State-of-the-Art Cuttings Transport in Horizontal Wellbores
- A.A. Pilehvari (Texas A&M U. at Kingsville) | J.J. Azar (U. of Tulsa) | S.A. Shirazi (U. of Tulsa)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- September 1999
- Document Type
- Journal Paper
- 196 - 200
- 1999. Society of Petroleum Engineers
- 1.15 Fundamental Research in Drilling, 5.3.2 Multiphase Flow, 4.2.3 Materials and Corrosion, 4.2 Pipelines, Flowlines and Risers, 1.6.6 Directional Drilling, 1.12.6 Drilling Data Management and Standards, 5.2.2 Fluid Modeling, Equations of State, 1.6 Drilling Operations, 1.7.7 Cuttings Transport, 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc), 4.6 Natural Gas, 1.6.1 Drilling Operation Management, 4.3.4 Scale, 1.11 Drilling Fluids and Materials, 1.10 Drilling Equipment, 1.11.2 Drilling Fluid Selection and Formulation (Chemistry, Properties)
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In the ever-growing use of, directional and horizontal drilling, hole cleaning is a common and costly problem. It can become particularly critical in the case of extended reach drilling where larger and longer wellbores are drilled. During the past two decades, especially in recent years, many laboratory studies as well as field observations have been directed towards addressing the cuttings transport problem. This has resulted in better understanding of the subject and some remedies to the problems. Several correlation/models have also been developed that give the field engineer a tool to better specify the hydraulic requirements in cleaning the hole. However, because of the complexity of the subject, a comprehensive and proven model, which does not exist at this time, requires much more laboratory research and field studies.
Inadequate hole cleaning in drilling horizontal wells may cause a number of costly problems, such as premature bit wear, a slow drilling rate, formation fracture, high torque and drag, and a stuck pipe. If the situation is not handled properly, the problem can lead to sidetracking or loss of the well. Several field studies have documented that hole cleaning is a frequently occurring problem that has to be watched carefully, otherwise it can and does develop into the serious problems mentioned.1-3 Persisting cuttings transport difficulties in the field and many studies with large-scale wellbore simulators have proven that cuttings transport in highly inclined wellbores is a complex problem. Over the last 20 years, many papers have been published that address the subject.
The majority of these publications provided qualitative studies and/or practical field guidelines, while some have presented modeling schemes that can be used to help the field engineer in optimization of drilling operations.
In this article, developments in cuttings transport over the years, the shortcomings of its present status, and future research needs are addressed.
Pioneering Experimental Studies
Large-scale cuttings transport studies in inclined wellbores were initiated at the Tulsa University Drilling Research Projects (TUDRP) about two decades ago with the support of a few major oil and service companies. A flow loop was built that consisted of a 40-ft length of 5-in. transparent annular test section and the means to vary and control the (1) angles of inclination between vertical and horizontal, (2) mud pumping flow rate, (3) drilling rate, and (4) drill-pipe rotation and eccentricity. Past results have revealed the marked difference between the cuttings transport in inclined wellbores and that of vertical wellbores. A cuttings bed was observed to form at inclination angles of more than 35° from vertical, and this bed could slide back down for angles up to 50°. Tomren, Iyoho, and Azar4 published the results of these studies. Mud velocities in the range of 3 to 4 ft/s were found necessary for high angles with no pipe rotation compared with the 1 to 2 ft/s normally used for vertical drilling. Eccentricity, created by the drillpipe lying on the low side of the annulus, was found to worsen the situation. Analysis of annular fluid flow showed that eccentricity diverts most of the mud flow away from the low side of the annulus, where the cuttings tend to settle, to the more open area above the drillpipe.
The above study at TUDRP was followed by another landmark work in cuttings transport by Okrajni and Azar,5 in which they investigated the effect of mud rheology on hole cleaning. Their work confirmed the earlier findings and established that the cuttings transport mechanism and flow behavior in high angle wellbores are quite different than those of vertical wellbores. It was observed that removing a cuttings bed with a high viscosity mud, a remedy for the hole cleaning problem in vertical wells, may in fact be detrimental in high angle wellbores (assuming a zero to low drill-pipe rotation), and that a low viscosity mud that can promote turbulence is more helpful. On the basis of this finding and on the previous study, hole cleaning was found to depend on the angle of inclination, hydraulics, mud rheological properties, drill-pipe eccentricity, and rate of penetration. Qualitative relationships for the effect of these parameters on cuttings transport were presented in Ref. 5.
Further analysis of Okrajni and Azar's rheological data by Becker, Azar, and Okrajni6 showed that the cuttings transport performance of the muds tested correlated best with the low end shear rate viscosity, particularly the 6 rpm Fann V-G viscometer dial readings.
The Awakening Years—Building Flow Loops
By the mid-1980's, a general qualitative understanding of the hole cleaning problem in highly inclined wellbores had been gained. However, the subject proved to be fairly complex and did not lend itself to a comprehensive analysis. Meanwhile, more directional and horizontal wells with longer lateral reaches were being drilled and they met with frequent difficulties in hole cleaning. The field problems were caused by lack of predictive models, slow transfer of the research results to the field, inadequate field experience, and limitations of the drilling equipment.
In light of the recurring hole cleaning problems, the oil industry felt the urgency for more research and development in cuttings transport. This led most of the major oil companies to form their own internal hole cleaning teams. The need for more and new experimental data created a high demand for additional flow loops. In partnership with Chevron, Conoco, Elf Aquitaine, and Philips, TUDRP built a new and larger flow loop, with 100-ft long test section of 8-in. annulus.7 By the end of the 1980's a few flow loops of various sizes and different levels of capability were built. The construction and commissioning of new flow loops were reported by TUDRP,7 Heriot-Watt University,9 BP,10 Southwest Research,11 M.I. Drilling Fluids,12 and the Institute Franc¸ais du Petrole.13 All the flow loops had a transparent part of the annular test section that allowed observation of the cuttings transport mechanism. These flow loops provided the necessary tools for collecting the badly needed experimental data.
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