Rigid centralizers improve cementing casing through caverns

Boris M. Kurochkin
Scientific Research Institute of Drilling Techniques
Moscow

The use of off-center rigid centralizers on casing maintains a minimum annular space on the low side of a deviated well, improving cementing operations in such wells with hole enlargement problems.

In deviated wells through carbonates or sloughing formations, caverns can form causing problems for hole cleaning and cementing operations.

In hole-enlargement areas, the flow of both drilling fluid and cement is predominantly on the high side of the well, between the pipe and the top wall of the well. Drill cuttings can accumulate around the pipe near these caverns. During casing-running operations, the cement may not flow completely around the pipe in these cavernous sections.

The study of cavern formation features in inclined wells can give a key to further prediction of technology for their washing and casing cementation.

In inclined wells, the drilling tools are nonconcentrically aligned in the borehole. The axis of the drillstring appears below the axis of the borehole. Therefore, in inclined wells there is a change in the flow hydrodynamics in the ring space.

This change is amplified because the drilled cuttings move up the well along the bottom wall of the hole, yet the main washing flow occurs in the space between the drillstring and the top wall of the hole. The flow may have a large erosion effect on the top wall of the hole.

If there are large caverns in inclined wells, the drilling tool usually passes free of them when it is run in the hole. Hence, on the bottom wall of an inclined well, a cavern is usually insignificant.

Cavern expansion usually occurs in the direction from the bottom wall to the top wall in an inclined hole. Anisotropy of the formation exposed by an inclined well at an angle to its bedding promotes this tendency.

Section gauge tool

In Russia, two types of devices are used for gauging well diameter: a caliper which measures average cavern diameter, and a section gauge which makes two measurements in perpendicular planes using four measuring levers.

In stable carbonate and sloughing rock intervals drilled with a bottom hole motor, the section gauge measurements have shown a different configuration (outline) in inclined wells than that for vertical well sections in the same interval.

To study this hole enlargement, a section gauge tool was modified with a device to misalign its center of gravity, such that the tool could more accurately measure caverns (Fig. 1 [29775 bytes]).^1-2 The entire tool connects to a logging cable by an articulated coupling. The tool thus retains its orientation relative to the bottom wall when it is pulled out of the well, despite any irregularities along the borehole.

The misalignment device is simply a section of pipe cut lengthwise, and the gauge tool is rigidly fixed to the pipe. The bottom part of the pipe section is filled with lead to keep the center of gravity below the tool axis and to fix a pair of measuring levers in the same direction the tool is pulled.

A slot on the bottom of the pipe section allows the bottom lever to reach through to the borehole wall.

Field runs

The first measurements with the tool were made in well No. 6056 in the Romashkinskoye field in Tatarstan. The bottom hole depth was 1,200 m, and the carbonate interval measured was at 330-400 m.1 The borehole inclination in this interval was 22°. This section was drilled with a downhole motor.

The gauging measurements showed that the cross section of the well was oval, extending to the top of the borehole wall. The oval height was 20-30% larger than the diameter drilled.

In subsequent runs, the bottom lever was removed from the gauge tool. The measurements were made by one lever directed upwards (designated R) and two lateral levers (designated D) located in a plane parallel to the bottom wall. The section gauge axis (r) was 75 mm from the bottom wall during the run. Fig. 2 [17858 bytes] shows the results of one of these measurements using well No. 32106 in the same field.

The gauging run in the top interval of the hole (260-280 m) was the control run to help determine the accuracy of the calculations for the device's subsequent inclination run below (740-780 m). In this well, casing was set at 265 m, and the interval of 265-278 m was then underreamed. A 215.9 mm bit drilled the well below 278 m; the borehole inclination angle in this section was 10-12°. A cavern was found around 768 m; section A-A in Fig. 2 [17858 bytes] shows the extent of the borehole enlargement.

In Well No. 5804 in the Beresovskaya field, the section gauge tool was again run in the three-lever configuration. The interval studied had sloughing formations which were drilled by a downhole motor. The borehole inclination was 12°.

Fig. 3 [19176 bytes] shows the cavern sections in reduced scale. The ovals are shown schematically, drawn through four points, to demonstrate cavern size and configuration. Three points were obtained from the gauge tool measurements, and the fourth is the point on the bottom wall a distance r from the tool's axis. Table 1 [5680 bytes] lists the gauge values for D, R, and r for each cavern in this well section.

These caverns developed a typical arch-type structure. The inequality R + r D/2 is true for the arch-structure caverns.

In these caverns, about 70-80% of the fluid circulating in a well will pass in the arch space above the drillstring.

Fig. 4 [24285 bytes] shows the measurements from the section gauge tool and a caliper tool (Dav) in Well No. 32106. In this inclined interval, the well was drilled with strict control over mud properties. The mud density was 1,350-1,450 kg/cu m, water loss was only 7 cc/30 min, and the funnel viscosity was 25-28 sec-1.

Table 2 [5546 bytes] presents the data from the caliper and section gauge tool runs across the cavernous sections.

Hole cleaning

Hole cleaning in extended cavern sections could become a problem. In formations susceptible to cavern formation, the drilling fluid should have high rheological properties to ensure adequate removal of cuttings. In some cases, it may be necessary to plug or bridge the caverns to reduce the area of the hole.

The washing of an inclined well plays an important role both during drilling and in preparing the well for running and cementing casing. Arch-type caverns will cause difficulty in transporting drill cuttings along the bottom wall of an inclined well. Across from these caverns, dunes of cuttings can form.

The space between the casing and the bottom wall of an inclined well will not be uniformly filled with cement slurry during cementing operations. Therefore, some changes to standard cementing operations are necessary to ensure a full, competent cement sheath around the casing.

One solution has been the use of rigid centralizers, which ensure a minimum standoff of the casing and the bottom wall.^3-4 The use of these rigid centralizers maintains enough annular space completely around the casing to allow better fluid flow.

Off-center rigid centralizers have two hard blades on one side of the casing and none on the opposite side. The pipe is run-in eccentrically. (The casing axis if offset to the well bore axis.) When the casing is run to the desired depth, the pipe is turned to place the centralizer blades on the low side of the well (Fig. 5 [30543 bytes]). The off-center centralizers increase the clearance between the casing and the wall.

In Well No. 3801 in Tatarstan, off-center centralizers were used on the casing opposite caverns in a 12° well. The off-center centralizers were set at 1,735 m, 1,725 m, and 1,715 m. Between them were placed standard spring centralizers. After the casing was run in to the design depth, it was turned so that the centralizers lay on the bottom wall.

The centralizers are oriented by use of an orienting ring with an offset aperture and a pump-down ball. The ring is set in the bottom of the casing such that the planes of the opening and the centralizer blades coincide, pointing downward. To orient the centralizers, a ball is pumped down the casing, and the casing is turned slightly until the ball seats in the hole on the low side of the well. When the ball blocks the ring aperture, the increased pump pressure shears pins in the ring. The ring and ball mechanism then fall to the bottom of the well, leaving a full-open port for cementing.

This casing cementing procedure was successful in Well No. 3801. Subsequent casing bond logs indicated a good cement bond across the cavern intervals.

References

1. Kurochkin, B.M., Bikchurin, T.N., and Muhamedov, R.S., "Research of Cavernous Sections in Incline-Directed Wells," Neftianoye Hoziaystvo (Oil Field Facilities), No. 7, 1991, pp. 7-8.

2. Kurochkin, B.M., Fatkullin, R.H., and Bikchurin, T.N., "Section Gauge Logging in Incline-Directed Wells with Fixing of Gauging Relative to Bottom Wall of a Well," Neftianoye Hoziaystvo (Oil Field Facilities), No. 1, 1995, pp. 18-21.

3. Kinzel, H., "Proper centralizers can improve horizontal well cementing," OGJ, Sept. 20, 1993, pp. 84-87.

4. Kurochkin, B.M., Fatkullin, R.H., and Zadvornich, V.N., "Well Casing and Cementing with Application of Off-Centre Centralizers on a Casing," Neftianoye Hoziaystvo (Oil Field Facilities), No. 3, 1989, pp. 10-12.

The Author