Solids Control in Weighted Drilling Fluids
Authors: Robinson L.H. , Heilhecker J.K. 
Published: December 2007

Introduction

The need for solids control for drilling mud systems has been widely discussed in the literature and is gaining acceptance in drilling operations. Some of the major advantages of reduced drill-solids content in drilling muds are lower plastic viscosity, improved filter cakes, longer bit bearing and pump life, and improved penetration rate.Increases in the drill-solids content of muds generally lead to higher filtration rates and increased filter-cake thickness. Thick filter cakes aggravate the problem of differential-pressure sticking and cause increases in the torque and drag on the drillstring. Field observations indicate that filter-cake properties can be controlled by controlling the amount and types of solids in the mud.Surprisingly, drill solids cause a drastic reduction in bit-bearing life. King performed a series of experiments using an 8 3/4-in. bit with 50,000 lb weight and rotating at 60 rpm. When water was the circulating fluid, the bearings lasted 36 hours. The addition of formation cuttings decreased the bearing life to 26 hours. When the mud was treated with caustic and quebracho, the bearing life was reduced to 12 hours. When the same experiment was conducted using bentonite instead of formation clays, the bearing life increased from 36 to 38 hours. Additions of caustic and quebracho to this system did not reduce the bearing life, but when drill solids were added to this mud the bearing life decreased to 15 hours.

New Mud-Treating System (Mud Cleaner)

To overcome many of the limitations of current solids-removal techniques, a new mud-treating system was developed for weighted muds. This new system, called the Mud Cleaner, removes most of the drilled solids while retaining most of the barite and liquid phase of the mud. Unlike most barite-reclaiming or solids-removal techniques normally employed only after the drilled solids have reached an undesirable level, the Mud Cleaner is most effective when used as soon as barite is added to the mud system, and continuously thereafter. Continuous operation removes solids before they disintegrate to silt or clay size, thus preventing drill solids from accumulating in the mud system.Briefly, the new mud-cleaning system involves processing the mud through three separate stages:mud processing the mud through three separate stages:mud at well-circulation rate flows through a first screen (shale shaker), which removes large-size solids;the mud passes through a centrifuging stage (desilters), which passes through a centrifuging stage (desilters), which separates the mud into a low-density effluent and a high-density underflow - the effluent, comprising most of the liquid phase, returns to the mud system; andthe underflow, containing barite and drilled solids, flows through a second, smaller screen that is sized to pass the barite but to reject most of the drilled solids.

The first screen can be provided by a conventional shale shaker that usually has a screen size between about 10 and 80 mesh. For most wells, shaker screens finer than 80 mesh are not capable of handling mud at normal well circulation rates. This means that mud passing the shale shaker contains large amounts of drilled solids. These solids upset mud properties and are the most difficult to remove. By pumping the mud tough desilters and screening the underflow, a large part of these fines can be removed without appreciable loss of barite.A sufficient number of desilters are used to process the entire mud stream. The desilters separate relatively light and fine particles from the heavier and coarser particles. Barite, which is present in the underflow along with drilled solids, is passed to the second screening stage. Most of the bentonite and liquid phase are discharged from the desilter in the effluent stream.For the second screen to reject additional drilled solids, its screen size must be much finer than the shaker screen, but must be sufficiently large to pass most of - the barite. The second screen is usually about 150 to 200 mesh. The flow rate of the underflow through the second screen is normally only about 10 to 20 percent of the well circulation rate. At such low throughput rates, the problem of screen blinding is not serious.For best performance, the fine screen requires special features. A vibrating screen that has performed quite well under field conditions is shown mounted on a portable skid below a bank of 4-in. desilters in Fig. 1. This device has a factory-tensioned screen that insures that screen life is not shortened by improper tensioning on the rig. A self-cleaning kit keeps drilled solids from plugging the screen and preventing passage of barite. This kit is a tray of short, plastic cylinders that are bounced against the screen by a 1,800-rpm motor; the rapid agitation keeps the screens clean and operative.The vibrating screens are adjusted at the factory to screen efficiently typical cyclone underflow material. The number and relative position of the upper and lower motor weights control the motion of the material on the screen surface. The horizontal throw of the material on each bounce is controlled by the number of top motor weights. The vertical amplitude is controlled by the number of bottom weight plates secured to the motor. The angle between the top and the bottom weights controls the spiral pattern on the screen. pattern on the screen. When the solids drop onto the screens, the vibration causes them to spiral away from the center. A "rope" of material forms and rotates around the outer perimeter of the screen. As the material passes the discharge spout, some of the rope is removed from the screen.

JPT

P. 1141