Other Flow Measurements
5.4 Cuttings Measurements
Measuring the amount of cuttings that are transported out of the well can be used to eval-uate hole cleaning and wellbore conditions. Based on this evaluation drilling parameters can be optimized. As described previously, flow parameters affect the cuttings transport, and measuring cuttings in the return line is therefore used in the evaluation of the flow parameters [28].
There are several methods for measuring the amount of cuttings. Comparing the theoreti-cal and actual decrease in the active volume of drilling fluid is one of them. The increased volume of the borehole due to drilling should lead to a decrease in the active volume as the cuttings are transported out of the well. When the actual decrease in active volume is considerably smaller than the theoretical decrease it is an indication of insufficient hole cleaning [29].
The cuttings flow meter can continuously measure the return of cuttings. A model of a cuttings flow meter can be seen inFig. 5.1. The meter is located at the outlet of the shale shakers. A tray-shaped gutter collects the cuttings and the weight is recorded. Conversion to volume is done by bulk density measurements. After a set time period the gutter is flipped, and the cuttings are dumped into a ditch. A large screw conveyor is located in the ditch, transporting the cuttings further in the rig cuttings collection system. To make the transportation easier lubrication is added [30]. The shaker room is a harsh working environment. Excessive vibrations, corrosive fluids, and erosion from solids lead to high requirements for the equipment. Gas can also be present, and all equipment must be within the safety regulations for equipment exposed to gas, because this presents an environment where explosions are a possibility. Rig space is often a limitation for equipment, and this is usually also the case in the shaker rooms. The cuttings flow meter can not take up too much space, and there also needs to be access for the crew to perform maintenance and repairs of the cuttings flow meter and the other parts of the shale shakers [28].
A new type of cuttings transportation system has been implemented on many rigs. An overview of the system can be seen in Fig. 5.2. After the cuttings have gone over the shale shakers, they are offloaded in the ditch with the screw conveyor. The cuttings are further transported into a pneumatic transfer unit called a cuttings blower. The cuttings are transferred in a dense phase to large tanks on the main deck using pressurized air. The
Fig. 5.1– Schematic of a cuttings flow meter [28].
bulk storage tanks are not pressurized and have a load cell to measure the weight of cut-tings. When the tanks are full another screw conveyor and pneumatic transfer unit are used to transfer the cuttings to similar bulk storage tanks located on supply vessels. There are many advantages to the new system. Offloading cuttings from the rig to supply vessels is usually done by transferring the cuttings in skips by the use of rig lifting cranes. The new system reduces the number of rig crane operations, both internally on the rig and between the rig and the boat. This is a major risk improvement when it comes to HSSE. The time it takes to transfer the cuttings from the rig to the boat is shorter using the pneumatic transfer system, so time-saving is also a benefit. The bulk storage tank system occupies less space than the cuttings skips and is beneficial for limited rig space. A challenge with the system is that the cuttings blower can be a bottleneck for cuttings transportation, limiting the rate of penetration (ROP) during drilling [31].
A lot of information can be extracted from looking at the shale shakers. Circulation before pulling out of hole is measured in how many times the full volume of the well is circu-lated from the bottom and up. The desire is to circulate all the cuttings out of the well to avoid packing off the BHA when pulling out of hole and to optimize hole conditions before running casing. Generally, there will always be a return of cuttings from the well when circulating, and the end point of circulation must be determined by when the amount of cuttings in the return line is sufficiently low [28].
When circulating after drilling to TD, the size of the cuttings often decreases after circu-lating approximately one and a half times bottoms up. Experiments have shown that larger cuttings arrive at surface before the smaller ones, because the larger cuttings are easier to remove. As drilling with a high ROP leads to larger cuttings, the amount of cuttings over the shakers can also vary with ROP. Variation in cuttings concentration over the shakers in inclined wells can be due to the position of the drillstring in a build-up section during rotation. Rotating while drilling or running in hole will push the drillstring to the low side of the well stirring up the cuttings that have accumulated there. Rotating while pulling out of hole will drag the drillstring along the high side of the well and there is no stirring up of the cuttings [28].
Fig. 5.2– Overview of the cuttings transfer system using cuttings blowers [31].
Studying the solids that are separated from the flow at the shakers can give important infor-mation, such as the origin of the solids. Rounded, smaller solids are often cuttings, created by the bit during drilling. Edged and elongated solids can often be cavings, which is a result of wellbore instability. However, due to long transport distances, the solids are often eroded to small, rounded pieces, making it different to interpret the origin. When large amounts of cavings are seen at the shale shakers, drilling parameters should be adjusted to manage the borehole instability [30].