CEMENTING PRACTICES Guidelines for cementing deepwater conductor strings

Proper use of gelled seawater, bead cements, and slurry design from formation data provided by experiences from 25 wells

James Griffith
Halliburton Energy Services

Operators in the US Gulf of Mexico and other offshore areas where wells are completed in extremely deep water have improved conductor casing cementing across overpressured sands by improving mud management techniques, shortening slurry transition times, and making mud and cement slurry weight compatible with formation pore and fracture gradients.

In 25 wells where conductor pipes were cemented using techniques described in this article, 24 were successful, saving the operator an average of $1.5 to 2 million per well.

The key elements in this success rate are as follows:

Use of gelled seawater to bring drill cuttings up for most of the drilling process, then pumping a bentonite gel mud with fluid loss additives periodically to deposit thin, tough mud filtercake on the formation face.
Cementing with foamed or ceramic bead cement containing additives that cause that decrease cement slurry transition time (TrT) at cool temperatures (around 50°F)
Obtaining formation characteristics with measurement while drilling (MWD)/logging while drilling (LWD) and formation test tools for optimal design of the cement slurry.

Background

Well completions in water over 2000 ft deep are considered to be deepwater, requiring special techniques to install the conductor casing, Once the conductor pipe is installed, drilling and completing to total depth (TD) are performed in a conventional manner.

In deepwater operations, formations where conductor pipe will be cemented - under 2000 ft below mudline (BML) - are relatively young geologically and are not well consolidated (Fig. 1). Makeup of the these formations is largely a product of erosion from the continental shelf. Consequently, conditions there hold the potential for (1) abnormally high saltwater flow formations, and (2) the existence of weak formations that can fracture and cause loss of cement/mud returns to the seafloor.

Fig.1 -- Deepwater conductor cementing in soft formations beyond the continental shelf.

A typical conductor pipe arrangement is shown in Fig. 1. A 30-in. OD pipe is driven 200 ft BML. A 20-in. OD conductor string is cemented by the innerstring method through the drillpipe, with cement returns back to the ocean floor. Since there is not a riser in place, annular returns must be taken at the sea floor.

Fig. 2 illustrates some of the challenges of deepwater conductor pipe cementing. Cool temperatures slow the cement hydration process and extend transition times, which allow fluid influx to begin. The term transition time (TrT) applies to the time lapse between onset of hydration until the cement gains compressive strength. During TrT, fluid (gas, oil, or water) can migrate up through the setting cement slurry, forming channels that destroy cement sheath integrity. Fluid migration is possible during TrT because the cement column begins to support itself and stops exerting hydrostatic pressure on the fluid source, but does not have enough compressive strength to prevent fluid migration.

The combination of fluid migration potential, formations of low fracture gradient, and low temperatures challenge the slurry designer.

Some water sands may be overpressured by water, so that they cause saltwater to flow into the cement sheath. Prevention of this flow is critical, since remedial steps to stop flow and repair damage are far more expensive than proper preventive steps. Containment of the overpressure problem is further complicated by weak zones (low fracture gradient) that can fracture and cause loss of mud and cement returns.

One cementing technique that contributed to catastrophic failure of deepwater wells in the past involved lightening the cement slurry by creating a high water-to-cement ratio. Slurries thus lightened with water have little or no useful strength at slurry densities below 11 lb/gal, and have long TrT, especially at cool temperatures.

Water-extending a cement slurry is the most common method of obtaining a lightweight cement slurry. It is completed by adding an extending material such as sodium silicate or bentonite to the normal-weight cement slurry, along with extreme volumes of seawater. The amount of water-extending material and seawater added to the normal weight cement slurry depends on the final desired density and requirements of little or no free water of the cement slurry. A typical design is shown in Table 1.

Table l: Typical conductor pipe slurry
Standard cement (94 lb)
1-3% (by weight of cement) sodium silicate powder
1-3% (by weight of cement) calcium chloride
13-14 gal seawater

Successful techniques

Three key techniques have been found to contribute to the total success of deepwater conductor cementing:

Proper drilling fluids and application: In typical deepwater drilling processes, gelled seawater is used to circulate drill cuttings out of the wellbore. The gelled seawater, also referred to as spud mud, removes cuttings that would otherwise settle out of the seawater.
Successful operators circulate, in addition to the spud mud, a good mud that contains fluid loss control and exhibits low gel-strength development. This mud is circulated in sweeps usually after each stand of pipe (30 ft of drilling) is added to the drillstring. The good mud is spotted in the openhole section before running casing. Properties of this mud are such that it leaves minimal filtercake deposits on the wellbore wall and is easily displaced at low annular velocities. Recommended mud system properties and placement techniques are listed in Table 2.
Table 2: Recommended mud system properties and placement systems
1. API fluid loss value of less than 7 cc/30 minutes
2. Yield point of about 20 lb/l00 sq ft and plastic velocity of about 15 cp
3. Flat gel strength development (10 sec/10 min/30 min) and less than 10 lb/l00 sq ft
4. Pump a controlled mud in sweeps to limit the development of excessive mud cake from the natural spud mud.
5. Use a high-viscosity spacer for a contact time of at least 10 minutes.
6. Incorporate rigid centralizers; these are less detrimental to seafloor equipment than bowspring centralizers.
Foamed cement with accelerators: To reduce hydrostatic pressure on weak formations, operators have successfully used slurries consisting of nitrogen-foamed cement. Accelerators, such as ultrafine cement, are added to these slurries to hasten the onset of hydration in the 40 to 60°F temperatures prevalent in the deep US Gulf waters.
Ultrafine/hollow ceramic bead slurries: Lightweight, but high-compressive strength slurries can be prepared by mixing conventional and ultrafine cement with hollow ceramic beads (HCB). To both of these slurry types, operators add cement additives that decrease TrT at the low temperatures found in deepwater wells.

Cementing considerations

Design parameters for deepwater cementing include pump time, rheology, free water, and compressive strength at a given temperature and pressure. For accurate measurement of downhole conditions, many operators use a bottomhole circulating temperature (BHCT) recorder. A variation of a few degrees at the mud line dictates slurry transition time, for example, if the designed temperature is higher than the actual temperature, significant delays in cement TrT will occur.

During TrT, rheological properties, or yield points, change so that the slurry no longer responds to a pressure change as it did immediately after pumping. To help prevent fluid migration through the cement sheath, gel strength (or yield point) of the slurry must be greater than 500 lb/l00 sq ft. Ideally, the slurry's gel strength should remain below 500 lb/l00 sq ft for no more than 30 minutes. If the gel strength remains low for a long period (3-6 hours would be considered excessive) a pressure low is created in the cement column. These design changes are recommended to help prevent fluid migration:

Decrease TrT chemically, in this case, by adding ultrafine cement and other additives to achieve TrT of less than 30 minutes.
Limit pressure loss by controlling volume loss of slurry system with:
- Near-impermeable mud filtercake (must use controlled mud systems)
- Cement fluid loss additives to reduce the volume leaving the cement slurry during cement TrT
- Reactive spacers to further tighten mud filtercake and reduce cement filtrate loss
- Compressible cements which expand as the pressure reduces in the setting cement column
Combine some or all of the above procedures.

Use of the above procedures will eliminate or localize the influx to an area near the formation. The cement maintains zonal isolation by helping to prevent fluid communication to a permeable zone or to the mud line.

Problem formations

As mentioned earlier, deposits within the first 2000 ft below the mudline consist of eroded material from the continental shelf. Because of this depositional environment, the overpressured water sand can be one massive body or innerbedded with shale stringers.

Also, the water sand varies in depth below the mudline, but remains consistent in abnormal pressure amounts. Therefore, the water sand could be located a few hundred ft BML or at the 20-in. casing shoe, but contain the same amount of abnormal pressure gradient.

By simple hydrostatic calculations, one can determine that the mud and cement slurry density can range from 10 lb/gal to 14 lb/gal to control the abnormally pressured sand, depending on the sands location BML. This is the point at which MWD, LWD, and seismic information allow drilling and cementing companies to design the mud and cement density to prevent an underbalanced drilling or cementing condition.

Seismic information identifies the sands more than 50 ft thick, but does not relay any pore pressure information. MWD/LWD information locates sand stringers or interbedded sand/shale formations that could not be seen by seismic methods, but does not evaluate the formation for pressure.

Therefore, some operators utilize a repeat formation test to obtain formation. Other operators

(1) assume an abnormal pressure gradient (9.2 lb/gal gradient to sea level),
(2) determine mud weight required to obtain the needed hydrostatic pressure, and
(3) spot the weighted mud in the drilled hole.

If the well does not flow (as seen by remotely operated vehicle pictures), the operator uses the mud weight determined throughout the drilling process. If the well flows, the operator will increase the mud weight until the annulus remains static when pumps are shut down. One should realize that the seawater gradient will dominate the mud weight calculation since annulus returns are at the seafloor, not at sea level. Once the mud weight is determined, the spacer and cement density should be increased by 1/2 lb/gal and 1 lb/gal, respectively, over mud weight.

Conclusion

Cementation of deepwater conductor strings requires cross-disciplined personnel or teams and total project management to obtain success. Taking shortcuts on logging, the mud system, or cement system cannot be justified when the probable result would be catastrophic failure of the cement supporting the conductor string, costing millions of dollars, depending on progress of the well.

In this case, the cost of remedial squeeze jobs at $0.5-2.0 million per job to contain a water flow on one deepwater conductor will more than pay for the incremental cost (about $300,000) of the techniques outlined in this article.