Polymer mud system improves well condition

June 12, 2000
In more than 250 horizontal wells in the Dukhan field, Qatar, the use of a thin, nondamaging XC-polymer mud system provided a more-efficient and economical system than using a thick nondamaging polymer mud system.

QATARI DRILLING FLUIDS-1

This first of two articles on Qatari drilling fluid shows how thin, nondamaging XC-polymers improved efficiencies and saved on costs as compared to thick, nondamaging polymers. The conclusion describes how bentonite water-base mud systems also improved penetration rates and reduced costs in relation to nondamaging polymer mud systems.

In more than 250 horizontal wells in the Dukhan field, Qatar, the use of a thin, nondamaging XC-polymer mud system provided a more-efficient and economical system than using a thick nondamaging polymer mud system.

Hole angle

The hole-cleaning principles of vertical wells also apply to directional wells with inclination less than 10°. Removal of cuttings generally becomes more difficult as the hole angle increases.

Holes with angles between 50 and 60° present most cleaning problems. Greater than an inclination of 60°, cutting beds development does not get any worse. Problems in hole cleaning occur because the cuttings have a tendency to slide down the annulus and cause packing off.

In wells deviated greater than 60°, the cuttings form stable beds because they are supported by sliding friction against the well bore. The angle range for cutting beds to slide depends largely on the mud rheology; however, high annular velocity is needed.

In holes with angle greater than 40°, turbulent-transitional flow is most effective in cleaning and evacuating cutting beds. Lower mud rheologies, at the corresponding annular shear rates, are required to obtain the Reynolds numbers greater than 2,100. In this case, the cuttings in the beds will be removed as ripples or dunes (saltation).

Turbulent flow cannot be achieved in most of the 445-mm (171/2 in.) holes and some of 311-mm (121/2 in.) holes. This is due to a variety of reasons, including limitation of surface downhole equipment and washouts.

In this case, a compromise for hole cleaning can be made by laminar flow that can be achieved by the highest possible pump output and annular velocities.

In addition, the low shear rheology must be optimized by using low shear-rate rheology modifiers in oil-base mud (OBM) and XC polymer-type material with water-base mud (WBM) to suppress the formation of cutting beds.

High initial gel strength gives rapid suspension of cuttings when the pumps are shutoff during surveys or trips. This should be combined with flatter gel strength development with time.

Use of mechanical means (for example, wiper trips, pipe rotation, reciprocation, back-reaming when top drive is available, etc.) is needed along with pills pumped in turbulence to assist hole cleaning.

ROP

An increase in penetration rate results in higher concentration of cuttings in the annulus. A maximum of 0.5% annular concentration is allowed to drill vertical and near vertical sections efficiently.

For deviated sections, deeper cutting beds develop as the penetration rate increases. Removing these deeper beds requires higher flow rates; therefore, it would be important to control and limit instantaneous rates of penetration (ROP) in deviated sections.

Fluid pills

For wells deviated less than 30°, conventional high viscosity-high weight slugs should be used to help rear cuttings from the annulus. For wells deviated beyond 30°, the pumping of thin turbulent fluids provides the best method for cuttings removal, followed immediately with high-viscosity and high-weight pills.

This will increase the effectiveness of carrying the cuttings from the rear of the cutting beds and out of the hole.

The size of the oil-base weighted pills must be matched to balance out the effective circulating devise. The effect on hydrostatic pressure of the oil-base (underbalanced) and the heavy mud (formation breakdown) in the annulus must be calculated in advance.

If possible, the heaviest weight pill (18 ppg) should be used. The size and frequency of these pills also ensure minimal contamination to the active system. Typical total pill volumes (low viscosity + high) are 50-100 bbl for 445-mm and 30-60 bbl for 311-mm hole.

Thin fluids can destabilize some formations. For example, highly laminated formations where laminae lie at acute angles to the well bore can be destabilized by the whipping act of the thin fluid when pumped at high speed. The thin fluid rheology should be increased in these cases.

Drill pipe size

Using larger-size drill pipe significantly reduces pressure drop by allowing higher flows. Experience shows that turbulence in the annulus is seldom achieved in deeper, horizontal wells when different drill pipe sizes are used.

The increase in annular velocity must be traded with the cost of changing out the drillstring for the smaller hole sections to follow.

Drill pipe rotation

Rotating the drillstring can assist in mechanically disturbing cutting beds in deviated sections. The rotating pipe forces drill cuttings upward to the high side of the hole and into the fast moving mud stream.

Drill pipe rotation also helps mudflow in the narrow gap between the pipe and the settled bed. When an oriented downhole motor is used in a deviated well, it probably will not disturb the cutting beds. Therefore, consider rotating the string prior to tripping.

Field experience shows that pipe rotation while drilling enhances the hole-cleaning efficiency by approximately 25%.

Periodic short trips or backreaming is used to remove drill cuttings by mechanical agitation. This is very effective with the use of a top drive as the short trip is enhanced hydraulically and rotationally.

A failure to clean the hole adequately can be avoided by increasing the frequency of wiper trips and increasing circulation time on connections and before trips. This can be achieved by maximizing pump rates.

Mud weight

Mud weight influences hole cleaning for both vertical and deviated sections by affecting the buoyancy of the drilled cuttings. For small changes in density, the flow rate required to maintain adequate clean hole is directly proportional to the cuttings and the mud-density differential.

The selection of mud weight in most cases should be based on pore pressures, rock mechanics, in situ tectonic stresses, density required to stabilize the hole, and the resulting fracture gradient.

It is very important to use the correct mud density based on rock mechanics to avoid borehole stability problems. Once the stability problem of the hole occurs, such as hole cavings, it is difficult to remedy.

Mud rheology

Mud viscosity affects cuttings slip velocity, which in turn greatly influences transport efficiency in vertical and near-vertical wells. However, changes in conventional mud rheology have little benefit once cutting beds form (>30°). Low-viscosity fluids are most effective at angles greater than 30° because they induce turbulence and help drill cuttings removal by saltation.

Plastic viscosity should be minimized to reduce pressure losses and obtain a flatter viscosity profile. This creates higher annular velocity in the outer portion of the hole for the same pump rate. This can be easily achieved for both 445-mm (171/2 in.) and 311-mm (121/4 in.) holes. The rheology modifiers provide the required low shear viscosities when using OBM.

Pump rate

Mudflow is one of the most crucial factors for efficient hole cleaning, especially for deviated holes. As a rule, the annular velocity needed for cleaning 50-60° deviated wells is approximately twice the required velocity for the vertical ones.

All reasonable steps should be taken to reduce frictional pressure. This will extend the range of available flow rate. In critical cases, careful consideration should be given to bottomhole assembly (BHA) design, nozzle selection, and additional losses due to mud motors and measurement while drilling (MWD) tools.

Cuttings

Rock density affects hole cleaning in the same way as changes in the mud weight. The increase in density of cuttings makes hole cleaning more difficult in both vertical and inclined sections.

The shape and size of the cuttings are also important in vertical transport. For instance, the larger, more-rounded particles are the hardest to remove.

In highly deviated wells, the shape and size of the cuttings have little influence in hole cleaning. This is because the cuttings move in blocks rather than discrete particles.

Bed suppression

Use of mud with good suspension characteristics can suppress the formation of cutting beds in deviated wells. High flow is still necessary, however, since the beds cannot be totally eliminated.

For WBM, the best conventional additive for this purpose is polymer. All the major service companies now offer low-shear enhancers, sometimes referred to as rheology modifiers, for OBM.

A simple force balance approach can be used to determine the minimum mud rheology needed to maintain drilled cuttings in suspension:

(2 x q3) - q6 = 110 x
(Cuttings sp gr - mud sp gr) Dp

where:
q6 = Fann 6 rpm reading
q3 = Fann 3 rpm reading
Dp = Cuttings diameter, in.

For example, a 1.4 sg gr (11.7 ppg) mud, with Fann 6/3 readings of 21/20 lb/100 sq ft, will support 6 mm (0.25 in.) cuttings with an sp gr of 2.1.

A thick mud with good suspension characteristics and laminar flow regimes is useful for horizontal wells.

Field data

Horizontal wells consist of vertical, deviated, and horizontal sections. In the Dukhan field in Qatar, with more than 150 horizontal wells, bentonite WBM is used to drill the vertical sections.

However, the deviated and horizontal sections have been drilled using a thick nondamaging polymer mud system implemented from July 1992 to March 1995.

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The lithologies encountered in the deviated section (about 1,300 ft) consist of 80% anhydrite and 20% carbonates, and in the horizontal section (about 3,200 ft) consists of about 100% carbonate rocks. The main mud rheology, flow regime, and drilling data of the thick mud system are given in Table 1.

Thick and nondamaging

In Qatar, the thick, nondamaging polymer mud systems used to drill the deviated and horizontal sections provided satisfactory results, with the following observations:

  • About 100 ft off bottom is reamed while tripping mainly in deviated section and commonly greater than 50° inclination.
  • While drilling the horizontal section, the hole is swept with a low viscous pill for solid dispersity action.
  • Mud must be continuously treated in order to maintain a reasonable yield point (YP) across the anhydrite section as the Ca++ concentration increases.
  • Average drag occurs mainly across the deviated section with 50,000 lb.
  • Mud has to be conditioned with appropriate YP and plastic viscosity levels prior to cementation.
  • ROP remains relatively slow due to low water loss or filtration (5 cc/30 min).
  • Chemical consumption and the treatment of the mud are costly.
  • Measurement while drilling (MWD) tool failure has been encountered due to high solid content.

As a result of these problems, it has been decided to replace the thick viscous XC-polymer mud with nondamaging, low-shear, thin muds - proposed in March 1995 and implemented in March 1997.

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In more than 100 horizontal wells to date, thin nondamaging, low-shear, XC-polymer mud systems (NDPMS) have been used in drilling deviated sections that consist of 80% anhydrite and 20% carbonates with horizontal sections containing almost 100% carbonates. The casing schemes of the various drilled sections are shown in Fig. 1.

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The rheology of the thin nondamaging polymer mud system is shown in Table 2.

Deviated sections include Hith, Upper Anhydrite, and Arab A and B formations, whereas Arab C reservoir rocks represent horizontal sections.

The Upper Jurassic Hith formation is about 250 ft thick and contains nodular anhydrite. The Hith and Upper anhydrite rocks may represent sabkha or subaqueous evaporite deposits. The Arab C reservoir rocks represent shallow water-shelf deposits.

The objective of using a nondamaging low-shear thin mud is to increase the ROP, improve hole cleaning, and decrease drilling mud costs while resolving the problems Qatar General Petroleum Corp. encountered while using thick viscous XC-polymer mud.

Thin and nondamaging

By using a thin nondamaging polymer mud system instead of the thick nondamaging polymer mud, the following results were achieved:

  • Differential sticking was minimized while drilling the troublesome section.
  • Less torque and drag. Thus, no reaming or back reaming was experienced while tripping. Average drag reduction is 50% or about 25,000 lb.
  • No filling or cleaning problems have been encountered in the bottom of the deviated and horizontal sections when the pumps are shut off.
  • By keeping all other drilling factors the same and by slightly reducing the water loss (

    ROP reached 200 fph across some porous intervals in the horizontal section. Additionally, no washout formation problems or deficiency in building up angle have occurred with directional tools.

    • With low gel, no special precautions are required. This avoids swab and surge problems.
    • Conditioning mud prior to cementation is no longer required.
    • Low-viscous pills in horizontal sections are no longer required.
    • The possibility of formation damage is minimized as the polymer consumption is reduced in 81/2-in. and 61/8-in. holes by 48.5% and 28.5%, respectively. The cost percentage savings on mud in 61/8-in. and 81/2-in. holes are about 23% and 42%, respectively.
    • The high solid content problem was controlled. Therefore, no tool problems regarding MWD have been encountered.

    Surface mechanical foam was the only problem encountered while using NDPMS, especially when 2% rap seed oil was added to the mud. This problem, however, has no effect on pump pressure, kelly hose, and downhole tools.

    Agitation and the use of mud defoamer controlled the foam problem.