This report describes the development, design and surface testing of a device designed to maintain pressure in a borehole during all phases of drilling. This borehole pressurisation device is designed to fit directly between the standpipe and the drill.
permit the taking of un-degassed chip samples.
The borehole pressurisation device has been tested successfully in a surface trial involving high pressure waterjet drilling through concrete. The device operated for several hours of flow time much of which involved the passage of cuttings.
The main focus of the trial was on the performance of the regulator which had to regulate cuttings and water flow to a preset pressure and in addition had to shut down completely below that pressure. How the abrasive nature of the cuttings affected the life of the regulator was the main concern.
The tool had through flow for several hours a fair proportion of which was spent drilling. The regulator was shut down for several days under conditions of no flow. The only malfunction of the device was caused by blockage when water jet drilling produced large particles of greater than 10 mm diameter. The regulator element itself showed slight scoring but negligible wear.
BOREHOLE PRESSURISATION SYSTEM
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1. BACKGROUND
In 1993 the need was perceived for geophysical devices to be used to give geosteering capabilities to in-seam drilling. At the time the three most valuable geophysical devices appeared to be resistivity, sonic and gamma tools. Only the latter would operate satisfactorily in a borehole which was not liquid filled. Therefore the concept of borehole pressurisation was proposed by which fluid pressures in a borehole could be maintained above sorption pressure thus ensuring a water filled borehole. Australian Coal Research provided a research grant to the AGA Consortium to develop such a device in its 1994 ACARP research round.
The tool was designed and built and displayed in early 1995 and then languished for lack of use in a mining application despite several attempts at getting it underground. These essentially were caused by a lack of a test mine. A site was chosen and then became unavailable. Replacing the site became more difficult as the economics of coal mining declined. In late 1997 the opportunity came to test the device in a surface trial. The requirement in this case was to provide adjustable back pressure to an artificial steel tube borehole. At the front of this borehole concrete was being cut using high pressure water jets. The drilling fluid and particles being cut travelled back along the steel tube and through the borehole pressurisation device. In general terms the device worked admirably.
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2. CONCEPT AND POTENTIAL USES
The concept of how the tool could be used evolved through its development. The design criteria was however clear from the outset, namely that the tool should be able to maintain pressure in a borehole during all phases of drilling. That pressure was intended to lie above sorption pressure and in virgin situations below seam water pressure. In this situation no gas would be expected to be emitted into the borehole and no drilling fluid lost into the seam. Clearly where unsaturated conditions exist the pressure would still have to be maintained above sorption pressure with a resulting loss of drilling fluid into the seam. The amount of fluid lost will be a function of the the seam fluid pressure, drilling fluid pressure and seam permeability. By maintaining pressure just above the sorption pressure threshold fluid loss could be minimised.
Keeping the borehole full of water brings with it several advantages described below.
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2.1 Borehole Wall Support
A net positive pressure against the wall of the borehole from the drilling fluid will help to support it. Normally drilling from underground is conducted in an under-balanced condition, meaning that the fluid pressure in the formation is less than that in the borehole. This means that there is a net flow from the seam to the borehole. Whilst this is good from the viewpoint of minimising formation damage (in coal this means clogging of cleats) it does however mean that a fluid pressure gradient exists from the formation to the borehole. This gradient, in combination with stresses which are concentrated around the borehole, may lead to borehole wall failure. In soft material erosion by fluid flow into the borehole may take place in the form of slumps or mini outbursts. Raising borehole pressure means that the gradient is minimised or reversed thus enhancing stability.
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2.2 Improved Operator Safety
Drilling in a gaseous environment is potentially dangerous. This danger arises from the escaping gases produced in drilling. Whilst there have been no instances of drilling operators being killed in Australia this has not been the case worldwide. Some deaths have been reported in cases where drilling has passed through gouge zones and the resulting expulsion of gas and particulate matter has lead to the deaths of drillers. The day to day problems of handling gas whilst drilling are a more regular issue. The ability to stop a borehole producing gas whilst it is being drilled has much to recommend it. The borehole pressurisation tool has that capability. The only gas expected to be produced is that issuing from the degassing cuttings.
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2.3 Use of Geophysical Tools
Some geophysical tools require an annulus filled with water to operate. Examples of this are sonic velocity and resistivity sondes.
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2.4 Sorption Pressure Measurement
Sorption pressure is the pressure at which the gas is stored in a coal seam. This pressure is a complex function of the coal seam history. It is described further in the next section. Suffice to say that it is an important coal seam reservoir property that needs to be measured. The borehole pressurisation device may make this possible.
If drilling is being conducted with the borehole pressurization device maintaining water pressure above the sorption pressure then the cuttings so produced will not be able to degas in the normal manner. Some gas exchange may take place from cuttings into solution in the drilling fluid, however this can be expected to be a small proportion of the gas content of the chips. The concept of sampling the cuttings held at pressure and measuring their sorption pressure is then attractive. This will allow semi-continuous sampling of sorption pressure throughout the length of a borehole.
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3. SORPTION PRESSURE
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3.1 Sorption Pressure in a Saturated Reservoir
Sorption pressure is the term used to define the pressure at which gas is stored in coal. In the case of a coal seam reservoir where the water pressure exceeds the sorption pressure no free gas exists, all of the gas being tied up in sorption (including adsorption, absorption and chemisorption) or in solution in the reservoir fluid. At water pressures exceeding sorption pressure the reservoir has a very stiff response to drainage, meaning that the value of storage (volume released per unit volume per unit head change in pressure) is very low. In this state the reservoir essentially behaves as a confined aquifer in which gas can be transported in solution. The exchange of gas between the coal and water is not fully defined but it is expected that the level of saturation of the water is controlled by the sorption pressure.
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3.2 Sorption Pressure in Unsaturated Conditions
When the water pressure drops below the sorption pressure free bubbles of gas form in the coal. This signifies the onset of very different reservoir behaviour. The compressibility of the gas means that the system becomes soft. At the same time the permeability changes due to multiphase behaviour. As the coal degasses and dries out it usually shrinks leading to changes in intrinsic permeability.
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3.3 Sorption Pressure as a Function of Gas Generation and Replacement
Sorption pressure is itself a complex relationship between the water and gas as well as coal types. It is history dependent, meaning that it is important which gas got to the coal chronologically and how it got there. Coal normally goes through a process of gas generation during coalification. The gasses generated are usually carbon dioxide and methane in that order and they can be expected to be generated with other hydrocarbons which are usually lost from the system in geological time. The gas stays there because of its affinity with the coal given adequate water pressure (hydrostatic head) to keep it in place and provided the seam is not subject to recharge and through flow of unsaturated water.
Consider the case of a seam which has generated methane which covers its micropore surfaces. If it is then subject to a flood of carbon dioxide or nitrogen rich gas in a hydrothermal process some of the methane is forced out and lost and some more is generated. There is then a competition for active sites, methane versus the invading gas. The extent to which replacement takes place depends on temperatures and concentrations. Following the replacement the coal may generate gas or lose gas in solution or by the lowering of the hydrostatic pressure.
What is left is a coal with potential sorption sites being occupied with water and different gasses. As different sites compete for gases and water in a manner which is history dependent it is not possible to define a single sorption isotherm of gas pressure versus gas content. Re-gassing a sample with a gas mixture corresponding to the seam gas composition to generate a sorption isotherm is therefore an invalid process.
Finding sorption pressure of the gas in the coal is a critical measurement because of the way in which reservoir properties change with the presence of free gas.
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3.4 Fluid Pressure, Sorption Pressure, Effective Stress and Outbursts
Fluid pressure is a component of the equation for effective stress:
Effective stress = total stress - fluid pressure
In a coal seam the failure of the seam is governed by the effective stress combinations. For outbursts to occur from solid coal the coal must fail (under effective stress) and then produce gas at a high enough rate to eject coal. In an outburst in already broken material (gouge zone etc) failure rapidly occurs with the approach of mining as the material is weak and the rate of gas evolution from this material governs how the material is ejected.
In a saturated coal the water pressure is the fluid pressure in the equation for effective stress. With dilation of the coal or drainage to the face, the water pressure drops and gas pressure takes over as the term in the effective stress equation. The volume of gas in coal for a given pressure drop is such that the gas pressure is much more readily maintained than water pressure.
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3.5 Measurement of Sorption Pressure
Sorption pressure is an important measurement in its own right. Gas content measurement is a different measurement. It is not possible to reliably measure gas content alone because of uncertainties associated with back calculations of Q1. This error and an uncertainty in a knowledge of the sorption isotherm mean that the sorption pressure cannot be estimated to a reliability much better than 50%. Sorption pressure therefore needs to be measured directly. Because of its importance to coal seam reservoir and failure of the coal it needs to be measured.
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3.6 The Use of the Borehole Pressurisation Tool To Measure Sorption Pressure
The borehole pressurisation tool is designed to give measurement of sorption pressure directly. In addition it is hoped that it will give an estimation of desorption rate.
Both these measurements can be derived from cuttings sampled from the return drilling fluid stream. It is expected that by operating the borehole pressurisation device at a pressure between hydrostatic and sorption pressure, drilling fluid will not be lost in the formation and neither will gas be lost from the coal. Thus when the fluid stream is sampled, cuttings at initial gas content and pressure will be present. The cuttings are designed to be retained on a filter in a pressure vessel. The fluid pressure in this vessel at the end of a sampling period is the same as that in the borehole pressurisation tool. A small amount of fluid can then be bled off and the pressure should return to sorption pressure. This process can be repeated for confirmation.
After testing for sorption pressure the sample vessel can then be opened to atmospheric pressure and the rate and volume of gas release measured. This will give an indication of gas release rate and the total gas content. The release rate is important because it is gas release rate that drives outbursts. Following degassing the samples can then be removed and weighed or the sample volume measured by displacement.
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4. THE TOOL
The borehole pressurization tool can usefully be described in oilfield terminology as a rotating tubing stripper fitted with an adjustable pressure regulator and an annular blowout preventer. A rotating tubing stripper means that tube can be rotated whilst it is being run or pulled under pressurized conditions. The adjustable pressure regulator is one of the main developments of this project. It is designed to permit flow of cuttings and drilling fluid when this is being pumped and to close down and seal when the pressure drops below a set value. The annular blowout preventer is a rubber section in doughnut form. It is actuated by a hydraulically operated piston to close down on the rods when desired.
The tool built is designed to pass drilling components up to 4 inches (101.2 mm) diameter.
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4.1 Overall Operation
Figure 1 shows the operation of the borehole pressurisation device. The drawing to the left shows the operation of the device whilst drilling. In this case water from the mud pumps is being passed through a two way valve, on through the water swivel and through the drill rods. It may operate a down hole motor before passing through the bit and back up the annulus carrying cuttings with it. The fluid and particle stream then pass out through the regulator which is designed to provide a constant and adjustable back pressure. The annulus around the drill string is sealed to the rotating assembly by a very heavy duty self energised top lip seal element which is connected to the rotatable spindle.
The central drawing shows the operation when adding or removing a drill rod. In this case the flow from the pump is diverted from the water swivel to the body of the borehole pressurisation device. Thus fluid flow is maintained in the annulus and the regulator is kept in operation. This is desirable to ensure constant fluid pressure and to prevent the regulator closing down unnecessarily whilst it contains cuttings. Back flow through the drill string is prevented by a check valve located behind the bit or downhole motor.
The rightmost drawing shows the situation when either the lip seal is worn or the regulator needs repair. In this case the lower seal or blowout preventer can be closed by a piston that compresses it axially so that it seals the annulus between the drill string and the tool body. Thus the borehole is closed and the lip seal or regulator may be removed for maintenance during the drilling operation. On completion of the repair the annulus of the tool is repressurised, flow established through the regulator and the hydraulic pressure on the piston driving closing the annular seal is removed. Drilling can then recommence.
In the event that pressures become excessive such as in the case where the regulator blocks, a pressure relief valve on the tool body will operate to protect the mechanism. This leads to a fairly rapid drop in pressure in the tool body. This is detected as a pressure difference between the desired pressure as set on the accumulator that controls the regulated pressure and that existing in the tool body. This pressure difference operates a valve that opens a connection to automatically actuate the annular seal piston. This should be linked to the drill rig to stop rotation and feed of the rods. A similar effect occurs if a lip seal cuts out leading to a drop in pressure.
On the completed tool the lip seal was designed to operate with rods between 65 and 85 mm diameter. Thus if smooth transitions are incorporated into the rod string it is designed that the drill string can be withdrawn right out of the casing/standpipe whilst operating under pressure and the ball valve closed thus shutting in the borehole. The borehole pressurisation tool can then be removed and the borehole bled down to a suitable pressure gradually through a suitable valve assembly that can replace the borehole pressurisation tool.
The main body of the tool weighs about 200 kg and is shown in plan in Figure 2. A pictorial view of the device is shown in Figure 3. Plate 1 and Plate 2 show the tool in the surface trial.
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4.2 Sampling for Sorption Pressure Measurement
Figure 4 shows the method of operation of the tool for chip sampling. In this case the drilling fluid stream containing cuttings is passing from the main body into the side arm of the tool and out of the regulator. In order to sample, a vessel connected to the side arm is opened to the drilling fluid through two isolating ball valves. The other end of the sample vessel is then opened to atmospheric pressure by an outlet ball valve to allow an inrush of particle-containing fluid. The particles are retained on the filter screen within the vessel. The valve to atmospheric pressure is closed and then so are the two isolating ball valves. The sample vessel may then be disconnected via the union for testing and another sample vessel put in its place.
The sample vessel is furnished with a bleed valve downstream of the filter. It is intended that this be used to bleed off a small proportion of fluid from inside the vessel. As the water is comparatively incompressible the removal of one or two drops of water will make that space available for gas to expand into. Gas from the coal will desorb into that space until equilibrium is reached. In the event that a reasonable sample of coal chips are collected and a small amount of drilling fluid is bled off the pressure should return to a value close to sorption pressure. Further bleeding of fluid from the vessel followed by a period of shut in should confirm this value.
In the event that a large enough chip sample is taken it may be possible to determine total gas contained in the vessel provided that the drilling fluid does not absorb it. This is expected to be quite possible for methane but not for carbon dioxide seam gas.
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4.3 The Regulator
The regulator is designed to operate whilst passing cuttings and to be able to shut down in the event that internal pressure drops below a threshold. These are difficult criteria to meet and required a significant proportion of the design effort to come up with a satisfactory solution. A quick calculation will show that at 150 litres/minute an orifice of 4.6 mm diameter is required to achieve a pressure drop of 5 MPa. To do this the flow velocity through the orifice would be 151 m/s. This type of choke has significant problems in as much as it will wear due to the high velocity, is not adjustable and will not shut down. The use of a gate type valve as a regulator is even more of a problem as the gap would be narrower leading to more blockage.
An alternative device was therefore sought. One of the key features of this was that the device should provide back pressure through friction over a substantial length of restricted size. By doing this the actual opening could be larger than it would be in the case of a choke. In addition it was essential that the device should be a true regulator and not have to be adjusted for different flow rates.
Experimentation on rubber types had showed that the annular seal could withstand particle damage when it shut down because rock was trapped between the rubber and the steel. In this mode the larger rock particles tended to break down to smaller ones which did little damage. This was unlike the case of rubber on rubber where the rock tended to cut the rubber. The concept of a gas filled packer inside a tube was considered as a regulator but the annular gap would have been minute to achieve the pressure drops being considered. An alternative of an externally pressurised tube was also considered and discarded because in a thick wall tube the rubber would be strained infinitely as it closed down to zero diameter internally. It was considered that such a load would lead to rupture. This approach was also undesirable because the closure of the hole would have trapped particles between rubber sides of the tube.
Bearing in mind the experimental results the tube idea was modified to a similar form by placing a rod through the centre of the tube. In this mode the tube was externally loaded by hydraulic fluid maintained at a preset pressure by a precharged accumulator. In this state the tube was forced closed on to the rod by the hydraulic fluid. A threshold pressure is required to close the tube on the rod and further pressure just squeezes it more tightly in place. When drilling fluid pressure increased beyond the precharge pressure minus the closing threshold pressure fluid begins to flow. Increasing drilling fluid pressure causes the tube to grow thus opening more area and maintaining the pressure. Provided particles are not too large to enter the gap between the inner rod and the metal tube entry, they simply pass down the regulator. Blockages cause a local pressure rise which leads to the tube expanding and the blockage being eroded away and released.
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4.4 Connections to the Drill
The borehole pressurisation device is designed to be connected to the drill hydraulics through a hydraulic supply and return. The hydraulic fluid is designed to be used to recharge the regulator hydraulics and the accumulator used to close the annular seal. The very low volume required is supplied via an orifice and released by a pressure relief valve back to tank. Draw-off flows from this line can be used for both designated purposes. In these cases the draw off is through orifices and check valves and through adjustable pressure relief valves. A manually operated valve will bleed the hydraulic circuit of the annular seal back to tank thus releasing the device. The pressure relief valve on the regulator circuit enables the pressure in the accumulator to be adjusted. A manual dump valve is also required to relieve back pressure.
Though it is not yet incorporated it is essential that a mechanism be used that will stop drill rotation and movement in the event of the annular seal being actuated. This mechanism is somewhat drill dependent.
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5. TESTING
The borehole pressurisation tool was tested on surface to provide back pressure in a water jet drilling trial being conducted by the CMTE Drilling Group. The test setup involved sending high pressure water up a drill string, through a water jet drill bit into a hole being drilled into concrete test cylinders approximately 1.5 m long and 300 mm diameter. The drill fluid and cuttings then returned via a steel tube used to simulate the drillhole and then through the borehole pressurisation tool and out to waste. The water flows were in the range 200 to 230 litres/minute (53 to 61 USGPM). The concrete samples being tested varied were of either 15 MPa or 30 MPa uniaxial compressive strength and contained aggregate up to 10 mm size. Cutting rates during drilling were up to 30 cm/minute.
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5.1 Regulator Performance
The regulator was found to require 1.0 MPa to close around the internal rod in static conditions. Flow tests showed that the regulator back pressure was a very good linear function of the regulator accumulator pressure. This is shown in Figures 5 to 7 (Fig 5; Fig 6; Fig 7). Two regulator elements were made. The first was a little large in diameter and had a tendency to block the hydraulic fluid part of the regulator and it was therefore replaced with a second element.
Figure 5 shows the regulating performance of the first regulator element using a range of accumulator pressures from 1.5 to 4 MPa and a flow range from 150 to 240 litres/minute. As can be seen a good linear performance has been obtained. It should be noted that flow does influence the accumulator pressure which drops back after operation. This characteristic is presumably a function of the hydraulic system volume. A larger accumulator would be expected to reduce this effect.
Figure 6 shows a very good linear match between accumulator pressure tested from 0 to 3.75 MPa and back pressure for the second regulator element at a fixed flow rate of 220 litres/minute. Figure 7 shows the behaviour of the second regulator element whilst drilling with cuttings flowing in a fluid flow rate of 240 litres/minute. It shows a little scatter but is of essentially the same shape as the case with only water flowing.
Dynamically measured pressures showed maximum pressure pulses of 0.1 MPa when particularly heavy cuttings loads were encountered.
The only problems experienced during drilling were an occasional blockage of the regulator inlet. This was designed to accept particles up to 10 mm diameter and when the cement/aggregate clumps from the water jet drilling trial exceeded this size the inlet blocked and the pressure relief valve in the system blew. Normal coal drilling would not be expected to produce particles this size or of the same strength as the aggregate. It is in fact comparatively easy to extend the inlet size of the regulator to take 15 mm particles.
Following approximately 10 hours of drilling and flowing trials the regulator element was removed and examined internally with a laparoscope. Some longitudinal score marks were observed but these were minor. No measurable wear could be discerned. The central rod similarly showed some scoring and would in future operations be switched from 316 stainless steel to a more wear resistant material.
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5.2 Seal Performance
The borehole pressurisation tool was sealed whilst containing a drill rod by the action of the annular seal and the lip seal. Static pressure could be maintained in the tool under these conditions. Tests during the drilling trial showed that the spindle seals had too high a friction which could have led to a drill rotating within the lip seal and causing that seal to fail. The internal spindle seals have since been replaced with PTFE units to reduce friction.
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5.3 Sampling Unit
The sampling unit was tested during the drilling trial and found to operate satisfactorily. As concrete was being drilled, not gassy coal sorption pressure testing could not be accomplished.
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6. CONCLUSIONS
The borehole pressurisation tool has been shown to be a functioning device capable of fulfilling the role for which it has been designed. Its real success will need to be determined by underground drilling when an opportunity arises. The performance of the regulator is particularly pleasing in its ability to cope with particles in a fluid stream. It has many potential uses.
The tool is not limited to underground drilling but has potential for use in surface open hole drilling where chips may need to be sampled to assess sorption pressures. Another possible use in surface drilling is where over pressures may be expected but it is undesirable to used weighted muds to control these pressures.
The success of any field use of the tool will depend on having a good wellhead/standpipe to sustain drilling pressure and a formation that will not permit too much fluid to be lost from the hole.
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7. ACKNOWLEDGEMENTS
The writer wishes to thank ACARP for financially supporting the project. Those individuals particularly involved were:
John Hanes, Jon Sleeman, Graham Muller, Laszlo Agoston, Tim Meyer and Evan Matthews. Also the Mater Private Hospital Operating Theatre, Brisbane, for use of their laparascope.
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8. LIST OF FIGURES AND PLATES