Graham Clydesdale
Security DBS
Aberdeen
Andre Leseultre
Etienne Lamine Security DBS
Brussels
A recently developed core bit reduces fluid invasion in the cut core by minimizing the exposure to the drilling fluid and by increasing the rate of penetration (ROP).
A high ROP during coring is one of the major factors in reducing mud filtrate invasion in cores. This new low-invasion polycrystalline diamond compact (PDC) core bit was designed to achieve a higher ROP than conventional PDC core bits without detriment to the cutting structure.
Field testing indicates that this core bit achieves higher ROPs than conventional PDC core bits and reduces the mud filtrate invasion in the cores.
The core bit has been used in numerous applications and is suitable for most coring applications. The field performance of this type of core bit was significantly better than the average achieved over a period of years using various PDC coreheads. The ROP was increased by a factor of 4.8 and bit life by 2.3. The bits were often reusable (Table 1).
The design uses a medium-heavy set PDC cutting structure developed with cutting models and balancing methods used for drill bits.
The bit's hydraulic configuration helps achieve the higher ROPS. Laboratory tests on the core bit design indicate that flow in the core bit throat is
Laboratory tests of these design features included measurement of pressure drop across the corehead and flow visualization studies (high-speed filming of the flow and paint tracing to indicate the flow pattern). A numerical simulation was also performed using fluid dynamics software to optimize hydraulic parameters.
Models were developed to understand the flow patterns of the experiments quantitatively and qualitatively. A simple Bernoulli approach has given good results regarding head losses and flow rates. A finite element study has shown promising results in the optimization of the design process of the low-invasion core bit.
Future work will involve refining the model and using sophisticated tracing technology in the field to determine invasion.
FILTRATE INVASION
Mud filtrate invasion into core samples significantly, affects laboratory measurements of certain reservoir parameters." Low-invasion coring is a technique that reduces the depth of penetration (invasion) of the mud into the core. The use of low-invasion coring allows laboratory measurements to be made on a native core, giving more accurate determination of wettability, relative permeability, and oil and water saturation. 2
Mud filtrate invasion is reduced by minimizing the exposure time of the core to the drilling fluid and creating a protective film around the core. 3 A nondamaging drilling fluid and reduction of the differential pressure between the mud and the core also help reduce invasion.
A low-invasion coring system has two essential components: a signed drilling fluid and a low-invasion core bit. A sponge core barrel may also be used for further reduction of the core's exposure time to the drilling fluid.
LOW-INVASION CORE BIT
A low-invasion core bit must reduce the exposure time of the core to the mud by increasing the rate of penetration during coring. The design of the core bit must permit the protective filter cake on the core to remain intact as the core passes through the core bit throat.
The low-invasion core bit performs both of these functions. In addition, its hydraulic configuration reduces the differential pressure on the core, reducing a major driving force of fluid invasion. A significant aspect of the design of this core bit is that the increase in ROP is achieved hydraulically, independent of the number of cutters on the bit.
Other low-invasion core bit designs rely upon a reduction in the number of cutters to increase the depth of cut and hence the ROP, but this design limits the application of these core bits to soft formations. Using a light-set core bit in medium or hard formations would result in premature cutter wear.
The PDC-cutter density on the bit is medium to heavy set. (The 8 1/2-in. x 4-in. bit has 39 cutters.) The face profile of the core bit is flat with a rounded shoulder. Fig. I shows the design features that provide the low invasion capability: flush internal diameter (ID), internal hp, and angled face discharge ports.
FLUSH ID
The ID of the core bit is smooth and has no exposed cutters. The circumference of the core is cut to size by shaped PDC cutters on the face of the bit. The core is not cut again after passing these cutters.
The protective filter cake is formed on the core after it is cut to size by the internal gauge cutters. The mud cake is maintained on the core surface by the smooth inner bore of the core bit.
INTERNAL LIP
Most core barrels have a pilot shoe at the bottom end of the inner tube. This shoe has an extended tip inside the core bit just above the throat. The drilling fluid is pumped down the annulus of the core barrel between the outer barrels and inner tubes and flows through the gap between the bottom of the lip of the pilot shoe and throat of the core bit (Fig. 1).
This flow impinges directly on the core as it passes through the throat of the core bit into the inner tube. Directing flow against the core creates a force against the core, resulting in an hydraulic pressure directed into the core. This pressure may be a driving force for invasion after the core has passed through the throat of the core bit. The force of the flow against the core may also erode the filter cake from the surface of the core.
The arrangement of the low-invasion core bit and inner tube pilot shoe differs from the conventional design to prevent mud flow through the throat of the core bit against the surface of the core.
The hp on the pilot shoe overlaps with an internal hp on the core bit, the internal hp having the smaller diameter (ID equal to the core outer diameter). This overlap creates a labyrinth-type seal between the shoe and the core bit. Elimination of direct flow impinging upon the core reduces the differential pressure between the mud and the core and prevents erosion of the protective filter cake by the mud.
DISCHARGE PORTS
Conventional core bits allow all the drilling fluid to flow through the throat of the core bit, along the core. Conventional face discharge core bits have flow ports through the crown of the bit. These bits split the fluid flow between the core bit throat and the flow ports.
The low-invasion core bit has face discharge ports that carry almost all the fluid flow. This flow pattern results from the sealing effect of the internal lip on the core bit overlapping with the lip on the inner tube lower shoe.
These ports are angled from the inside of the core bit towards the outer diameter. The angled ports direct the flow of drilling fluid away from the core. The exit of the flow ports is tightly curved to point nearly perpendicular to the axis of the hole. This curvature of the flow port, in combination with the profile of the core bit body creates the hydraulic characteristic of the core bit. The laboratory testing indicates that this hydraulic characteristic creates a suction that removes the drilled cuttings from the face of the bit. The flow patterns observed also indicate that the pressure of the mud surrounding the Core is lowered because of this suction effect.
The effective removal of drilled cuttings is key to achieving the high ROP required for low-invasion coring in any application.
The reduction of the mud pressure around the core will lower the tendency for invasion.
LAB TESTS
Laboratory tests and computer modeling were performed to evaluate the pressure drop and the flow repartition to ensure low invasion of the core.
The lab testing was designed to measure the effectiveness of the internal hp in preventing flow through the throat of the core bit.
A full-sized bottom section of core barrel and core bit was made up with inner tube and internal hp lower shoe. The core bit was placed in a replica of its bottom hole profile, including the core. Pressure transducers were placed upstream and downstream of the core bit to measure the pressure loss across the core bit.
The gap between the core bit and the shoe was sealed with an O-ring to ensure that all the flow was transmitted through the face discharge ports. The pressure loss through the core bit was recorded for incremental increases in the flow rate.
The O-ring seal was removed, and the same process was carried out for incremental increases in the gap between the core bit and the shoe (Fig. 1).
The curves of pressure loss vs. flow are virtually coincident for the conditions from sealed to an 8-mm gap (Fig. 2). The curves for 10.5-mm and 14.2-mm gaps separate from the group, giving lower pressure drops for the same flow rates.
The pressure/flow regime was similar for the tests with gaps from 0 mm to 8 mm. In other words, the total flow area of the core bit does not change from the sealed condition until the lower shoe/core bit gap exceeds 8 mm. Thus, the flow ports conduct virtually all of the fluid flow if the gap remains below 8 mm. In the worst case, there was
PRESSURE DROP
The pressure drop computation was performed using Bernoulli's equation. The head loss is divided into a uniform part and a singular part which includes bend, sudden enlargement, sudden contraction, divergence, and convergence.
The results of the computation are in good agreement with the test results. The calculated pressure drops corresponded to the test values. The secondary flow rate, Q2, is very limited, even for high values of principal flow rates (Fig. 3).
FLOW VISUALIZATION
These tests, which involved an 8 1/2-in. x 4-in. core bit, were designed to establish the nature of the flow pattern on the core bit face. The results of the tests are qualitative rather than quantitative and enable the flow pattern to be described.
The test apparatus allows flow visualization testing on full-sized bits and core bits up to 12 1/4-in. diameter.
The apparatus consists of two encased tubes and a jack system to set the bit in the desired position (Fig. 4). An hydraulic motor provides the driving force for bit rotation. The test bench allows flow rates up to 3,500 1./min (925 gpm). The tests were carried out in static position with freshwater in a Plexiglas counterform providing a transparent environment for visualization studies.
Two types of tests were conducted to produce still and moving pictures of the flow patterns.' The still pictures were produced by covering the core bit with paint and flow testing while the paint was still wet. The pattern produced by the removal of some of the paint gives an indication of the flow pattern. For moving pictures of the flow pattern, a high-speed camera filmed the particles that were injected into the fluid flow through the core bit.
STANDARD CORE BIT
The first core bit tested had a rounded profile and inner waterways (Fig. 5). Figs. 6 and 7 show that the flow coming from these inner waterways follows the profile until the nose has been reached, then unsticking occurs. The flow comes back to the surface of the core bit and hits the center of the outer waterway. Starting at this impact point, a greater recirculation occurs through the front face of the bit.
The major drawbacks of the configuration are the washing of the core at the level of the inner waterways, the impact point with zero velocity flow, and the risk of cuttings balling up in the outer waterways. Furthermore, the recirculation of the cuttings to the face of the core bit leads to an extensive recrushing effect, limiting performance.
PARABOLIC-PROFILE BIT
The washing of the core could be eliminated by using bottom discharge, but the angle of the profile vs. the angle of the discharge forces the majority of the flow to go to the center of the corehead, badly invading the sample (Fig 8).
The parabolic-profile core bit also has an impact point in the waterway and a recirculation, but to a lesser extent. Figs. 9 and 10 clearly show the maximal cleaning of the center of the core bit and a poor cleaning of the waterway.
The bottom discharge is located parallel to the face of the blade to maximize the cleaning and cooling of the cutters, contributing to the asymmetry of the flow.
LOW-INVASION CORE BIT
The low-invasion core bit remedies the drawbacks of the other configurations: the combination of the angle of the port and the angle of the profile forces the flow to keep its momentum in the direction of the outer waterways (Fig. 11).
Recirculation is minimized. The port is located in the center of the interblade space, close to the outer waterway. The configuration is totally symmetrical, leading to an optimal cleaning of the waterway and preventing any balling up (Figs. 12 and 13). A central flow is surrounded by two symmetrical vortices with the impact points outside the outer waterways in the annulus where a new recirculation cannot be created.
FLUID DYNAMICS
A numerical simulation was performed on finite difference software in three-dimensional geometry. 7 The program solves the full steady-state and transient Navier-Stokes equations and models laminar and turbulent flows with a k-e model.
The simulation was restricted to the space between two blades because of the axisymmetrical geometry and establishment on the test bench of flow independence between the different waterways. The simulation was performed to determine the following:
- Flow distribution
- Influence of port positioning
- Influence of flow rate and port section
- Influence of flow repartition (between the bottom discharges and inner space).
The following parameters were used in the simulation:
- Flow rate of 750 1./min (83.33 1./min for the modeled sector)
- 8-mm port diameter with a jet velocity of 27.6 m/sec (the flow is turbulent with a Reynolds number of 6,631)
- Flow repartition of 99% through bottom discharges and 1% through the inner space.
PORT POSITIONING
The flow coming out of the port is defined by two angles: alpha (orientation) and beta (inclination). The flow orientation is the angle between the flow direction and the blade. The flow inclination is the angle between the flow direction and the rotational axis of the corehead.
The low-invasion core bit has an alpha of 28 and a beta of 55.
FLOW DISTRIBUTION
The flow from the bottom discharge exits nearly tangential to the corehead face. After impact, the flow splits with part going to the gauge and part going behind the port.
This part of the flow goes along the blade towards the core but is sucked up by the flow coming from the port and thus goes back to the gauge along the cutting blade. So, this region between the port and the core is cleaned by the recirculation.
This suction avoids a zone with weak velocities, so the exposure time of the core to the mud is reduced. The flow is characterized by high static pressures along the PDC of the cutting blade (from 12,240 Pa to 27,134 Pa). The static pressure along the core is low (4,900 Pa).
INCLINATION ANGLE
The following inclination angles were tested: 4, 12, 20, 40, 55, 75, and 90. The higher the angle, the more important the recirculation behind the jet. At higher angles, the recirculation is quicker and the suction is stronger.
The static pressures along the core are three times less (4,900 Pa/15,500 Pa) for configurations with deflection (p = 55) than without deflection (p = 12). In fact, this static pressure reduction is high for angles up to 400 but is low for angles 40.
Flow inclination also influences cuttings removal through the outer waterway. When the inclination is high enough, two descending helical flows occur. The eddies generated around the jet move to the outer waterway and produce these two vortices.
The velocity along the cutting blade decreases with inclination angle. With a deflector configuration of 0 = 55 and (x = 28, this velocity is about 6 cm/sec, which is enough for cuttings removal.
ORIENTATION ANGLE
A weak variation of the orientation angle (a around 28) does not produce any modification in flow. For higher angles (56-90), more fluid goes behind the jet.
Moreover, the recirculation changes and is then in the direction of the cutting blade, which means the cuttings are directed to the core and have a longer path to the gauge.
For the descending helical flow in the waterways, increasing the orientation is not successful. High orientation angles are not adequate to direct the fluid to blade faces.
FLOW RATE AND PORT
Doubling the flow rate (166.66 1./min) and doubling the port section (11 mm diameter) were also tested. Three cases were considered: standard flow rate and doubled section, standard section and doubled flow rate, and doubled section and flow rate.
The flow pattern was not modified for the doubled port section, and there was only a flatter recirculation. The velocities decreased because of reduced injection velocity. The suction was weaker which should have a detrimental influence on the static pressure along the core.
For the doubled flow rate, the results were contradictory. A better suction should have enhanced a pressure decrease along the core, but a general pressure increase masked this advantage.
The flow pattern was the same for a doubled flow rate and port section as it was for the standard parameters. The velocities and pressures were greater than those for standard parameters.
FIELD RESULTS
The low-invasion core bit has been used in numerous field applications. The most useful data from the field are those which indicate a valid comparison with conventional core bits. Table 1 compares a low-invasion core bit to standard core bits that drilled an 8 1/2-in. hole in Jurassic sandstones (firm to hard) with oil-based mud, and it also compares a low-invasion core bit to standard core bits that drilled a 12 1/4in. hole in Eocene sandstones with oil-based mud.
Fig. 14 compares the fluid invasion from two types of bits. The low-invasion core bit has been run after a conventional core bit. A tracer was used to measure the invasion in the core. During the pipe trip, the bottom of the hole was invaded, resulting in poor quality of the first few feet of core of the second run.
ACKNOWLEDGEMENT
The authors thank Security DBS for permission to publish this article.
REFERENCES
- Rathmall, J.J., Tibbitts, G.A., Gremley, R.B., Warner, H.R. Jr., and White, E.K., "Development of a Method for partially uninvaded coring in high permeability sandstone," SPE paper 20413 presented at the Society of Petroleum Engineers 65th Annual Technical Conference and Exhibition, New Orleans, Sept. 23-26, 1990.
- Pallatt, N., Stockden, I.L.M., Mitchell, P.S.H., and Woodhouse, R., "Low Invasion Coring gives 'Native' Reservoir water Saturations," European Society of Professional Well Log Analysts, London, 1991.
- King, I., Delbast, B., Besson, A., and Chabard, J.P., "Hydraulic optimization of PDC bits," SPE paper 20928, Europec 90, The Netherlands.
- Coruse, R., and Chia, R., "Optimization of PDC bits hydraulic by fluid simulation," SPE paper 14221.
- Bisson, P., Choux J.C., and Provo, A., "Hydraulic optimization of PDC bits by visualization methods," SPE paper 18370, 1988.
- Myer, M., and Funk, J.E., "Fluid Dynamics in a Diamond Drill bit," SPE paper 1696, 1967.
- Meunier, L., "Contribution a l'etude du comportement hydraulique des tetes de carottage," internal report of Mons Polytechnique School, 1992-1993.
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