GB2184149A

GB2184149A - Well injection system

Info

Publication number: GB2184149A
Application number: GB08629498A
Authority: GB
Inventors: Stephen Paul Major; Robert Wynn Stephens
Original assignee: BP PLC
Current assignee: BP PLC
Priority date: 1985-12-17
Filing date: 1986-12-10
Publication date: 1987-06-17
Also published as: GB2184149B; GB8531001D0; GB8629498D0

Abstract

A well injection head 1 comprises two co-axial cylinders 2, 3 and an injection line 6 for injecting fluid and sand into the annulus 4 between the two cylinders, the annulus 4 being sealed (5) upstream of the injection line and the inner cylinder 3 being adapted for the reception of a wireless instrument. The head is suitable for introducing wireless instruments such as bottom hole pressure gauges into a well during fracturing operations to obtain surface read-outs. <IMAGE>

Description

SPECIFICATION Well injection system This invention relates to an injection head for injecting fracturing fluid into a well.

In natural flowing wells the reservoir pressure forces the oil from the bottom of the hole up the casing or production tubing to surface. The gas which is dissolved in the reservoir crude also assists in raising the oil.

As the oil rises up the well bore, the back pressure imposed on it by the column above becomes less and progressively more gas is released from solution. This gas forms minute bubbles which grow in size and number as the oil continues on its upward journey. The resuiting oil and gas mixture is lighter than the oil itself and so less pressure is needed at the bottom of the hole to lift the gas-laden column to surface and out of the well.

The well will continue to produce by natural flow as long as the combination of reservoir pressure plus the energy resulting from the release of solution gas is sufficient to overcome the downward pressure of the column and the frictional losses in the casing or tubing all the way up the well.

The reservoir pressure at the bottom of a flowing well is, of course, less than the general average reservoir pressure due to the friction loss suffered by the oil in passing through the formation surrounding the hole. This varies according to the type of formation and in the case of low permeability rocks it can be considerable. In such cases it is usually necessary to improve the effective well bore radius by a stimulation treatment.

In the case of limestone formations this is performed by pumping hydrochloric acid into the formation, while, in the case of sandstones, and, in particular, low permeability sandstones, fractures are induced in the formation by hydraulic pressure applied at the wellhead and transmitted to the sand face.

After a fracture breakdown of pressure has been indicated, sand is introduced into the fluid being pumped into the formation and this sand props open the fracture, thus maintaining it as a new flow channel when the pump pressure is released. Such stimulation treatments can successfully increase the flowing capacity of a well by as much as two-fold or more.

To ensure the successful execution of hydraulic fracturing treatments it is necessary to monitor continuously bottomhole pressures on a realtime basis. At present there are three possible methods of attempting this: 1. using bottomhole gauges suspended on wireline.

2. using bottomhole gauges on cabling strapped to a test string, and 3. relying on computer generated pressures.

The presence of sand in the fracturing fluid prevents the use of wireline suspended gauges unless very low pumping rates are utilised. At high rates erosion of the wireline is likely to occur with the subsequent loss of both wireline and gauge into the wellbore. It is also highly likely that this would result in the premature termination of the fracture treatment itself.

Bottomhole gauges can be used to monitor fracturing pressures by strapping electric cable onto the outside of the tubing to protect it from erosion. This practice, however, is time consuming, and hence costly. It is therefore generally avoided on all but very shallow wells.

The third method uses a computer to generate "expected" bottomhole pressures based on wellhead pressures and calculated pressure drops. In the past this method has been considered, but the accuracy of the computer generated pressures cannot be determined until any downhole gauges have been retrieved.

Since there is always the risk of the computer generating erroneous values this method cannot be used with any degree of confidence.

We have now devised a well injection head suitable for introducing wireline instruments such as bottom hole pressure gauges into a well during fracturing operations to obtain surface read-outs.

Thus according to the present invention there is provided a well injection head comprising two co-axial cylinders and an injection line or lines for injecting fluid and sand into the annulus between the two cylinders, the annulus being sealed upstream of the injection line and the inner cylinder being adapted for the reception of a wireline instrument.

Preferably flow vanes are provided in the annulus below the injection line or lines to assist in straightening the flow of fluid.

The length of the inner cylinder should be such that the flow of the fracturing fluid is parallel to the wireline cable when the fluid emerges from the annuius.

The head preferably cooperates with a base located in use near the bottom of the well, the base comprising a wireline stop having a central aperture and circumferential grooves, the diameter of the central aperture being sufficient to allow the passage of todl string weights but insufficient to allow the passage of the wireline instrument, the circumferential grooves being adapted to allow the passage of fracturing fluid and sand.

The base preferably abuts against a "nogo" shoulder near the bottom of the tool string of the well.

Apart from the base and shoulder, no other modifications need be made to a conventional down hole assembly.

A second function of the injection head is that it also acts as a "tree-saver" in that the fracture fluid no longer passes through the flowhead assembly, or Christmas tree. This in turn reduces wear on the internal components of the tree and so prolongs its life.

The well injection head can also be used during well clean-up after fracturing until sandproduction rates are non-damaging and fluid can be resumed through the tree.

The invention is illustrated with reference to Figs. 1, 2 and 3 of the accompanying drawings wherein Figure 1 is a schematic diagram of an injection head, Figure 2 of the down hole assembly and Figure 3 is a plan of the wireline stop shown in Fig. 2.

The injection head 1 comprises an external cylinder 2 and a protective insert 3. An annulus 4 is formed between the two cylinders 2 and 3. The annulus is sealed off at its upper end 5. An injection line 6 for the injection of fracture fluid and sand is fitted to the external cylinder 2. A wireline 7 passes down through the axis of the injection head 1. The insert 3 is sufficiently long to ensure that the flow of the fracture fluid is substantially parallel to the wireline 7 before impinging on it. Flow vanes 8 are provided to assist in straightening the flow of fluid prior to meeting the wireline 7.

A pressure gauge 9 is suspended from the wireline 7 and lowered down a tool string 10 containing a wireline stop 11 which rests against a no-go shoulder 12.

The stop 11 comprises a central aperture 13 which is large enough to permit the passage of toolstring weights 14, but not the pressure gauge 9. It also contains circumferential grooves 15 to permit the passage of fluid and sand.

Thus during fracturing operations, fracture fluids and sand are able to flow past the pressure gauge 9 and wireline stop 11. This prevents the build-up of proppant above the gauge.

Claims

1. A well injection head comprising two co-axial cylinders and an injection line or lines for injecting fluid and sand into the annulus between the two cylinders, the annulus being sealed upstream of the injection line and the inner cylinder being adapted for the reception of a wireline instrument.

2. A well injection head according to claim 1 comprising flow vanes provided in the annulus below the injection line or lines to assist in straightening the flow of fluid.

3. A well injection head according to either of the preceding claims wherein the length of the inner cylinder is such that the flow of the fracturing fluid is parallel to the wireline cable when the fluid emerges from the annulus.

4. A well comprising a well injection head according to any of the preceding claims and a base located in use near the bottom of the well, the base comprising a wireline stop having a central aperture and circumferential grooves, the diameter of the central aperture being sufficient to allow the passage of tool string weights but insufficient to allow the passage of the wireline instrument, the circumferential grooves being adapted to allow the passage of fracturing fluid and sand.

5. A well according to claim 4 wherein the base abuts against a "no-go" shoulder near the bottom of the tool string of the well.

6. A well injection head as hereinbefore described with reference to Fig. 1 of the accompanying drawings.

7. A well as hereinbefore described with reference to Figs. 1 to 3 of the accompanying drawings.