Abrasives for well cleaning

Novel abrasive particles for cleaning subterranean wellbores are described. The particles are rounded, preferably spherical, and has a hardness of 80 to 200 Vickers. The particles are preferably made of non-metallic material such as Calcite pellets. The new abrasives cause significantly less damage to the well tubulars than sand.

The present invention relates to abrasives and an improved method for 
cleaning a hydrocarbon well using a fluid jet loaded with said abrasives. 
BACKGROUND OF THE INVENTION 
It has been common practice for many years to run a continuous reeled pipe 
(known extensively in the industry as "coil tubing") into a well to 
perform operations utilising the circulation of treating and cleanout 
fluids such as water, oil, acid, corrosion inhibitors, hot oil, nitrogen, 
foam, etc. Coil tubing, being continuous rather than jointed, is run into 
and out of a well with continuous movement of the tubing through a coil 
tubing injector. 
Coil tubing is frequently used to circulate cleanout fluids through a well 
for the purpose of eliminating sand bridges, scale, and similar downhole 
obstructions. Often such obstructions are very difficult and occasionally 
impossible to remove because of the inability to rotate the coil tubing 
and drill out such obstructions. These well tubulars vary from 
unperforated and perforated pipe, large diameter casing, production 
tubing, and slotted or wire-wrapped well liner. Well tubulars often become 
plugged or coated with corrosion products, sediments and hydrocarbon 
deposits. The deposits may consist of silicates, sulphates, sulphide, 
carbonates, calcium, and organic growth. 
It is desirable to perform drilling type operations in wells through use of 
coil tubing which can be run into and removed from a well quickly in 
addition to performing the usual operations which require only the 
circulation of fluids. The same types of well servicing can also be 
performed with various small diameter work strings. The present invention 
may be used with such work strings and is not limited to coil tubing. 
High pressure fluid jet systems have been used for many years to clean the 
inside diameter of well tubulars. Examples of such systems are disclosed 
in the following U.S. Pat. Nos.: 3,720,264, 3,811,499, 3,829,134, 
3,850,241, 4,088,191, 4,349,073, 4,441,557, 4,442,899, 4,518,041, 
4,919,204, 5,181,576 or 5,337,819. 
The abrasive of choice in current practice of well cleaning is sand, though 
other abrasive particless are known from different technical fields. For 
example, the use of non-spherical flint or steel shot is disclosed in the 
U.S. Pat. No. 4,482,392. The hardness of the material described is well 
above 50 on a Rockwell C scale. 
A well cleaning method using coiled tubing is described in the 
International Patent Application WO 91/11270. It comprises the use of an 
abrasive mixture of carrier fluid and abrasive particles, a pumping system 
to pressurise said mixture and coiled tubing unit with a jetting head. The 
abrasive is characterised as rounded and its effect on the pipes is 
described as being confined to a beneficial shot-peening action. No 
specific example of such an abrasive is given. 
In view of the above cited prior art it is an object of the invention is to 
provide a improved abrasive for well cleaning applications. 
SUMMARY OF THE INVENTION 
The objects of the invention are achieved by abrasives and methods as set 
forth in the appended independent claims. 
The abrasive particles in accordance with the invention are round and have 
a hardness of 80 to 200 Vickers (as measured with a 50 g load). This value 
is below the hardness of the steel shot disclosed in the U.S. Pat. No. 
4,482,392 referred to above. 
It was found that the novel abrasives, while effectively removing scale, 
cause only limited erosion of the well tubulars. 
The erosion of the well tubulars can be limited further by ensuring that 
the abrasive particles are essentially spherical. Essentially spherical in 
the context of this invention is defined as having no systematic 
preferential shape other than an ideal sphere, even though each single 
particle may deviate more or less from that shape. 
It was further found that the removal of the solid deposits can be 
accelerated by choosing material from within the range of 120 to 190 
Vickers, even more preferably from within the range of 155 to 185 Vickers. 
Furthermore, preferred abrasives in accordance with the invention have a 
material or SG density of more than 2000 kg/m.sup.3, more preferably in 
the range of 2000 kg/m.sup.3 to 5000 kg/m.sup.3. It should be noted that 
the density given refers to the density of a single pellet of the abrasive 
material. 
The abrasives are preferably selected from non-metallic materials, such as 
minerals or ceramics. 
Ceramics can be for example clay type particles which are produced by 
processes which include rolling and spray drying to make spherical shape. 
The requisite hardness can then be generated by calcining to temperature 
for specified period. 
Minerals are taken from earth deposits as rock, then crushed to produce 
particles. These particles (e.g. Calcite, Dolomite, Barite) can be 
acquired with the right size and hardness, but usually tend to be angular. 
However using for example a wet rolling process, it is possible to produce 
spherical particles. 
Materials like Calcium, Barium, and Zinc or derivatives, thereof, such as 
Sulphates, Carbonates, Phosphates can be produced as spherical particles 
by precipitation, or in rotary bomb type reactors. They have the correct 
hardness and can be made in the correct shape and size. Importantly pellet 
reactors are used for reduction of Carbonate (CaCO.sub.3) or Phosphate 
levels in cold water. These produce spherical particles with the correct 
properties (including particles normally known as Calcite Pellets, 
comprising precipitated Calcium Carbonate) Calcite Pellets are 
specifically advantageous for the purpose of this invention as they are 
available in large quantities and for economical prices. 
Furthermore, the pellets are preferably graded so as to select a size range 
of 0.1 mm to 1 mm diameter. 
It should be noted that the abrasives in accordance with the present 
invention are rounded so as to limit the damage to the steel tubulars to 
be cleaned. If however such damage is tolerable the above-mentioned 
materials, specifically the calcite based materials could also be used in 
other, e.g. angular, shapes. 
These and other features of the invention, preferred embodiments and 
variants thereof, and advantages will become appreciated and understood by 
those skilled in the art from the detailed description and drawings 
following hereinafter.

MODE(S) FOR CARRYING OUT THE INVENTION 
The invention is now described with reference to the attached drawings. 
The respective performance of different abrasive materials was tested using 
a standard testing set-up. The results of which are illustrated by FIGS. 1 
and 2. 
For the tests, a 2.5% (by weight) water--abrasive mixture was prepared. The 
tested materials included Olivine with Vickers hardness of around 700, 
Dolomite (hardness: 200) and Calcite (hardness: 150), as well as graded 
Calcite pellets (hardness: 180) with spherical shape. 
The slurries were pumped through a nozzle of 2.8 mm diameter at a pressure 
of 180 bars (18 MPa) (jet speed approximately 200 m/s). The jet was 
targeted at a steel plate and, after the jetting, the hole depth was 
measured to quantify the damage caused by the abrasives. 
The results illustrated by FIG. 1 were measured at a constant distance 
(stand-off) between jet nozzle and steel plate of 15 mm. The jetting time 
varied between 40 and 105 seconds (as marked on the abscissa). The 
measured hole depth in the steel plate (in mm) is marked on the ordinate. 
Results related to Olivine slurry are labelled by squares, those for 
Dolomite with a triangle, and for Calcite and the Calcite pallets with 
circles and crosses, respectively. 
Notably the damages caused by the rounded pellets are about an order of 
magnitude smaller that those cause by the angular Olivine (sand) and still 
less that the damages caused by the angular Calcite, which has 
approximately the same or even a lesser hardness. 
The efficacy of the abrasives regards the removal of deposits was tested on 
a Barium Sulphate sample. Barium Sulphate, together with Calcium Sulphate 
and Calcium Carbonate, is a typical component of well deposits (scales). 
During these tests, illustrated by FIG. 2, the jet travelled in a circular 
path over the Barium Sulphate at a constant speed of 60 mm/s, while the 
stand-off varied between 6 and 10 nozzle diameter (2.4 and 3.2 mm) (on the 
abscissa). The ordinate shows a normalised groove depth. Results for the 
different materials are labelled as in FIG. 1. 
Surprisingly, the Calcite Pellets displayed a higher cutting rate than even 
the much harder and angular Olivine sample, even though the performance at 
increased stand-offs seemed to drop off at a faster rate. Also, the 
performance of the pellets compared favourably with that of the angular 
calcite and Dolomite. 
Other possible abrasive material may comprise steel shots annealed to 
control their hardness. This material shows a performance similar to the 
Calcite Pellets, however, it is significantly more expensive and heavier. 
Another alternative could be beads of plastic material loaded with a 
heavier mineral, typically Barium Sulphate. 
Typical applications of the novel abrasives include well cleaning 
operations as illustrated by FIG. 3. The subsurface equipment for well 
cleaning comprises a coiled tubing reel 31 usually mounted on a truck 32. 
Connected to the reel there is a cleaning fluid tank 33, a reservoir and 
feeder for the abrasive material 34. A mixer 35 generates the abrasive 
slurry applied for deposit removal. A pump unit 36 generates the pressure 
to circulate the slurry through the coiled tubing 37 and the wellbore 38. 
The coiled tubing 37 is fed through the Blow-out Preventer (BOP) stack 381 
into the well tubulars 382. A return pipe 371 at the upper end of the well 
tubulars closes the flow loop through which the cleaning fluid is pumped. 
Also included in the flow loop (but not shown) are separators to recover 
the cleaning fluid and/or the abrasives. 
In operation, the coiled tubing with a jetting head 372 at its end is 
lowered into the well 38 to a predetermined depth at which deposits 383 
are to be removed. Then the abrasive containing slurry is discharged 
through the nozzles of the jetting head removing scale at a rate depending 
on the deposits, jetting speed and stand-off.