Case Study Annular Cuttings Transport

This project investigated a method for removal of cuttings from a drill bit and out of the new hole during an oil well drilling operation.  The two main objectives were to identify acceptable operating conditions with the lowest possible power requirement and perform an endurance test on the pumping equipment. The first requirement ensured that sufficient power could be transferred from a drilling platform to the drill bit which could be located several thousand meters away. The endurance test demonstrated the resilience of the equipment to the abrasiveness of the cuttings and long term effects like system blockage caused by the gradual accumulation of cuttings within the system.

Working from a basic specification consisting of the well diameter and the target drilling speed, an experimental programme was defined. This covered everything from specification of a range of test conditions through to design of all the experimental equipment, sourcing of specialist oil abstraction pumps, assembly and  testing of an automated flow rig and eventual reporting.

The test rig was essentially a recirculating system where a water / sand slurry was continuously pumped around a series of pipes. A reservoir of slurry was generated in a continuously stirred 1500 litre vessel. The concentration was controlled by manual addition of sand, the speed of the mixing paddle and sand removal by a hydrocyclone. The slurry was then pumped 1200 meters along 5 different types of pipe, a transparent section of flow visualisation tubing, through a flow meter and finally back to the reservoir. The transparent tubing revealed sand particles rolling along the bed of the pipe and the presence of travelling waves of sand. These observations were used to determine minimum acceptable mean fluid velocity.

 A major element of the project was the initial specification of both test conditions and suitable hardware. Identification of the minimum power requirements required the rig to run with the lowest fluid velocity and highest cuttings concentration that would guarantee cuttings transport without blockage occurring. A study of sand pumping technology provided the background data to specify a range of potentially suitable fluid velocities. This data, combined with the target drilling rate, allowed the system flow rates to be determined. A more accurate simulation of test conditions involved insertion of another plastic tube along the length of the first one. The second tube, being denser that water, fell to the lowest point inside the pipe and hence created a crescent shaped flow passage. A computational fluid dynamics (CFD) model of the crescent was constructed to calculate the pressure drop per meter for a range of flow rates. This data, along with the potential length of the well, allowed calculation of the pumping pressures. With knowledge of both operational pressures and flow rates the pump sizes were then specified.

A Labview based control and data logging system was built to monitor and log component performance as well as control rig operating conditions. This successfully allowed the rig to run in stand alone mode 24 hours a day. Fluid velocities were measured by a rubber lined electromagnetic flow meter and critical pressures were measured via a series of strain gauge differential pressure transducers. By linking their outputs, along with those from motor speed sensors and motor inverters to the control system it was possible to measure and log the actual pressures needed to drive the slurry around the system as well as observe variation in pump performance due to the abrasive effects of the sand particles. The control system turned out to be surprisingly effective - to the extent that the effect of daily temperature variations on the geometry of some of the plastic pipe work could be measured.

Under normal abstraction conditions pumps used to raise crude oil already cope with a degree of sand contamination. In this study the concentration levels far exceeded the norm and pump manufacturers were unable to predict the longevity of their products. Two pumps, both commonly used to raise crude oil but working on quite different principles, were therefore tested to observe characteristic wear modes. The first was a progressive cavity positive displacement pump and the second was an multistage centrifugal system. The test results easily demonstrated the effects of wear on performance efficiency and it’s decline with running hours. These results, combined with post test inspection of each pump’s key components provided data suitable for design optimisation.