By Colin Rice, Colin Rice Exploration and Training (Pty) Ltd
Recirculation of drilled solids is probably the most common cause of in- hole and equipment problems in diamond core drilling operations and, in many cases, the application of "above ground" sump systems has exacerbated this problem. Effective solids control is therefore a critical element of any core drilling operation. In this article I will examine some of the theory and then some practical solutions to this serious problem.
This is the first article of Part 2 of our Technical Series on Drilling Fluids. Click here for an outline of the entire Drilling Fluids, Oils & Greases Series.
The rate of cutting development
Very seldom have I come across a driller, or even a supervisor, who knows how much cutting is generated in drilling a borehole. In this article I will not discuss how to calculate of the mass of cutting that is generated in drilling a borehole, if you would like to know how to do the calculation send me an e-mail and I will show you how. Obviously, the mass of cutting generated is dependent upon the kerf area of the drill bit and the density of the rock being drilled, the table below shows the mass of cutting generated per meter for a number of the more popular diamond drilling configurations and for two full-hole borehole sizes.
It is astounding how much cutting we generate in drilling borehole and, in the case of a cored borehole, the fact that all of the cutting is extremely small in size makes solids control very challenging indeed. The solids control system is therefore critical to ensure that cuttings are not recirculated.
The nature of drilled solids
The size of solids produced while drilling will depend upon the method of drilling and the physical characteristics of the rock being penetrated. The table below shows a generally accepted classification of drilled solids by particle size.
In an oil and gas drilling operation where the borehole is drilled with a tricone or PDC bit, cuttings vary in size from fine to coarse with a very small proportion of ultra-fines, oil and gas operations therefore are able to remove a very great proportion of cutting from the drilling fluid using a combination of mechanical seperators, shale shakers, desanders, desilters and sometimes even hydro-cyclones. Each of these mechanical seperators efficiently remove progressively smaller sized solids from the system and so it is possible to successfully drill with relatively small circulating volumes without recirculating drilled solids.
In a diamond coring operation however all of the cutting will be colloidal to ultra-fine and so the industry has relied on large surface sumps to remove drilled solids through sedimentation. Mechanical solids control systems have not be applied in core drilling operations . In recent years, there has been a significant move towards the elimination of surface sumps and the use of "above ground" sump systems to reduce environmental and other risk and to reduce rehabilitation costs. These "above ground" sump systems are generally based on a series of steel or plastic tanks and, because of the logistics in moving and setting up these tanks, most systems have very small total circulating volumes 3000 to 4000 litres only, and so the efficiency of solids settling is extremely poor resulting in very high rates of cutting recirculation.
The rate of settling of drilled cuttings
We can predict the rate of settling of drilled cuttings in a stationary liquid by using Stoke’s Law which is expressed below in a very simplified form:
We can think of slip velocity as the speed of descent of a cutting in the stationary liquid. Stoke’s Law expresses some readily observed principles:
- the greater the size of the cutting (d), the greater the rate of settling,
- the greater the density of the cutting the greater the rate of settling,
- the greater the viscosity of the drilling fluid the more slowly the cuttings will settle and,
- the greater the density of the drilling fluid the more slowly the cuttings will settle.
We can now understand why we use polymers to flocculate the cuttings and so effectively increase their size. Similarly, we can understand why desanders and desilters are effective - they spin the drilling fluid at very high speed and so artificially increase “g”, the acceleration due to gravity. As we progress through the rest of this series of articles, it is important to remember Stoke’s Law.
Total surface volume and turn-over time
Stoke’s Law allows estimation of the rate of settling of solids in a static liquid – it does not however predict the effect of movement in the fluid body on settling rates. In any fluid system, the drilling fluid is re-circulated and so the surface volume is constantly being “turned over”. What do I mean by "turned over"? If the total circulating volume of a fluid system is 5 000 litres and the pump rate is 50 litres per minute then the “turnover time” is 100 minutes - in other words, the entire circulating system will be circulated through the borehole in 100 minutes and so the total possible settling time will be 100 minutes. The shorter the turnover time, the less time the drilling fluid will have to settle cuttings and so the efficiency of solids settling will be poor. Conversely, the longer the turnover time, the more efficient solids settling will be.
It is pretty obvious that if we want cuttings to settle out of the drilling fluid, we want the fluid to be as quiesent as possible and this is why we use a series of meandering settling troughs to slow the rate of flow of the fluid. In small volume above ground systems however this is not possible and so the drilling fluid is constantly moving resulting in poor rates of settling.
As borehole is advanced, a portion of the drilled cutting settles in the sump system and effectively consumes some of the circulating volume resulting in the turnover time getting less and less as the borehole deepens. The rate of settling therefore becomes less and less efficient resulting in increasing amounts of cutting being recirculated. we can explain this as follows:
Assume we are drilling an NQ borehole and the total volume of the sump system is 5 000 litres. Let’s assume that our pump rate is 35 litres per minute.
At the start of the borehole, the turnover time of the system will be approximately 140 minutes (5 000 divided by 35). As the borehole depth increases, much of the cuttings generated are deposited into the surface system and these cuttings will gradually occupy an increasing proportion of the surface volume – the “effective” surface volume therefore gradually reduces as the borehole deepens and so the turnover time similarly reduces.
At 400 meters depth for example, we would have produced over 3 MT of cutting and this mass of cutting will have a density very much less than that of the host rock. The cutting will therefore occupy a much greater volume than if it were solid rock.
Let’s assume that the density of the cutting is half the density of the original rock – the 3 MT of cutting will therefore occupy 2 140 litres of the surface volume. The “effective circulating volume" will therefore no longer be 5000 litres, it only be 2860 litres. The turnover time will therefore only be 82 minutes, settling rates will be poorer and solids recirculation will occur at an increasing rate.
High solids content in a drilling fluid has a number of detrimental effects;
- centrifugal forces produced by the rotating drill rods throws out these solids onto the inside of the drill rods and over a period of time this deposition of material can lead to stuck inner tubes, greatly impaired bit life and excessive torque.
- ultra-fine material presents a very great surface area and drilling fluid additives are absorbed onto these solids resulting in increased usage and therefore increased drilling fluid cost.
How do we prevent recirculating solids
If we use surface sumps then it is relatively easy to keep or drilling fluid clean:
- make sure you have sufficient circulating volume and make sure that you have a series of settling troughs that slow down the velocity of the drilling fluid as it flows to the suction sump.
- ensure that sumps are cleaned regularly to ensure that the designed turnover time is maintained.
If we use above ground sumps it is a bit more challenging:
- carefully consider the total circulating volume available (number and volume of tanks used) and what the turnover time will be when using different drilling configurations. Turnover time when drilling HQ will be significantly less than when drilling NQ.
- above ground systems are used because we do not want to disturb the surface of the earth but a series of settling troughs leading to the pumping sump will help significantly to settle solids and if carefully designed will not add significantly to rehabilitation costs.
- because of the small circulating volumes, tanks must be cleaned very regularly to maintain turnover time.
- the cleanest fraction will always be at the surface of the sump, it is a good idea therefore to float the suction hose in the suction tank so that it sucks the cleanest possible fluid.
Solids build-up in cored boreholes has been shown to be a major cause of "in hole" problems and effective control of solids is an essential part of any drilling program.
Above ground systems have severe limitations and, in my opinion, potentially create some significant problems for the contractor. A very elegant solution is provided by mechanical solids removal units, or SRU's - the efficiency of these systems has increased dramatically over the past 4 or 5 years and they now offer an economical solution to a serious problem. In addition, they have several other advantages - a summary of the working and advantages of these units is provided in the following article and a video of one in operation is also included.