WO2002048505A1

WO2002048505A1 - An arrangement and method for drilling with optimal penetration rate

Info

Publication number: WO2002048505A1
Application number: PCT/NO2001/000491
Authority: WO
Inventors: Per H. Moe
Original assignee: MOE, Per, Thomas
Priority date: 2000-12-11
Filing date: 2001-12-11
Publication date: 2002-06-20
Also published as: NO313468B1; AU2002222825A1; NO20006302D0; NO20006302L

Abstract

This invention relates to a method and arrangement for drilling in the subsurface. The drilling is done with a rock-drilling hammer, and, according to the invention, the pressure is measured in the drill fluid that is circulated in the borehole. by controlling the feed of drill string into the hole as a function of the pressure, the drilling hammer will be forced towards the bottom in the borehole with an optimal high force.

Description

AN ARRANGEMENT AND METHOD FOR DRILLING WITH OPTIMAL PENETRATION RATE

Field of the invention

The present invention relates to drilling in the subsurface .

Technical background

Today, all drilling for oil and gas is done by means of Polycrystalline Diamond Compact (PDC) bits that cut and rolling bits that crush. PDC bits are used in soft rocks while rolling bits are used in hard rocks.

During drilling, the bit is rotated while it is forced towards the bottom of the hole with a certain force . If this force becomes too high, the cutters on PDC bits are torn off while the bearings on roller bits are destroyed. In either case, the bits will be short lived and debris (often hard parts that have to be removed) will remain in the hole. If the weight on the bit becomes too low, the drilling will become ineffective.

In order to assess the weight on the bit during drilling, a weight cell is used which measures the weight in the lifting block that suspends the drill string with the bit.

The drill string is rotated with the bit positioned just above the bottom, and the weight is read. When the bit touches the bottom, the weight on the block is relieved and in theory it is possible to see which degree of relief that corresponds to the weight on the bit . The problem is that when drilling at 4,000 meters, the weight of the string is 100 tons and the force on the bit should be 5 tons ± 1 ton. The pointer on the scale is vibrating with an amplitude of more than ± 1 ton, so the weight can be in the interval 0-10 tons as the regulation is performed by feeding more or less string according to the driller's experience. Additional difficulties will arise when drilling from a vessel heaving in the waves. This is intended to be counteracted by the heave compensators, but the uncertainty in the weight/force on the bit increases.

During drilling for gas and oil, water (commonly with clay (mud) added) is circulated down the drill string and up the annulus between the drill string and the rock wall. This water (mud) circulation is motivated by many reasons :

o 1. It is primarily intended to lift drill cuttings to the surface,

2. To cool the drill bit,

3. To prevent the hole from collapsing,

. By adding a lot of clay, the specific weight can be s increased up to 2.2 kg/dm³. Sudden increases in pressure when running into oil or gas are choked by the heavy fluid and blowouts prevented,

5. If the string is stationary, like when using coiled tubing, a motor can be driven by the fluid flow that o again rotates the drill bit,

6. In soft rock, the fluid may be scavenged through nozzles in order to obtain a doubling of the penetration rate.

Circulation is obtained with mud pumps with pistons, which 5 often have a maximum pressure approaching 500 bar.

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It seems that this instability problem can be solved in one way only; with full continuous control over the weight exerted on the drill bit.

Brief summary of the invention

The object of the invention is to provide an arrangement and method for drilling holes in the subsurface that avoids the drawbacks mentioned above. In particular, the invention relates to an arrangement and method for drilling with a steady and high penetration rate, achieving a longer life span for the bit.

Another aspect of the invention is that feedbacks regarding the work conditions of the bit/hammer is obtained more rapidly, and these feedbacks are more accurate and unambiguous than in prior art systems. Thus, the drilling can be done under safer conditions as error conditions can be discovered more rapidly. Further, one is not so dependent on the drilling crew's experience for empirical control of the drilling/ and the drilling process can be more easily automated. In addition, the arrangement and method according to the invention is more cost favourable than alternative solutions according to prior art.

This is obtained in an arrangement and method for optimal drilling as defined in the appended patent claims.

Brief description of the drawing

The present invention will now be described in relation to the enclosed drawing, which shows an arrangement for drilling holes in the subsurface, and which makes use of the invention. ft⁾ * Ω rt . -. « cr Hi cr ft⁾ rt >d oo rt CQ μ- σi ( ■ t Hi Hi rt H3 μ- 3 O Hi rt rt μ- ⁽-3 a

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stream up the annulus can be up to 10 bar. Values for this so-called friction loss can be found by measuring the pressure loss over the system while the drill bit is held just above the bottom, i.e. before contacting the rock.

When using a stationary string and drilling motor, the drilling motor also has 10 bar resistance when rotating without the bit touching the bottom.

The hammer has also a certain internal resistance, about 10 bar, when the water is streaming there-through and the hammer is not striking due to the bit being above the bottom.

Thus, the total pressure loss is 100 + 3*10 bar = 130 bar.

When the hammer reaches the bottom, the bit is forced in and the hammer begins to strike. At the same time, the resistance against the fluid flow, i.e. the pressure loss over the tool, will increase. Ideally, the hammer should have a resistance of 108 bar. This value is, of course, dependent on the brand and type of the hammer. When the hammer is striking, the rotational resistance will also increase somewhat, the resistance in the rotation motor will increase from 10-25 bar.

If a greater force is exercised on the tool, the pressure will build up over the tool until it stalls. There is also a safety valve in the tool, set to 350 bar. However, the hammer is working optimally when it is forced towards the rock with as large a force as possible, i.e. just before the drilling stops.

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Claims

P a t e n t c l a i m s

1. A method for drilling holes in the subsurface, the drilling being done by means of a rock drilling hammer suspended in a drill string, with an arrangement for s feeding drill string up and down the borehole, as well as a circulation system including a pump pumping drill fluid down the drill string, the drill hammer being powered by the fluid circulation, c h a r a c t e r i z e d i n that the pressure in the o drill fluid is measured in the drill string at the surface, and the force from the drilling hammer towards the bottom of the bore hole is controlled by setting the feed rate of drill string in or out of the hole so that the pressure is kept inside predefined limits.

s 2. A method as claimed in claim 1, c h a r a c t e r i z e d i n that said pressure measurement is compensated for influence from the length of the drill string by performing an initial measurement of the pressure before the drill bit is touching the 0 bottom of the borehole.

3. A method as claimed in claim 1 or 2, c h a r a c t e r i z e d i n that the feed of drill string in or out of the bore hole is set to achieve as large a force from the drill bit towards the bottom of the 5 bore hole as possible without the drilling being stopped.

4. An arrangement for drilling holes in the subsurface by means of a rock drilling hammer with a drill bit, suspended in a drill string, as well as a pump for circulating drilling fluid through the drill string, and o the drilling hammer being driven by the fluid circulation, c h a r a c t e r i z e d i n a sensor gauging the pressure in the injected drill fluid on the surface downstream the pump, a control circuit receiving the signal from the sensor, which control circuit is adapted to control the feed of drill string in and out of the bore hole as a function of the pressure, in order to keep the force exerted from the drilling hammer towards the bottom of the bore hole inside predefined limits.

5. An arrangement as claimed in claim 4, c h a r a c t e r i z e d i n that said control circuit is adapted to control the feed of drill string in and out of the borehole in such a way that the force from the drilling hammer towards the bottom of the borehole is so large as possible without the drilling stopping.