Multi-slug staged method for plugging fractured formation

Abstract

A multi-slug staged method for plugging a fractured formation includes: determining an average opening of fractures around a well as D, an average particle size of bridging particles for first-stage plugging as D.sub.1 that is slightly less than D, and average particle sizes of plugging particles for second to last-stage plugging as D.sub.2-D.sub.n, where D.sub.n is small enough to form a tight plugging layer; and sequentially injecting a plugging slurry only containing the bridging particles having the average particle size of D.sub.1, plugging slurries containing the plugging particles having the average particle sizes of D.sub.2-D.sub.n-1, and a plugging slurry containing the plugging particles having the average particle size of D.sub.n into the fractures to achieve the fractured formation plugging.

Claims

1. A multi-slug staged method for plugging a fractured formation, sequentially comprising steps of: 1) determining an average opening of fractures around a well as D according to logging data and a leakage amount of the fractures, determining a number of stages of a multi-stage plugging as n according to the average opening of the fractures, determining an average particle size D.sub.1 of first plugging particles of first stage plugging of the multi-stage plugging, and respectively determining average particle sizes D.sub.2-D.sub.n of second to n.sup.th plugging particles of second to n.sup.th stage plugging of the multi-stage plugging, wherein the first plugging particles are bridging particles; D.sub.1 is smaller than D; D.sub.2 is larger than D.sub.1/4 and smaller than D.sub.1; the n.sup.th plugging particles having the average particle size D.sub.n are configured to form a tight plugging layer; and one stage plugging of the multi-stage plugging corresponds to one slug; 2) injecting a plugging slurry only containing the bridging particles having the average particle size D.sub.1 into the fractures through a first slug to form a bridging particle layer, wherein the bridging particles are rigid particles; 3) injecting plugging slurries respectively containing the second to (n−1).sup.th plugging particles having the average particle sizes D.sub.2-D.sub.n-1 into the fractures in a plurality of times through respective second to (n−1).sup.th slugs to form a particle filling layer; and 4) finally injecting a plugging slurry containing the n.sup.th plugging particles having the average particle size D.sub.n into the fractures through an n.sup.th slug to form the tight plugging layer.

2. The multi-slug staged method for plugging the fractured formation according to claim 1, wherein in step 1), the average particle size D.sub.2 of the second plugging particles used in the stage plugging approaches D.sub.1/4.

3. The multi-slug staged method for plugging the fractured formation according to claim 1, wherein in step 1), the n.sup.th plugging particles used in the n.sup.th stage plugging are an ultrafine plugging agent having the average particle size D.sub.n being smaller than 0.1 mm.

4. The multi-slug staged method for plugging the fractured formation according to claim 1, wherein the rigid particles are walnut shells or calcium carbonate.

5. The multi-slug staged method for plugging the fractured formation according to claim 1, wherein in step 3), the second to n.sup.th plugging particles of the second to n.sup.th stage plugging are selected from the group consisting of fibers, flaky materials and elastic particles to improve plugging stability, wherein a particle size of the fibers, a particle size of the flaky materials or a particle size of the elastic particles corresponds to the average particle sizes D.sub.2-D.sub.n.

6. The multi-slug staged method for plugging the fractured formation according to claim 5, wherein the fibers are animal hairs or plant fibers, the flaky materials are mica flakes or rice husks, and the elastic particles are rubber particles or asphalt.

7. The multi-slug staged method for plugging the fractured formation according to claim 1, wherein in step 4), bentonite or lime milk is added to the n.sup.th plugging particles used in the n.sup.th stage plugging to increase a compactness.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a flowchart of a multi-slug staged plugging method of the present invention (before plugging).

(2) FIG. 2 is a flowchart of the multi-slug staged plugging method of the present invention (after plugging).

(3) FIG. 3 is a structural schematic diagram of a multi-slug staged plugging area.

(4) FIG. 4A and FIG. 4B are comparison diagrams of the effects of the method of the present invention and a conventional plugging method during the fracture opening process.

(5) FIG. 5A and FIG. 5B are comparison diagrams of the effects of the method of the present invention and the conventional plugging method during the fracture closing process.

(6) In drawings, reference symbols represent the following components: 1—drill pipe; 2—well wall; 3—drilling fluid; 4—plugging particles; 5—filling particles; 6—bridging particles; 7—fractures around a well; 8—multi-slug staged plugging area; 9—bridge particle layer; 10—filling particle layer; 11—tight plugging layer; 12—fracture opening location; 13—fracture closing location; 14—crushed particles.

DETAILED DESCRIPTION OF THE EMBODIMENTS

(7) The present invention is further described below with reference to the accompanying drawings, for those skilled in the art to understand the present invention. However, it should be clear that the present invention is not limited to the scope of the specific embodiments. To those of ordinary skill in the art, as long as various changes are within the spirit and scope of the present invention as defined and determined by the appended claims, they are all protected.

(8) Referring to FIG. 1 and FIG. 2.

(9) FIG. 1 and FIG. 2 are flowcharts of a multi-slug staged plugging method of the present invention before plugging and after plugging. During the drilling process, a drill pipe 1 is located in the center of a well wall 2, and a drilling fluid 3 enters the bottom of the well through the drill pipe 1, and then flows through an annulus between the drill pipe 1 and the well wall 2 and returns to the ground. When a formation is drilled open, natural fractures with sufficient openings are formed around the well, or when a fluid column pressure at the bottom of the well is excessively high to crush rocks around the well to a certain state and induce the formation of fractures, the leakage of the drilling fluid 3 will occur. As shown in FIG. 1, bridging particles 6, filling particles 5 and plugging particles 4 are sequentially injected into the drill pipe 1 through a plurality of slugs. The drilling fluid 3 free of plugging particles is located above the plugging particles 4. The plugging process is as follows: the bridging particles 6 first enter the fractures 7 around the well, and the filling particle 5 fill a flow space of the fractures 7 around the well stage by stage based on the bridging formed by the bridging particle 6; and finally, after the filling is fully completed, a tight plugging layer 11 is formed by plugging of the plugging particles, that is, a multi-stage staged plugging area 8 is finally formed, as shown in FIG. 2.

(10) Referring to FIG. 3.

(11) FIG. 3 is a structural schematic diagram of the multi-slug staged plugging area 8 formed in the fractures 7 around the well. In FIG. 3, the drill pipe 1 and the well wall 2 are shown in the middle part in sequence from inside to outside. In the fractures 7 around the well, the bridging particles 6 are formed outside to form a bridging particle layer 9, the filling particles 5 are located inside to form a filling particle layer 10, and the plugging particles 4 are located closest to the well wall to form a tight plugging layer 11.

(12) Referring to FIG. 4A and FIG. 4B.

(13) FIG. 4A and FIG. 4B are comparison diagrams of the effects of the method of the present invention and a conventional plugging method during the fracture opening process. In FIG. 4A and FIG. 4B, dotted lines indicate fracture opening locations 12. As shown in FIG. 4A, after the plugging area is formed by a conventional plugging method, the bridging particles 6, the filling particles 5, and the plugging particles 4 are randomly distributed at various locations in the fractures 7 around the well. When the fractures around the well 7 are opened, part of the filling particles 5 and the plugging particles 4 in the plugging area will be lost along with passages of the fractures 7 around the well, causing the original plugging layer to be destroyed, and the plugging fails when the loss is serious. In this method of the present invention, as shown in FIG. 4B, because the front section of the plugging area is completely composed of the bridging particles 6 having a larger particle size, when the fractures 7 around the well open, the bridging particles 6 continue to move toward the front ends of the fractures 7 around the well and continue to bridge at the front ends of the fractures 7 around the well. Since the rear filling particles 5 and plugging particles 4 cannot be directly lost, most of the plugging particles remains still in the newly formed multi-slug staged plugging area 8, so the original plugging state can be maintained better.

(14) Referring to FIG. 5A and FIG. 5B.

(15) FIG. 5A and FIG. 5B are comparison diagrams of the effects of the method of the present invention and a conventional plugging method during the fracture closing process. In FIG. 5A and FIG. 5B, dotted lines indicate fracture closure locations 13. As shown in FIG. 5A, after the plugging area is formed by the conventional plugging method, when the fractures are closed, the fracture wall surface will squeeze the plugging layer that has been formed, such that the rigid bridging particles 6 at the front part are crushed and the front end of the plugging layer is destroyed. A part of the filling particles 5 and the plugging particles 4 will be lost subsequently, and the plugging area will be destroyed when the loss is serious. However, in this method of the present invention, as shown in FIG. 5B, after the bridging particles 6 at the front end are crushed, the subsequent bridging particles 6 will move towards the front ends of the fractures and replace the locations of the crushed particles 14 and continue to form bridging, such that the rear filling particles 5 and the plugging particles 4 will not be lost. Therefore, compared with the conventional plugging, the multi-slug staged plugging area 8 is more stable.