foraco reverse circulation drilling

7/23/2019 Foraco Reverse Circulation Drilling

1/4

REVERSE CIRCULATION DRILLING

USING ROTARY TRICONE BIT OR

DOWN THE HOLE HAMMER

Drilling techniques can be distinguished by :

the type of the tool cutting the ground : Tricone bit or DTH ;

the type of the flushing fluid (mud, water, air) which carries the cuttings from the bottom of the hole

to the surface ;

the way the flushing fluid circulates :

direct circulation, where the fluid is injected by the drilling rods and removes the cuttings to the

surface through the annulus between the walls of the drill hole and the rods,

reverse circulation, where the fluid circulates downwards and carries the cuttings upwards inside

the drilling rods.

Classical drilling techniques are :

rotary drilling with tricone bit, using mud and direct circulation (generally the case of oil drilling

techniques) ;

drilling using air and DTH, with direct circulation (generally the case of water well drillings in

igneous rock, Brittany, and West-Africa ).

The Boniface company from Lunel (France) has developed a drilling technique using reverse

circulation, either for rotary or DTH hammer tools.

The development comprises a patented new type of double walled drilling rod and a special air

distributor.

Double Walled Drilling

Rod and Air Distributor


2/4

The Principle :

Compressed air is injected inside the annulus of the double walled rod (1),

arriving inside the special air distributor (2) to feed the drilling device (3) and

allow the removal of the cuttings through the internal hole of the rods. Aclimbing fluid speed of about 1 000 m per minute is maintained in the central

hole of the rods. This speed is independent of the ratio between the drill hole

diameter and the rod diameter.

Equipment :

The use of the robustly manufactured double walled rods is simple as well as

fast. The external tube has a diameter of 7"5/8 and the internal tube has a

diameter of 5"1/2, with conical thread API type (5"1/2 IF). Screwing andunscrewing operations are similar to those with conventional rods. The inner

tube diameter is 4"3/4, and is sufficiently large to minimise head losses.

The special distributor (2) has several functions :

it allows the air flow to feed the air lifting for rotary or the DTH, from the

double walled rods,

it allows, with a special crossing joint tool, the removal of the air and the

cuttings through the inner tube of the double walled rods,

it ensures good verticality of the hole (stabilizer),

it isolates the toe of the drill hole from its upper section, located above the

distributor/stabilize, which has the same diameter as the drilling tool and forms

a key part of this reverse circulation drilling method.

The classical drilling device (3) is equipped with a rock bit that has a cutting

part containing tungsten carbide buttons inserted in the rock bit head (photo 2).

The average rock bit diameter comprises between 6 and 30 (152,4 up to 763

mm).


3/4

A better geological investigation tool :

The advantages of this reverse circulation drilling method, using Rotary or DTH, are as follows :

Geological information is accurate and instantaneous. The cuttings arrive at the drilling head at a

high speed from the bottom of the hole. Their dimensions are of centimetre order and in addition

there is no mixing of these "bottom of the hole" cuttings with others, which can be the result of

erosion of the wall of the hole.

The geological information is continuous (100 % recovery). When using a conventional drilling

technique (direct circulation), drilling through fissured or fractured ground, or through cavernous

rock is hindered by partial or total fluid loss. On the contrary, removing the cuttings by the rods

(reverse circulation) cancels possible fluid and cutting losses and their associated risks (plugging,

jamming, pollution of water table).

The successive fluid arrivals are individualized. This allows us to identify the respective physical-

chemical characteristics and their relative importance contrary to the direct circulation method who

only provides general information on the aquifers.

Financial Savings :

The air output to remove the cuttings is significantly reduced when using the reverse circulation

method. For example, to drill a 17"1/2 (444.5 mm) diameter hole, the air consumption is in the range of25 m

3/minute when using reverse circulation and 5 to 6 times more with direct circulation. In other

words, the reverse circulation method allows economical large diameter drilling (smaller air

compressors).

Drilling with no turbulence above the distributor/stabilizer allows much greater hole stability, thus

increasing the durability of the rods, which do not suffer from external erosion. The flush surface of the

internal pipes of the rod leads to less turbulence than when removing the cuttings through the hole.

Drilling with the reverse circulation method is generally linked with the use of Rotary or DTH

hammers. The technique leads to cost savings which increase with rock hardness. The savings are due

to faster progress, increased durability of the tools and lighter equipment than direct circulation.


4/4

All ground conditions

Reverse circulation techniques can be used in all geological formations, from silty grounds to rock. This

technique is an important step in solving problems in large diameter drilling in hard rock (Limestone,granite, etc...) and in fractured or karstic rocks, where total drilling fluid losses are encountered with

direct circulation.

Some examples of its applications are given below :

nUnderground water

deepening of existing wells in order to increase production ;

large diameter water wells in igneous rock (Africa).

nOil drilling techniques :

large diameter drillings in hard rock conditions. As an example

the Boniface company has performed in Saint Hippolyte du Fort

(Gard, France) a pre-boring 400 meter deep in 17"1/2 (444.5

mm) diameter in Karstic limestones (Kimmeridgien period) with

an average commercial output of 8 m/hour. This work has been

done for Total Oil Company.

n Ground investigations for mining with accurate full cutting

recovery giving a continuous geological information.

nCivil works : Piling - Bored pile construction.

foraco reverse circulation drilling

Documents