Proper application and utilization of mechanical solids control equipment enables the driller to maintain the desired drilling fluid properties which in turn makes it possible for the drilling operation to be carried out in an efficient and possible for the drilling operation to be carried out in an efficient and economical manner.
Drilled solids have a direct and pronounced effect on the drilling fluid. Since the drilling fluid is itself so closely related to a wide range of factors which determine both daily and total drilling costs, it is obvious that there exists a strong relationship between the control of drilled solids and the control of drilling costs.
This paper will present methods to calculate the mechanical efficiency of solids control equipment on site during drilling operations and will demonstrate from case histories a method of determining the economic justification for the use of mechanical equipment.
Data presented in this Paper is based on information gained in the field evaluation of various types of solids control equipment, including sophisticated fluids processing systems, under a wide variety of weather, geographic and geologic conditions.
The drilling industry as we know it today has evolved through a combination of trial-and-error and technology, of hard work and luck. Through more than a century of development many of the problems encountered by the pioneers in the business have been problems encountered by the pioneers in the business have been solved, or at least made bearable. The technological advances of the drilling industry are on a par with those of virtually any other industry. But there is one serious problem still facing us and it has been a problem since the earliest days. Very simply, that problem is how to get the dirt out of the hole. The removal of drilled solids from drilling fluid, or drilling mud, has been a problem since the introduction of rotary drilling rigs, The early day driller used gravity settling to accomplish solids removal, and earthen pits were a standard part of the drilling site. Drilling mud became more complex, however, part of the drilling site. Drilling mud became more complex, however, and with the use of barite as a weighting agent in the early 1930’s the need for better methods of solids removal became apparent. Vibrating screens designed to remove the undesirable drilled solids while salvaging the barite and liquid mud were introduced to the drilling industry. These “shale shakers”, as they came to be called, were the first attempt by the drilling industry to use mechanical equipment for more effective and economical solids removal.
During the past half-entry there have been numerous types of solids removal equipment developed and tried in the industry. When judged solely on an engineering evaluation, much of the equipment was found to be satisfactory for applications in the drilling industry. Nevertheless, the equipment was not widely accepted until economics dictated a reassessment of our drilling practices. Solids removal equipment which had been judged to be too costly and too bothersome during an earlier era of relatively inexpensive drilling and minimum regulation is now being seriously considered as a means of reducing total drilling costs and complying with new environmental regulations.
Drilling fluid, whether it be water-base or oil-base mud, can be described simplistically as being a suspension of solids in a liquid medium – Liquid Phase. This may be either water, oil or salt solution. Solids.These may be described as either additives or contaminants. The additive materials are further differentiated on the basis of their specific gravity. High gravity solids, such as barite, have a specific gravity of 4.2 or more and are used to increase the density of the drilling fluid. Low gravity solids, such as bentonite have a specific gravity of 2.6 and are used as a viscosifying agent. Drilled solids, also have a specific gravity of 2.6. and are also referred to as low gravity solids. Drilled solids are the most common contaminant found in drilling fluids and they are responsible for the majority of problems associated with drilling fluids. These drilled solids, by their sheer volume, can significantly alter the density of a fluid and both the volume and the size of the drilled solids can drastically affect the viscosity of the drilling fluid.
Over the years, drilling fluids have been developed to the point that they do an excellent job of transporting drill cuttings up the bore and to the surface. However only part of the job has been done when drilled solids reach the surface. At this point the problem becomes that of removing the drilled solids from the drilling fluid. In as much as the volume and type of solids in a drilling fluid directly influence the density and viscosity – – and indeed all the rheological properties of the drilling fluid, it obviously follows that drilled solids greatly influence mud costs, well costs, rates of penetration, hydraulics, possibilities of kicks” and or returns, and an penetration, hydraulics, possibilities of kicks” and or returns, and an almost endless list of both small and large drilling problems. From both an engineering and an economic viewpoint, solids control becomes one of the most important aspects of drilling fluid treatment. It has been estimated that more money is spent annually to control drilled solids and to correct problems created by inadequate solids control measures than is spent on all problems created by inadequate solids control measures than is spent on all other problems encountered in drilling operations.
This indicates the need for a practical approach to the problem of solids control, or solids removal as we should more properly state it.
Methods of Solids Removal
Four basic methods of handling the problem of drilled solids removal from drilling fluids have been developed:
- Dilution displacement
- Accelerated gravity
The use of chemical additives is sometimes considered to be an additional method of handling the drilled solids problem. In general, chemicals are used to increase the ability of drill fluids to tolerate solids rather than decrease the actual volume of solids in the drilling fluid.
Settling Settling continues to be a commonly used method of solids removal in drilling operations throughout the world although it is no longer common to have the drilling site covered with earthen settling pits. Settling by itself, is too slow and inefficient–and therefore too expensive and impractical — to be considered as the primary method of solids removal in modem drilling applications.
The settling rate of drilling solids in drill fluids can be estimated by Stokes’ Law, which demonstrates mathematically that the settling sate of a particle in a drilling fluid is dependent on such factors as particle size and density and fluid density and viscosity. The settling rate of drilled solids can be changed by manipulating any of the factors expressed in Stokes’ law. The settling rate can be increased in drilling fluid by the usc of chemical additives (such as flocculants or “thinners”) or by using mechanical equipment to increase the “G” forces acting on the particles and thereby increase the settling rate.
Dilution displacement Dilution basically involves the addition of water to reduce the solids concentration to an acceptable level. The cost of the dilution water is an important factor in determining the economics of dilution, but it is not the only cost factor. Sufficient chemicals (bentonite, barite, etc.) must be added to maintain the basic mud properties required for the drilling operation. In essence, each barrel of dilution water added has a value equivalent to the cost per barrel of the drilling fluid in the circulating system.
Screening – Vibrating screens are the most common type of mechanical solids removal equipment used on modem drilling rigs, being used on virtually all drilling rigs today. These vibrating screens, commonly referred o as side shakers, are used to remove solids of a particle size larger than the screen openings. Screens used in drilling applications vary from those as coarse as 10 mesh to those as fine as 200mesh.
A number of factors, such as circulating sate and type of drilling fluid to be used, should be taken into account when determining the number of shaken and the screen mesh to be used in a drilling operation. A sufficient number of shakers with the finest mesh screen possible should be employed in order to remove the largest volume of drilled solids possible. The vibrating screens do remove solids larger than the screen openings, but particles smaller than the openings remain in the drilling fluid. Those particles not removed by screening or settling must be removed by some other method in order to prevent the problems associated with excess drilled solids.
Accelerated gravity – As shown by Stokes’ Law. solids separation can be substantially improved over normal gravity settling by mechanically increasing the “G” forces acting on the particles in the fluid, Mechanical devices employing this concept are routinely u=d in fluid processing and solids removal equipment, Such pieces of equipment are generally referred to as accelerated gravity devices and include the liquid cyclone or hydrocyclone and the decanting centrifuge.
Many drilling rigs incorporate a desilter into their solids control systems. The desilter is a smaller hydrocyclone than the desander and is placed down steam from the desander. The desilter normally processes the overflow, or clean mud, from the desander. The cut point for desilters is in the 25 micron range. These particles are discharged from the desilter through the under flow and are normally directed to the waste pit. As with the desander, the under flow can be directed across a fine-mesh vibrating screen to salvage the liquid mud from the underflow.
Centrifuges are required to remove the lower range of the ultra-tine drilled solids particles. When used to handle unweighed muds, the centrifuge underflow contains drilled solids and is discarded while the overflow is returned to the active system. In treating weighted muds the process is reversed, with the underflow (containing mostly barite) being returned to the active system and the overflow being discarded. Decanting centrifuges make a cut in the 5-10 micron range and are available as decanting barrei type centrifuges and hydrocyclone centrifuges.
A recent refinement in solids removal systems is the introduction of the ultra-high speed centrifuge. It is primarily used to proms the overflow from a conventional centrifuge. Making a cut in the 1 -2 micron range, the ultra-high speed centrifuge removes solids in a very dry condition. The overflow from this device can be used as make-up water or, when u~d on weighted muds, can be used as dilution water for conventional centrifuges. This utilization reduces both the volume of waste fluid ad the volume of dilution and/or make-up water.
Equipment Evaluation procedures
Material balance studies are required to properly evaluate the efficiency of either a single piece of solids removal equipment or a comprehensive solids control system. A number of methods have been formulated and published which arc mathematically valid and which, when properly carried out, give accurate results. Most of these methods are seldom used however because of the difficulty of training rig-site personnel to carry out the complicated and/or time consuming procedures required to obtain accurate measurements. The method outlined below has been developed through extensive field testing and is the refinement of several successful techniques. It has been found to k both a practical and accurate means of collecting the required information to carry out an evaluation of the solids removal efficiency of solids control equipment in use.
- Collect the solids discard into a 5 gallon pre-calibrated bucket and mead the time in minutes and seconds to fill the bucket 1/2 to 3/4 full.
- Weigh the pre-tared bucket on a wale to determine net amount of wet solids to nearest 5. + pounds.
- Add water to 5 gallon mark of bucket while stirring to remove entrapped air. Record the weight of the wet solids plus water. From this weight subtract the weight of the wet solids and bucket to obtain the weight of the water. The weight of the water divided by 8,33 will give gallons of water added. Subtracting the gallons of water from 5 gallons will give gallons of wet solids discard collected in the measured time interval.
- Calculate the weight of the wet solids in lbs/gal. (Weight of the wet solids — gallons of solids collected)
- For unweighed muds, assume the specific gravity of solids to be 2.6. For weighted muds, determine average specific gravity of solids in the sample by using mud report mud stiff retort data (if reasonably accurate. More complicated laboratory analysis procedures ears be used if preferred and/ or available.
- Percent low gravity solids and percent barite in the sample can be calculated as follows:
Vc=Vs(Sb-Ss)/(Sb-Ss) and Vb=Vs-Vc
Where Vc = % low gravity solids; Vb = % barite; Vs = Total % volume solids; Sb = Specific gravity of barite; Sc = Specific gravity of low gravity solids; Ss = Average specific gravity of solids in sample.
- Using the measured volume rate of solids discarded in gallons per minute of each piece of solids control equipment being used, the equipment efficiency can be calculated.
Percent efficiency of solids control equipment = (Drilling solids discarded, lbs/hr) ⁄ (Drilling solids generated, lbs/hr)
After a brief training session, rig-site personnel have been able to collect and record the required rate and weight information for making an equipment efficiency evaluation. Using prepared data forms and either standard hand held calculators or programmable calculators with attached printers, rig-site personnel can calculate the solids removal efficiency of any or all solids control equipment in use and they eats do this on a routine basis.
Two separate Solids Evaluation data forms have ken developed for rig-site testing of solids removal equipment. One form is designed for usage with standard hand-held calculators and the other for programmable calculators with attached printers. Both types of data forms (for evaluating shale shakers. cyclones and centrifuges) are included in this paper, although the actual program listings for the programmable calculator are not included. Two Solids Evaluation data forms have been developed for laboratory analysis evaluations of mechanical equipment efficiency. These procedures are more complex and require more equipment and more time to awry out than the field evaluation procedures. The labotatory data forms are available for use with standard or programmable calculators.
Economical Evaluation Methods
T he need to control the volume of drilled solids in a drilling fluid has been accepted. The means by which they should be controlled is still a subject of debate and undoubtedly will remain so for years to come. One central problem is that the majority of people do not agree about what they are trying to accomplish with solids control or solids treating techniques. Solids control, like all aspects of drilling, has become mom complicated and sophisticated. Very simply, however, drilling involves the boring of a hole into the earth to a required depth without damaging either the formations penetrated or the environment of the rig site, and doing all this in a safe, efficient and economical manner. Equally simply, solids control involves the maintenance of the solids content in a drilling fluid at a tolerable level. In order to do this, excess solids must be removed or the drilling fluid system must be diluted. Solids control eats then be simplified to (a) Solids removal or (b) Dilution.
As stated above, safety, efficiency and economics are three important considerations in any phase of drilling. Many in the drilling industry would insist that economics has always been the main determinant in the type of solids control employed and the extent to which it has been used, However, not all the cost factors involved have been properly considered in previous evaluation procedures, Recent inflationary trends have also had considerable impact on the drilling industry and such factors have been particularly significant in solids control cost evaluations.
Dilution Costs – The economic evaluation procedure discussed here involves a cost comparison between the use of mechanical solids removal equipment and the practice of dilution, The primary cost factor in dilution k the additional volume of whole mud required to dilute the drilling fluid system so as to contain the same percent volume of drill solids as has been accomplished by the removal of a measured volume of drilled solids by the mechanical equipment. The total cost of such dilution practices is the cost of the additional volume added plus any additional miscellaneous costs required (transportation, additional labor, etc.).
Mechanical Equipment Costs – There are only three basic factors to be considered in determining the cost of mechanical equipment usage for solids removal:
- The replacement cost of that volume of material lost from the active drilling fluid system through the use of mechanical equipment, This volume is comprised of both the drilled fluid solids removed from the drilling fluid and that portion of the drilling fluid which is inevitably discarded along with the undesirable drilled solids.
- Equipment costs this factor includes daily rental costs operating costs, service and maintenance costs and costs for auxiliary equipment, such as generators, required for equipment usage.
- Miscellaneous costs: transportation costs (trucking, shipping etc.) are the primary items in this category. In some areas labor costs may also be a significant factor.
Economics is one of the primary considerations in the planning and implementation of the vast majority of wells drilled today. Inflationary cost increases and ecologic regulations halve forced the driller to look more closely at every aspect of the drilling operation. Traditional methods of solids control are now being prevented or made difficult by governmental regulations in many areas of the world. It is in the best interest of the industry to adapt to these changes and to determine the most efficient and economical methods to accomplish their tasks.
Economic evaluations from other wells, regardless of mud types and costs, have indicated similar results in savings through the use of mechanical equipment. One other obvious result of these tests is the finding that economic efficiency of mechanical equipment tends to be directly proportional to the mechanical efficiency of the equipment. Properly maintained equipment is a good investment in reducing the cost of drilled solids removal.
The proper application of mechanical solids control equipment is an effective means of solving a troublesome drilling problem in an economical way. Equipment efficiency evaluations and economic evaluations can be of benefit in justifying the use of such equipment and in determining the degree of success in removing drilled solids.