CHARTS DETERMINE HOLE CLEANING REQUIREMENTS IN DEVIATED WELLS

Yuejin Luo, Peter Berns
BP Exploration
Sunbury, U.K.

Daryl Kellingray, Bryan Chamber
BP Exploration
Aberdeen

A set of simple charts can help engineers at the rig site quickly determine hole cleaning requirements for drilling deviated or horizontal wells. The charts can also quantify the effect of hole enlargement, which adversely affects hole cleaning.

These charts have been derived from a computer model based on both laboratory and field measurements.

The mud flow rate, penetration rate, and mud rheology are the key variables for optimizing hole cleaning. An example problem illustrates how the charts can determine the influence of these major drilling variables on hole cleaning requirements.

In planning or drilling a deviated well, one of the key parameters that must be determined is the minimum flow rate required to transport drilled cuttings up to surface to keep the hole clean.

This minimum flow rate is the critical flow rate (CFR). If an inadequate flow rate is used, cuttings will settle on the low side of the hole. A large stationary bed may form, possibly resulting in severe drilling problems such as high torque and drag, hole packing-off, and stuck pipe. These problems may subsequently require expensive remedial operations.

Hole cleaning has been investigated by numerous researchers, and in some of the early studies, the effects of a range of variables on cuttings transport and bed formation in deviated wells were investigated.1-3 The later studies have tended to concentrate on developing mathematical models for predicting the CFR.4-8 Most of the models have been based on small-scale experiments.

A physically based model for predicting the CFR in deviated wells was presented in a previous paper.8 This model was developed based on the analysis of forces acting on the cuttings and the associated dimensionless groups. The model was validated initially with experimental data obtained from an 8-in. well bore simulator and further validated with actual drilling data from six different hole sizes from 8/2 in. to 17 1/2 in. This article presents a set of charts derived on the original model and based on typical North Sea drilling conditions.8

These charts are applicable for wells deviated greater than 25. The charts are useful for drilling engineers at the rig site to optimize hole cleaning requirements.

MAJOR VARIABLES

A number of drilling variables affect hole cleaning in a deviated well. Some of these variables (controllable) can be specified during the planning stage or controlled while drilling. Other variables (uncontrollable) can be neither designed nor controlled, however.

The controllable variables that can be modeled include mud flow rate, rate of penetration (ROP), mud rheology, mud flow regime, mud weight, hole angle, and hole size.8 These variables are the most important parameters for optimizing hole cleaning and therefore have been included in the hole cleaning charts.

The uncontrollable variables include drill pipe eccentricity, cuttings density, and cuttings size.

To keep the hole cleaning charts relatively simple, these uncontrollable variables have been fixed at default values based on typical North Sea conditions.

The influence of changing drill pipe rotation is not presently modeled. The existing validation has been based on field data gathered under normal rotary drilling conditions. Greater levels of drill pipe rotation have assisted cuttings transport in deviated wells in both laboratory experiments and field operations.1-3 6 8-10

Similarly, if the drill pipe is not rotated (during oriented drilling, for example), cuttings removal may worsen and the charts may underpredict hole cleaning requirements. Under these circumstances, increased flow rate or changes in operational practices (rotary wiper trips, for example) may be necessary to compensate for the effects from no rotation.

HOLE CLEANING CHARTS

Based on the typical drilling conditions in BP Exploration's operating areas in the North Sea, a sensitivity analysis was carried out by using the original model to examine all the controllable variables. The effects of each variable on hole cleaning were established for each of the hole sizes.

The influences of mud plastic viscosity (PV) and yield point (YP) have been grouped together in a single parameter referred to as the rheology factor (RF). The greater the RF, the more effective the mud rheology for hole cleaning.

To determine the value of the RF from the mud PV (in cp) and YP (in lb/100 sq ft), charts were derived for each of the hole sizes. Figs. 1a, 2a, and 3a show these values for 17 1/2-in., 12 1/4-in., and 8 1/2-in. holes, respectively.

The effect of the hole angle was approximated by a group of factors called the angle factor (AF), shown in Table 1. As the hole angle increases, the AF value decreases, and hole cleaning becomes more difficult.

The effect of mud weight (MW) was combined with the RF and AF to form a single parameter called the transport index (TI):

 TI = RF x AF x MW

In this equation, MW is in specific gravity. The RF and AF are obtained from Figs. 1a, 2a, 3a, and Table 1 and can be considered as dimensionless.

At a given set of drilling conditions (hole size, angle, mud weight, and mud plastic viscosity and yield point), TI is a direct indication of the hole cleaning condition during drilling. The greater the TI, the easier the hole is to clean. Likewise, a small TI indicates more difficult hole cleaning.

The remaining controllable variables are the rate of penetration (ROP) and the critical flow rate (CFR). The interaction between the ROP and the CFR can be plotted on a chart, but a link to the TI should be established to reflect the effects of all the other variables. Figs. 1b, 2b, and 3b show the relationship of ROP and CFR to the TI for 17 1/2-in., 12 1/4-in., and 8 1/2-in. holes, respectively.

RHEOLOGY, FLOW REGIME

In deviated wells, cuttings are removed by a combination of saltation (bouncing particles) and bed sliding. The driving mechanism results from the fluid lift and the drag forces acting on the cuttings bed.

In laminar flow, the drag force dominates, whereas in turbulent flow the lift force is more important. Thus, the effects of the mud rheology and the flow regime are mutually dependent (Fig. 4).

In turbulent flow, a smaller YP results in a greater turbulent intensity and thus a greater lift force for transporting cuttings by saltation. Reducing YP in turbulent flow reduces the CFR and improves hole cleaning. In laminar flow, however, a higher YP corresponds to a higher fluid drag force, which removes cuttings as a sliding bed. Thus, high-YP muds are preferred in laminar flow for hole cleaning.

This mud rheology effect has been accurately depicted in the rheology factor (RF) charts. For example, from Fig. 3a for 8 1/2 in. holes, there is a minimum RF line from which both increasing or reducing the YP will increase the RF value and thus improve hole cleaning. A similar effect occurs in in. holes (Fig. 2a). For 17 1/2-in. holes, however, the annular flow is dominated by the laminar flow regime under normal drilling conditions. Thus, the RF increases continuously from a YP of about 18 lb/100 sq ft, and there is no minimum RF line in this range of YP (Fig. 1a).

The rheology factor directly indicates how effective the mud rheology is in terms of hole cleaning: the higher the RF, the more effective the mud rheology. Therefore, the RF charts can be a useful tool to optimize the mud rheology in the field.

The charts show that the RF value is much more sensitive to a change in YP than a change in PV (Figs. 1a, 2a, and 3a). Thus, YP has much greater effect than PV on hole cleaning in deviated wells. In-house experiments and field experience have confirmed that adjusting mud YP is much more effective than adjusting PV to prevent or minimize hole cleaning problems.

ROP

The relationship between ROP and CFR is approximately linear (Figs. 1b, 2b, and 3b). At a given ROP, increasing the transport index (TI) reduces the critical flow rate (CFR) and therefore improves hole cleaning.

At a given maximum mud flow rate, as is often the case during drilling a deviated well, increasing the TI increases the maximum allowable ROP at which the well can be safely drilled.

Therefore, the ROP charts are a useful tool for optimizing hole cleaning while maximizing drilling efficiency.

HOLE WASHOUT

These hole cleaning charts have been prepared for three in gauge hole sizes. However, if part of the open hole section is enlarged (washed out), the flow rate required to clean the hole section will be higher than that determined from the charts.

Table 2 shows a group of correction factors (derived for the three in-gauge hole sizes) for determining the flow rate for washouts.

The flow rate for the gauge hole should be multiplied by the correction factor to obtain the flow rate for a washed out section:

 CFRwashout = a X CFRgauge

In this equation, a is the correction factor obtained from Table 2.

PROCEDURE

Use the mud PV and YP to read the rheology factor (RF) from the appropriate chart (Fig. 1a, 2a, or 3a).
Obtain the angle factor (AF) from Table 1.
Calculate the transport. index (TI), based on the RF, AF, and MW values.
With the TI and the desired ROP (or maximum flow rate), read the CFR for hole cleaning from the appropriate ROP chart (Fig. 1b, 2b, or 3b).
If the hole is washed out, find the flow rate correction factor (a) from Table 2 and calculate the CFR for the washed out hole section.

EXAMPLE

A horizontal 8 1/2-in. hole is drilled with a 1.45 sp gr mud. The mud PV is 25 cp, and the YP is 18 lb/100 sq ft.

What is the maximum safe ROP if the mud pumps can deliver a maximum flow rate of 450 gpm?
From the RF chart (Fig. 3a), RF = 0.91, and from Table 1, AF = 1.0. The TI can then be calculated: TI = 0.91 x 1.0 x 1.45 = 1.32. From the ROP chart (Fig. 3b) at a TI of 1.32, if the maximum flow rate achievable is 450 gpm, then the maximum ROP without causing hole cleaning problems is about 23 m/hr.
If it is anticipated that the well can be drilled at an ROP of 20 m/hr, what flow rate will be required to clean the hole?
At this ROP, the flow rate required to clean the hole is 440 gpm.
If the hole is suspected to be washed out to 10 in., what should be the flow rate?
With this washout and with the planned ROP of 20 m/hr, then from Table 2 the flow rate should be corrected by a factor of a = 1.38: CFRwashout = 1.38 x 440 = 607 gpm. Under this circumstance, measures must be taken either to increase the maximum achievable flow rate (by using larger drill pipe, for example) or to adjust drilling parameters (mud YP, for example).

ACKNOWLEDGMENT

The authors would like to thank BP Exploration for permission to publish this article. Thanks are also due to all the operations personnel, with BP and its contractors, for their considerable support throughout this work.

REFERENCES

Tormren, P.H., Iyoho, A.W., and Azar, J.J., "An experimental study of cuttings transport in directional wells," SPE paper 12123, presented at the Society of Petroleum Engineers 58th Annual Technical Conference and Exhibition, San Francisco, Oct. 5-8, 1983.
Brown, N.P., Bern, P.A., and Weaver, A., "Cleaning deviated holes: New experimental and theoretical studies," SPE/IADC paper 18636, presented at the Society of Petroleum Engineers/International Association of Drilling Contractors Annual Drilling Conference, New Orleans, Feb. 28-Mar. 3, 1989.
Sifferman, T.R., and Becker, T.E., "Hole cleaning in full-scale inclined wellbores," SPE paper 20422, presented at the SPE 65th Annual Technical Conference and Exhibition, New Orleans, Sept. 23-26, 1990.
Gavignet, A.A., and Sobey, I.J., "Model aids cuttings transport prediction," SPE paper 15417, presented at the SPE 61st Annual Technical Conference and Exhibition, New Orleans, Oct. 5-8, 1986.
Martin, M., et al., "Transport of cuttings in directional wells," SPE/IADC paper 16083, presented at the SPE/IADC Annual Drilling Conference, New Orleans, Mar. 15-18, 1987.
Peden, J.M., Ford, J.T., and Oyeneyin, M.B., "Comprehensive experimental investigations of drilled cuttings transport in inclined wells including the effects of rotation and eccentricity," SPE paper 20925, presented at Europec 90, The Hague, Netherlands, Oct. 22-24, 1990.
Larsen, T.I., Pilehvari, A.A., and Azar, J.J., "Development of a new cuttings transport model for high-angle wellbores including horizontal wells," paper 25872, presented at the SPE Rocky Mountain Regional Symposium on Low Permeability Reservoirs, Denver, Apr. 12-14, 1993.
Luo, Y., Bern, P.A., and Chambers, B.D., "Flow rate predictions for cleaning deviated wells," SPE/IADC paper 23884, presented at the SPE/IADC Annual Drilling Conference, New Orleans, Feb. 18-21, 1992.
Stewart, C.D., and Williamson, D.R., "Horizontal drilling aspects of the Helder field redevelopment," paper 17886, presented at the 20th Annual Offshore Technology Conference, Houston, May 2-5, 1988.
Alfsen, T.E., et al., "Pushing the limits for extended reach drilling: New world record from platform Statfjord C, Well C2," SPE paper 26350, presented at the 68th SPE Annual Technical Conference and Exhibition, Houston, Oct. 36, 1993.