SLIM HOLE MWD TOOL ACCURATELY MEASURES DOWNHOLE ANNULAR PRESSURE

Feb. 14, 1994
Measurement while drilling of downhole pressure accurately determines annular pressure losses from circulation and drillstring rotation and helps monitor swab and surge pressures during tripping. In early 1993, two slim hole wells (3.4 in. and 3 in. diameter) were drilled with continuous real time electromagnetic wave transmission of downhole temperature and annular pressure. The data were obtained during all stages of the drilling operation and proved useful for operations personnel.

Measurement while drilling of downhole pressure accurately determines annular pressure losses from circulation and drillstring rotation and helps monitor swab and surge pressures during tripping.

In early 1993, two slim hole wells (3.4 in. and 3 in. diameter) were drilled with continuous real time electromagnetic wave transmission of downhole temperature and annular pressure. The data were obtained during all stages of the drilling operation and proved useful for operations personnel.

The use of real time measurements demonstrated the characteristic hydraulic effects of pressure surges induced by, drillstring rotation in the small slim hole annulus under field conditions.

The interest in this information is not restricted to the slim hole geometry, (2# in. drill pipe in a 3 in. hole). Monitoring or estimating downhole pressure is a key element for drilling operations. Except in special cases, no real time measurements of downhole annular pressure during drilling and tripping have been used on an operational basis. The hydraulic effects are significant in conventional geometry wells (3 in. drill pipe in a 6 in. hole).

DOWNHOLE PRESSURE

Downhole pressure is a function of hydrostatic load and pressure losses in the annulus. Calculations of well bore pressure are based on pressure loss models with constant mud density. These assumptions are acceptable in most wells because the mud density in the well does not differ significantly from that measured at the surface, and, in conventional wells, annular pressure losses are often negligible.

The difference between formation and fracture pressure is often sufficiently great to allow a high mud column pressure, which overwhelms any uncertainties introduced into the calculations as a result of the assumptions.

Downhole pressure is a critical parameter in slim hole drilling, however. Because the annular area is relatively small in slim holes, annular pressure losses are not negligible.

When mining drilistrings are used, the annular pressure losses become predominant, accounting for up to 90% of the total pressure losses. In such cases conventional pressure loss calculations do not work.1

In many high pressure, high temperature wells, the formation and fracture pressure gradients converge. Consequently, close monitoring of the mud column pressure is necessary. A similar control of the mud column pressure is needed when depleted formations are drilled. Under such circumstances, the availability of realtime downhole pressure is extremely useful.

In the field tests, the annular volume was represented by a 5 mm thick cylinder (66 mm drill collar in a 76 mm borehole). This configuration induces specific hydraulic effects. The drillstring vibrations, high rotational speed, and transversal motions produced highly irregular flow instabilities.

Slain hole bits require high rotational speeds (300 1,000 rpm), and the annular volume between the drillstring and well bore is considerably smaller than that in conventional wells.

The annular eccentricity and the resultant fluid flow regime cannot be established with certainty. The effects of variation in fluid temperature and annular geometries on annular pressure losses are complex and these effects are not easily incorporated into standard models. Centrifugal instabilities, such as Taylor vortices, contribute to the increase in annular pressure losses with increased drillstring rotation.2

Previous on site measurements showed that pressure losses increase with drillstring rotational speed, and the recent field tests confirmed this observation. " At present, only downhole measurements can provide adequate information during field operations.

EUROSLIM

In 1991, the Euroslim project began development of a new slim hole drilling system, including the design and construction of a purpose built rig and drillstring. The project also worked on well control, drilling monitoring, and mud system improvements.

Forasol SA, Diamant Boart Stratabit SA (DBS), Geoservices SA, and the Institut Francais du Petrole (IFP) tested well control systems, drill monitoring systems, and measurement while drilling (MWD) tools and technology.

The results were practically applied in Elf Aquitaine's FAX (Forage Allege d'eXploration) project. In the FAX project, two slim hole wells were drilled to evaluate drilling, logging, and testing technology on site.

MWD TOOL

The two vertical wells were drilled in known formations. Thus, directional and formation evaluation measurements were not required. Consequently, the prototype slim hole electromagnetic MWD tool was built to record annular pressure only.

The MWD tool was derived from proven downhole production gauge technology. The tool consisted of a 1 11/16 in. (43 mm) probe suspended inside a conventional 2.6 in. (66 mm) OD x 2 in. (52 mm) ID drill collar (Fig. 1). The strain gauge sensor was connected to the drill collar annular pressure port by a grease fined buffer tube.

Pressure measurements were scanned every 2 sec and transmitted to surface in a data package every 24 sec. One temperature record was transmitted for every eight pressure measurements. All MWD measurements were transmitted directly to the mud logging unit (ALS 2, level 5), integrated into the data base, and displayed in real time on work station video monitors on the rig. Measured data were also stored every 6 sec in a downhole memory module with a 4 day recording capacity. These data could be downloaded upon tool recovery at surface.

The electromagnetic transmission technology uses geological formations as transmission media. Bidirectional (surface to bottom or bottom to surface) transmission can be achieved without any interference from rig operations. Mud circulation is not required, and transmission may be established as soon as the MWD tool enters the hole, regardless of mud flow rate or drilling and tripping operations.

This transmission technology has been applied to downhole directional and gamma ray acquisition since 1987 and works even in air or foam drilling. For this particular application, data transmission was possible even in the critical conditions caused when minimum flow rate conditions are not maintained (such as swab and surge effects during tripping and circulation breaks during connections).

In 1994, directional and gamma ray sensors will be added to this slim hole MWD tool (which is a 2# in. OD drill collar).

FAX WELLS

The two vertical wells in the FAX project were drilled in known geological formations in the Paris basin. Fig. 2 shows the hole and casing programs. Both wells reached 2,160 m (7,086 ft) without major problems.

A total 2,850 m (9,350 ft) of borehole was drilled with bit diameters of 120 mm (4 # in.), 86 mm (3.4 in.), and 76 mm (3 in.). For the Euroslim project, DBS manufactured two complete drillstrings designed for normal destructive drilling and wire line coring. Table 1 lists the dimensions of the string used with the MWD tool.

The MWD tool was installed 12 m (40 ft) above the drill bit in a slick bottom hole assembly. Several runs were made in 86 mm (3.4 in.) and 76mm (3 in.) holes with no failures recorded during the 200 hr of operation, 120 hr of which were in harsh drilling conditions with rotational speeds in excess of 350 rpm.

FIELD RESULTS

The MWD tool provided useful operational information that was highly appreciated by the drilling crews. In one instance, despite an increase in injection pressure from 30 to 53 bars (430 760 psi) in 10 min, downhole pressure measurements clearly showed that the annulus was free from any pressure surge (Fig. 3). Therefore, drilling could continue without damaging the formation. Flow in the drillstring was obstructed for 20 min but returned to normal, although no definite explanation was found.

Without the downhole pressure information, several hours of rig time could have been lost by pulling out of hole to clean the well of any hypothetical annular restriction from a cuttings accumulation or formation instability.

Several tests were performed during the drilling operations to study the effects of slim hole conditions on pressure losses in the well bore. The influence of flow rate, drillstring rotational speed, and weight on bit were investigated. These results are presently being analyzed by IFP.

An initial review of the data shows that annular pressure losses increase with increased drillstring rotational speeds. The pressure loss is particularly significant with rotational speeds above 100 rpm.

Fig. 4 shows the pressure drop at the bit during a connection. During drilling, the downhole pressure remained steady at 207 bars (3,002 psi), or 1.09 sp gr (9.1 ppg) equivalent circulating density (ECD). Before the connection was made at 1,800 m (6,000 ft), the hole was conditioned by circulation but without drillstring rotation.

When the rotation stopped (from 350 to 0 rpm) the downhole pressure dropped by 4 bars to 203 bars (2,493 psi) or 1.07 sp gr (8.9 ppg) ECD. This change in annular pressure drop because of drillstring rotation is typical for slim hole wells. When the circulation was stopped later, there was a further drop in downhole pressure by 9.5 bars to 193.5 bars (2,806 psi) or 1.02 sp gr (8.5 ppg) ECD.

These measurements were made for 56mm (2.2 in.) OD drill pipe, 65 mm (2.5 in.) OD drill collars, and a hole diameter of 86 mm (3.4 in.). The high rotational speed induced an increased pressure loss equivalent to 40% of the pressure loss resulting from circulation only. In this particular case, the downhole pressure dropped a total of 13.5 bars (196 psi) or 0.07 specific gravity (0.6 ppg) ECD.

During pipe connections, bottom hole pressure is reduced by both the circulation and rotational pressure components. This total reduction in equivalent circulating density (ECD) amplifies the risk of a kick. This effect has been observed recently in an exploration slim hole well in which two kicks occurred during pipe connections, but losses occurred during drilling.

SWAB AND SURGE

Swab and surge effects during tripping often cause fluid losses or gains. Pressure effects may be estimated from pressure loss models, but these pressures are seldom calculated in real time and are rarely, if ever, actually measured. The MWD tool in these test wells supplied the relevant pressure measurements from just above the bit throughout the tripping operation. Fig. 5 shows the swab effects. Note the difference between the computed downhole hydrostatic pressure and the measured downhole annular pressure.

As the string is pulled out of hole at an average speed of 0.25 m/sec (0.8 fps) between 1,600 and 1,700 m (5,250 5,580 ft), the downhole pressure drops below hydrostatic pressure levels by approximately 6 bars (90 psi) or 0.03 sp gr (0.25 ppg) ECD.

The surge effects are shown in Fig. 6. Before 4:00 a.m., the downhole pressure is increased above hydrostatic pressure by almost 3 bars (44 psi) or 0.03 sp gr (0.25 ppg) ECD in the interval between 1,100 and 1,200 m (3,600 3,950 ft) when the string is run into the hole at an average rate of 0.5 m/sec (1.6 fps). After the crew change at 4:00 a.m., the pressure surge is approximately 1.5 bars (22 psi) or 0.01 sp gr (0.1 ppg) ECD because the running speed is only 0.3 misc (1 fps). Overall running speeds were 360 m/hr (1,200 fps) before 4:00 a.m. and 250 m/sec (800 fps) afterwards.

By continuously monitoring annular pressure at the bit, trip speed could be adjusted to control swab and surge effects in real time.

KICKS

Kick prevention results from a thorough knowledge of hole conditions. By permanently monitoring downhole pressures during drilling and tripping, this MWD tool has the potential to prevent losses or gains.

Kick detection is another possible application for the real time transmission of downhole annular pressure. Further field testing is under way to evaluate influx detection.

6 IN. HOLES

Table 2 shows dimensions and the Couette flow shear rate at the drillstring wall for different annuluses and types of hole. Because no standard pressure loss models have been widely accepted for slim hole conditions, the comparison between different types of hole is based on the Couette flow shear rate (that is, flow between two cylinders, one stationary and one rotating).

The shear rate increases by a factor of two from a conventional 8 in. hole to a small 6 in. hole and by a factor of three from a small 6 in. hole to the 3.4 in. slim hole with a Euroslim drillstring. The shear rate with a mining drillstring (4 4 in. continuous coring) increases by a factor of nine compared to the Euroslim drillstring.

These figures indicate that the geometry of the Euroslim drillstring and hole is close to that of a 6 in. small hole. Consequently, the hydraulic effects detected during these field tests may be considered significant in a 6 in. small hole.

This 6 in. configuration is now common in re entry wells drilled in depleted formations, where pressure surges can initiate costly and detrimental mud losses. The 6 in. hole is also common in the last section of deep high pressure, high temperature wells. In many high pressure, high temperature wells, the difference between the pore pressure and the fracture pressure is small making drilling operations difficult to conduct without an influx or fluid losses.

In these cases, the real time monitoring of downhole annular pressure would be helpful.

ACKNOWLDEGMENT

The authors thank Geoservices SA and Elf Aquitaine Production, particularly A. Sagot and Y. Galy, for permission to publish this article.

REFERENCES

  1. Bode, D.J., Noffke., R.B., and Nickens, H.V., "Well Control Methods and Practices in Small Diameter Wellbores," SPE paper 19526, presented at the Society of Petroleum Engineers 64th Annual Technical Conference and Exhibition, San Antonio, Oct. 8 11, 1989.

  2. Marken, C.D., Xiaojun, H.E., and Aarild, S., "The Influence of Drilling Conditions on Annular Pressure Losses," SPE paper 24598 presented at the Society of Petroleum Engineers 67th Annual Technical Conference and Exhibition, Washington D.C., Oct. 4 7, 1992.

  3. Vighetto, R., and Dachary, J., "Slim hole drilling proven in remote exploration project," OGJ, June 22, 1992, pp. 62 67.

  4. Dupuis, D., and Fanuel, P., "Well Cost System Approach: Achieving Well Cost Reduction Through Slim Hole Drilling With a Purpose Built Drillstring And Rig," IADC/SPE paper 25721, presented at the International Association of Drilling Engineers/Society of Petroleum Engineers Drilling Technology, Conference, Amsterdam, Feb. 23 25, 1993.

  5. Soulier, L., and Lemaiter, M., "E.M. MWD Data Transmission Status and Perspectives," IADC/SPE paper 25686, presented at the International Association of Drilling Engineers/Society of Petroleum Engineers Drilling Technology Conference, Amsterdam, Feb. 23 25, 1993.

  6. Deliac, E.P., Messines, J.P., and Thieree, B.A., "Mining technique finds applications in oil exploration," OGJ, May 6, 1991, pp. 85 90.

  7. Vighetto, R., Delwich, R.A., Lejeune, M., and Mawet, P., "Slim Hole Drilling Hydraulics," SPE paper 24596 presented at the Society of Petroleum Engineers 67th Annual Technical Conference and Exhibition, Washington D.C., Oct. 4 7, 1992.

  8. Walker, S.H., and Milheim, K.K., "An Innovative Approach to Exploration Drilling: The Slim Hole High Speed Drilling System," SPE paper 19525 presented at the Society of Petroleum Engineers 64th Annual Technical Conference and Exhibition, San Antonio, Tex., October 1989.

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