Cuttings transport is a topic of great interest in the oil and gas drilling industry. Insufficient cuttings transport leads to several expensive problems. Knowledge and selection of the drilling fluids is one of the important factor for efficient hole cleaning. It has been observed, however, that the hole cleaning performance of drilling fluids can be different even if the fluid rheological properties are similar as measured in accordance with API specifications. The reasons for stated difference in the behavior of drilling fluids are not well understood. The main objective of present work is to evaluate hole cleaning efficiency of an oil-based drilling fluid (OBM) and a water-based drilling fluid (WBM) whose viscosity profiles are similar as per API specifications.Hole cleaning efficiency of an oil-based drilling fluid and a water-based drilling fluid whose rheological properties are similar was investigated. The fluids tested were industrial fluids used in the field and were sent to us after reconditioning. Experimental studies were performed on an advanced purpose-built flow-loop by varying flow velocities and drill string rotation rates. The flow loop had a 10 m long annulus section with 4" inner diameter wellbore and 2" outer diameter fully eccentric drill string. Pressure drop and sand holdup measurements were reported. Rheological investigations of the same fluids were used to understand the difference in the behavior of the drilling fluids tested. Higher pressure drop was observed for WBM compared to OBM, and for both fluids, the pressure drop increased with drill string rotation speed. In case of no drill string rotation, better hole cleaning performance was observed with the oil-based fluid compared to the water-based fluid. With the presence of drill string rotation, hole cleaning performance of both the fluids was nearly the same.

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One of the most critical operations during well construction is the cementing procedure. Due to the curing nature of the cement slurry there will be only one opportunity to cement the well properly. Although one for top hole cases can fill cement in from the top in a remedial operation, this possibility cannot fully compensate for a non-optimal initial cement job. Furthermore, it cannot be applied to other well sections. In those sections, complex squeeze cementing operations may be necessary. Consequences of improper annular cement can be leakage during production phase and extensive costs when the well is to be plugged for abandonment after the production phase. To ensure that the risk of poor cement is minimised it is important to use the best procedures to place the cement properly. To be able to select the optimum procedures, it is necessary to improve the understanding of the displacement in the wellbore annulus. All wells will be cemented in several sections. Findings and improvements that can reduce risk of poor cementing results are thus highly relevant for a large number of operations every year. The article is based on analysing experimental results that illustrates a drilling fluid being displaced by a cement slurry. These fluids are represented by realistic model fluids and circulated through a transparent annular section. The geometry used is a 6,5″outer diameter with an inner string of 5″that also can rotate. The selected pipe sizes may normally be found in the lower parts of a well and often in deviated sections where the inner pipe cannot be assumed concentric at all times. Both concentric and eccentric inner pipe positions have therefore been selected. The test section was run both in horizontal and in inclined position. The test section was 10 meters long and instrumented with conductivity probes in an array around the perimeter at 4 separate positions along the pipe. Together with cameras along the test section the fluid interphases was observed along the test section. Results presented in the article show that inner string rotation provides a steeper displacement front, On the other hand such rotation will also cause more mixing at the interphase. Results also show that the displacement front in a concentric annulus is significantly affected by gravity. While for an eccentric annulus, with the low side at the bottom, the narrow gap is poorly displaced when realistic fluids are applied. It was also observed that the displacement front in concentric annulus was more stable when the test section was inclined than in horizontal position.

A drilling fluid for drilling deviated wellbores must provide adequate hole cleaning efficiency for all well angles relevant to the operation. For angles near vertical, experience show that hole cleaning is straight forward. In wellbore angles larger than, say, 45 degrees hole cleaning is more difficult. Cuttings beds are formed and at some well angles these beds may avalanche during circulation stops etc. This paper presents results from laboratory tests with injected cuttings using a low viscosity oil based drilling fluid with micronized grained barite as weight material. The fluid is designed for highly deviated wells with low ECD requirements and the cuttings transport performance through relevant wellbore inclinations was investigated. The experiments have been performed under realistic conditions. The flow loop includes a 10 meters long test section with 2” OD freely rotating steel drill string inside a 4” ID wellbore made of steel, representing a cased wellbore. Sand particles were injected while circulating the drilling fluid through the test section. Experiments were performed in three wellbore inclinations: 48, 60 and 90 degrees from vertical. Results show that hole cleaning in absence of drill pipe rotation is significantly improved if the well angle is less than a critical angle. This critical angle appears to be less than 60 degrees from vertical. Further result show that this critical inclination angle is dependent to the drill string rotation rate and the annular flow velocity.

One of the most critical operations during well construction is the cementing procedure, where drilling fluid is displaced by cement, normally with one or more spacer fluids in between. Due to the curing nature of the cement slurry there will be only one opportunity to cement the well properly. Although one for top hole cases can fill cement in from the top in a remedial operation, this possibility cannot fully compensate for a non-optimal initial cement job. Furthermore, it cannot be applied to other well sections. In those sections, complex squeeze cementing operations may be necessary. Consequences of improper annular cement can be leakage during production phase and extensive costs when the well is to be plugged for abandonment after the production phase. To ensure that the risk of poor cement is minimised it is important to use the best procedures to place the cement properly. Most models in use assume that the annulus is homogeneous. This is not always the case since washout sections appear during drilling. The effects of these on cementing are not sufficiently studied and considered in models and procedures. Here we present and discuss results from fluid displacement experiments in a laboratory flow loop, illustrating annular displacement of drilling fluid by spacer (or spacer by cement). Model fluids with realistic densities and rheological properties have been used in a test setup with a transparent annular section. The wellbore is represented by a 10 m long test section, where the annulus has a 6,5” outer diameter and an inner string of 5” that can rotate. A washout section is represented by a 2 m long section of the outer pipe with a larger diameter of 11”. These diameters are representative for the lower parts of a well were high wellbore inclinations are common. In these sections the inner pipe cannot be assumed concentric at all times, so both concentric and eccentric positions have been tested. Experiments reported here were conducted at 60 degrees inclination. The test section was instrumented with conductivity probes in an array around the perimeter at 4 separate positions along the pipe, including the inlet and outlet of the washout section. Together with a camera along the test section, this provided information about the motion and shape of the liquid-liquid interface through the test section. Results show that the displacement front changes significantly when entering the washout zone compared to the regular annular section. Due to the larger flow area the density differences between displaced and displacing fluids become more important in the washout section, while momentum effects dominate in the regular section.