fracturing fluid is added according to the fracturing design plan.

[0129] 10) Stop the pump appropriately

[0130] Based on step 9), the pump was stopped for 5 minutes, and the magnitude and speed of the wellhead pressure reduction were observed. The pressure decreased by 2.8 MPa.

[0131] 11) Extension construction of horizontal joints newly connected to vertical cracks

[0132] Repeat steps 6) to 8).

[0133] 12) Conventional low-density large-size proppant filling vertical joint construction

[0134] In this stage, the fracturing fluid with a viscosity of 50mPa.s is used, and the amount of fluid used is 150m 3 , the displacement is the maximum designed displacement of 4m 3 / min. The proppant particle size is 30-50 mesh and the density is 2.64g / cm 3 The proppant is added in continuous sand adding mode with a sand-to-liquid ratio of 15% to 18% to 21% to 24% to 27% to 30%. The amount of liquid and proppant dosage in each sand-to-liquid ratio stage is added according to the fracturing design plan.

[0135] 13) Replacement work

[0136] After the fracturing construction is completed, the fracturing fluid with a viscosity of 5mPa.s is used for displacement. The maximum displacement design is 4m 3 / min.

[0137] The present invention has been described in detail above in conjunction with specific implementations and exemplary examples, but these descriptions cannot be understood as limiting the present invention. Those skilled in the art understand that, without departing from the spirit and scope of the present invention, a variety of equivalent substitutions, modifications or improvements may be made to the technical solution of the present invention and its implementation methods, all of which fall within the scope of the present invention. The scope of protection of the present invention shall be subject to the attached claims.

Claims

1. A volume fracturing method for secondary diversion of horizontal fractures, the volume fracturing method comprising the following steps: 1) Using medium-viscosity fracturing fluid to create horizontal fractures; in step 1), when performing horizontal well fracturing, the viscosity of the medium-viscosity fracturing fluid is 15mPa.s to 20mPa.s; when performing vertical well fracturing, the viscosity of the medium-viscosity fracturing fluid is 25mPa.s to 30mPa.s; 2) Small-size ultra-low-density proppant plugs the primary diversion; in step 2), the viscosity of the fracturing fluid used is 3mPa.s to 5mPa.s; 3) Filling the horizontal cracks with small-size and low-density proppants and performing secondary diversion; in step 3), the viscosity of the fracturing fluid used is 9mPa.s to 12mPa.s, and in the last 30% of the construction stage, the viscosity of the fracturing fluid is increased to 40mPa.s to 50mPa.s; 4) Vertical seam extension construction; In step 4), continue to use 40mPa.s to 50mPa.s fracturing fluid for construction; 5) Stop the pump for 3 to 5 minutes; 6) Extension construction of horizontal joints newly connected to vertical cracks; 7) Low-density large-size proppant filling vertical seam construction; In step 7), a fracturing fluid with a viscosity of 30mPa.s to 40mPa.s is used; 8) Replacement work.

2. The volume fracturing method according to claim 1, characterized in that: In step 1), when performing horizontal well fracturing, the volume of the medium viscosity fracturing fluid is 20% to 30% of the total volume in the fracturing section, the displacement takes the optimized maximum value, and is increased to the optimized maximum value within 1 to 2 minutes.

3. The volume fracturing method according to claim 1, characterized in that: In step 1), when performing horizontal well fracturing, the volume of the medium viscosity fracturing fluid is 80m 3 ~100m 3 .

4. The volume fracturing method according to claim 1, characterized in that: In step 1), when performing vertical well fracturing, the volume of the medium viscosity fracturing fluid is 200m 3 ~250m 3 , the displacement takes the optimized maximum value and is increased to the optimized maximum value within 1min to 2min.

5. The volume fracturing method according to claim 1, characterized in that: In step 2), the apparent density of the small particle size ultra-low density proppant is 1.05 g / cm 3 ~1.15g / cm 3 , particle size is 140 mesh ~ 210 mesh.

6. The volume fracturing method according to claim 1, characterized in that: In step 2), a continuous sand adding mode is adopted, the sand-to-liquid ratio is 1% to 3% to 5% to 7%, and the amount of fracturing fluid used in each stage is 10m 3 ~12m 3 .

7. The volume fracturing method according to claim 1, characterized in that: In step 3), the apparent density of the small-size low-density proppant is 1.15 to 1.30 g / cm 3 , particle size 70 mesh ~ 100 mesh.

8. The volume fracturing method according to claim 7, characterized in that: In step 3), a continuous sand adding mode is adopted, the sand-to-liquid ratio is 3% to 6% to 9% to 12% to 15%, and the liquid volume for each sand-to-liquid ratio is 80m 3 ~120m 3 .

9. The volume fracturing method according to claim 7, characterized in that: In step 3), the sand-liquid ratio is increased to 20% to 25% or higher than 25%, and the rising speed of the wellhead pressure is controlled to be above 1 MPa / min.

10. The volume fracturing method according to claim 1, characterized in that: In step 6), steps 1) to 3) are repeated.

11. The volume fracturing method according to claim 1, characterized in that: In step 7), the amount of liquid used is designed to be 110% to 120% of the vertical fracture volume, and the displacement is the optimized maximum displacement.

12. The volume fracturing method according to claim 11, characterized in that: In step 7), the particle size of the low-density large-particle proppant is 30-50 mesh and the density is 3.0 g / cm 3 Within.

13. The volume fracturing method according to claim 11, characterized in that: In step 7), a continuous sand adding mode is adopted, and the sand-to-liquid ratio is 15-18-21-24-27-30%. The amount of liquid and proppant dosage in each sand-to-liquid ratio stage is added according to the fracturing design plan.

14. The volume fracturing method according to any one of claims 1 to 13, characterized in that: In step 8), the displacement fluid volume of the vertical well is calculated as 100% to 105% of the wellbore volume of the target layer, and the displacement fluid volume of the horizontal well is calculated as 110% to 115% of the wellbore volume of the target section.

15. The volume fracturing method according to claim 14, characterized in that: In step 8), the initial 30% to 40% is replaced with a fracturing fluid having a viscosity of 30 mPa.s to 40 mPa.s, and thereafter, a fracturing fluid having a viscosity of 1 mPa.s to 3 mPa.s is used for displacement, and the displacement takes the optimized highest value.

16. The volume fracturing method according to claim 14, characterized in that: Before step 1), perform the following steps: (1′) Evaluation of key reservoir parameters; (2′) Calculation of geological engineering sweet spots and determination of segment cluster locations; (3′) Optimization of fracturing fracture parameters; (4′) Optimization of fracturing operation parameters; (5′) Vertical well perforation operation or horizontal well bridge plug perforation combined tool operation.

17. The volume fracturing method according to claim 16, characterized in that: After step 8), the following steps are performed: (1″) Construction of other layers or sections, repeating steps 1) to 8) until all sections are constructed; (2″) Post-pressure drilling and plugging, flowback, testing and production, etc., in which vertical wells are not drilled and plugged.

18. Application of the horizontal seam secondary diversion volume fracturing method according to any one of claims 1 to 17 in fracturing construction of shallow tight sandstone oil and gas reservoirs.

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