1. Completion of tight and fractured reservoirs
Tight reservoirs generally refer to low permeability sandstone layers with permeability between 0.01~0.001μm2. The reservoir is mainly of pore-fracture type and fracture-pore type, and there are few pure pore type.
A fractured reservoir is one in which the fractures in the rock provide oil storage space and permeability. The rock strata can be limestone, mud, shale, silicate and other sedimentary rocks. The permeability of reservoirs is controlling by fractures, which can vary in a wide range. Fractures in sedimentary rocks are mostly vertically developing.
These two reservoir completion methods differ in many ways from those of pure pore reservoirs, depending on the characteristics of the reservoir.
- 1) Completion of tight reservoirs
Tight reservoirs are characterizing by high rock strength, low permeability, low productivity, and extremely low oil recovery by the porosity of the reservoir.
Fracturing and acidification are often requiring to increase the porosity of the formation to increase production.
The well’s ability to withstand acidification and fracturing should be considering in the completion of such Wells.
- The principles of completion in tight reservoirs are:
high casing strength, good cementing quality, high cement return, and high cement strength.
The measures to be taken are: use of casing centralizers and mud scrapers, good performance of the preliquid.
displacement fluid, high speed displacement of cement slurry, displacement of the casing movement, casing thread seal good. Select high quality silk buckle sealing oil or silk buckle and glue. With thick wall casing above N80, its internal pressure strength should be more than 1.2 times of formation rupture pressure.
The wall thickness of casing string should be the same as far as possible to prevent the sudden change of wall thickness, and the casing should have no deformation as far as possible. To avoid harmful operations such as casing milling in the casing, upper and lower packers should be using to seal the fractured well section during fracturing.
- 2) Completion of fractured reservoirs
The characteristic of fractured reservoirs is that the distribution and development of fractures are not uniform, and the fractures are directional.
Only when the borehole intersects with the fractures can the well produce oil. The more the intersection, the higher the well production. Such Wells are drilling so that the borehole meets as many fractures as possible.
The characteristic of well completion,is that the fracture should be well connecting with the borehole to prevent.
the closure and blockage of the fracture in the process of drilling and completion.
The principle of well completion in fractured reservoirs.
is to make the borehole pass through the reservoir fractures as much as possible.
Horizontal drilling is a widely using technique to make the horizontal borehole intersect with the vertical fractures as much as possible.
Open-hole completions are the most common because in perforated completions, casing is using to seal the reservoir and then shoot it open. Cement slurry will plug the cracks and greatly reduce the permeability of the pay zone; The possibility of intersecting perforating channels with unevenly distributed fractures is greatly reducing.
The best completion method to maximize the communication between the well and the fracture is the open hole completion and its variants. When the reservoir strength is relatively strong, the open hole completion method can be using, and when the reservoir strength is slightly poor, the screen and liner completion method can be using. Packers can also be placing at appropriate locations in screens and liners to isolate weak formations, or gravel pack completions can be using to minimize perforation.
2. Horizontal well completion
The purpose of drilling horizontal Wells is to expose the borehole in the approximately horizontal pay zone.
or to make the borehole as much as possible to intersect with the high inclination of the pay zone, so that the well can produce more oil.
In recent years, horizontal Wells have become an effective means to increase reserves and restore productivity of old oil fields.
and are widely using in various oil fields at home and abroad. Due to the characteristics of the horizontal well, the completion of horizontal Wells is different from that of vertical Wells. Horizontal well completions can be dividing into two categories: open and closed, including open hole completions, screen or liner completions, screen or liner with packer completions, liner or casing cementing completions, etc.
The specific structure is shown in the following figure. The completion method of horizontal Wells depends not only on the nature of the reservoir but also on the radius of curvature of the well. Due to the stress on the bending section of the casing, it affects the running of the casing, making it impossible for some Wells to run the casing for completion.
- 1) Long medium radius of curvature horizontal well completion methods
Horizontal Wells with long radius of curvature have a slope of (2°~6°) /30m and a radius of curvature of more than 280m.
The drilling method is the same as that of conventional drilling, and all the methods of vertical well completion can be adopting.
For horizontal Wells with medium radius of curvature.
the slope of the inclined section is (6°~20°) /30m, and the radius of curvature is 85~280m.
Due to the small radius of the inclined section, flexible drill pipe or ordinary drill pipe, bend joint.
downhole power drill tool and other inclined drill tool assembly should be using to drill the inclined section.
Becaus e the radius of the inclining section is small, the friction of the drilling tool increases when it is set up. When running the casing, the bending stress of the casing should be checking to ensure the running of the casing. All completion methods can be using for vertical Wells.
When casing can be successfully run into the well, conventional cementing methods can be using.
In order to improve the cementing quality of horizontal Wells, multiple centralizers are using to improve the centralization of casing in horizontal Wells.
usually adding a centralizer to each casing in horizontal Wells. Often, high-flow washing and casing movement can disrupt the cuttings bed in the lateral section.
In the process of cement injection.
the movable casing is strengthening, and the lightweight displacement fluid is using in the displacement of cement slurry.
The floating effect causing by the density difference between cement slurry and displacement fluid makes the casing float up and center, which is commonly known as “casing floating collar down method”.
Light diesel oil can be using as the displacement fluid. When perforating horizontal Wells, drilling or tubing is usually using to run the perforating gun smoothly.
- 2) Short radius of curvature horizontal well completion methods
Construction slope of horizontal well with short curvature radius is, (30°~150°) /30m, and the curvature radius is 10~60m.
When drilling, only flexible drill pipe, bending casing downhole power drill tools.
bending joints and other inclined tools can be using to drill the inclined section.
Limiting by the radius of curvature, the casing has to bend sharply when passing through the inclining section.
which will be subjecting to great bending stress.
and the casing often gets stuck on the wall and cannot be run. If forced in, the casing will be damaging.
Therefore, short radius horizontal Wells are generally difficult to be completing by casing cementing.
and are mostly completing by open hole, screen and other methods.
