State-Of-The-Art Cuttings Transport in Horizontal Wellbores

Abstract

In the ever growing 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 the recent years, many laboratory studies as well as field observations have been directed to address the cuttings transport problem. This has resulted in better understanding of the subject, and some remedies to overcome the problems. Several correlations/models have also been developed which give the field engineer a tool to better specify hydraulic requirements to clean the hole. However, due to the complexity of the subject, a comprehensive and proven model, not existing at this time, requires much more laboratory research and field studies. This will be the major topic addressed in this article. Introduction Inadequate hole cleaning in drilling horizontal wells may cause a number of costly problems such as premature bit wear, slow drilling rate, formation fracture, high torque and drag, and stuck pipe. If the situation is not handled properly, the problem can lead to side tracking 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 to the serious problems mentioned above. Persisting cuttings transport difficulties in the field and many studies with large scale wellbore simulators have proven that hole cleaning in highly inclined wellbores is a complex problem. Over the last twenty years, many papers have been published addressing 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 paper 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 which consisted of a 40-ft long of 5-in. transparent annular test section and means to vary and control:angles of inclination between vertical and horizontal,mud pumping flow rate,drilling rate, anddrillpipe rotation and eccentricity. Past results using the facility 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 beyond 35 degrees from vertical, and this bed could slide back down for angles up to 50 degrees. The results of these studies were published by Tomren, Iyoho, and Azar. Mud velocities in the range of 3 to 4 ft/s were found necessary for high angles with no pipe rotation as compared with 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, where 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 is quite different than that of vertical wellbores. It was observed removing a cuttings bed using a high viscosity mud, a remedy for hole cleaning problems in vertical wells, may in fact be detrimental in high angle wellbores (assuming zero to low drillpipe rotation), and that a low viscosity mud which can promote turbulence is more helpful. P. 389

Review of Hole Cleaning in Complex Structural Wells

“…For a long time, many scholars has been studying the hole cleaning problems of complex structural wells, and some achievements were made. Considering some major achievements reviewed by Pilehavri et al [3], this paper made a brief summary of hole cleaning before 2000 years, and the latest experimental researches, CFD simulations, correlations and models, and future study needs in hole cleaning were mainly addressed.…”

Challenges of cuttings transport in micro borehole coiled tubing drilling for mineral exploration

“…This is in agreement with experimental work of Peden et al [28] but contradicts the idea presented by Hyun et al [29] that less viscous fluid with high velocity under turbulent flow regime result in better cuttings transport performances. Pilehvari et al [30] recommended that turbulent flow regime in horizontal and highly deviated wells, regardless of the viscosity of the fluid, will lead to good cuttings transport. This shows that a decisive rheological property of the drilling fluid for the purpose of cuttings transport is not available.…”

Hole Cleaning Performance of Water vs. Polymer-Based Fluids Under Turbulent Flow Conditions

“…Inner pipe rotation enhances cutting transport but it is absent in coiled tubing application (Leising and Walton 1998, Kelessidis and Bandelis 2004, Li, Misselbrook et al 2008. Hole inclination angle has a profound influence on cutting transport (Larsen, Pilehvari et al 1997, Pilehvari, Azar et al 1999. For angles of inclination less than 10 degrees, cutting transport remains similar to vertical case.…”

“…(Martins and Santana 1992) experimentally showed that smaller cuttings (smallest tested size was 2 mm) are easier to erode than bigger ones. According to (Pilehvari, Azar et al 1999) and based on the results of experiments by (Larsen, Pilehvari et al 1997), small cuttings (0.1 in) are more difficult to remove than bigger cuttings.…”

“…Applicability of these models is limited by the assumptions under which they were developed. Another shortcoming of these models are the coefficients involved, which mostly must be determined experimentally ( (Pilehvari, Azar et al 1999). According to (Li and Walker 1999) fluid mechanics of flow in eccentric annulus is very different than in pipes or channel flows.…”