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Pitless drilling


Pit construction and rehabilitation, water cartage and slurry disposal costs represent an ever-growing part of a drilling budget. With environmental and safety regulations becoming more stringent and increasing pressure from landholders to reduce the impact of drilling operations, there is increased scrutiny placed on the use of surface settling pits.

Historically, wireline coring operations in Australia have used multiple earthen pits or sumps as a reservoir for the circulating fluid. These pits allow the fine solids and rock particles produced by the impregnated or surface-set bits during the drilling process to be gravity-settled. The cutting-size fraction produced depends on both the geology and the bit type, but fine solids (2-60µm) are the most difficult to gravity-settle. As such, the smaller the cuttings, the larger the volume of earthen pits required.

Solids control techniques

Conventional solids-control equipment such as de-sanders/de-silters and shale shakers have long been used for open-hole mud rotary applications to recycle the drilling fluid and effectively manage solids content (i.e. mud weight).

When fine solids become problematic, centrifuges have been employed successfully. However, the primary limitation of centrifuge technology for wireline-coring is the capital and operating and maintenance costs relative to the number of diamond drills requiring attention.

The accumulation and eventual overloading of low-gravity solids in the drilling fluid can significantly affect the drilling operation and increase the mud weight to >1.08SG, which results in a range of drilling and down-hole problems. Wireline retrieval can be affected by development of mud rings and stuck tube and, in the worst case, a broken wireline.

High solids-loading increases the circulating pressures, resulting in possible damage to weak core and formations (circulation losses). Reduced rate of penetration (ROP) and poor core recovery are also symptomatic of a drilling fluid that has become laden with solids. If the solids can be removed, the drilling fluid can be maintained; however, if the fine solids continue to accumulate, the fluid will probably have to be completely replaced, resulting in additional water and product usage.

Better life expectancy

Baroid Industrial Drilling Products’ System 360TM uses a combination of mechanical and chemical separation to improve the quality and increase the productive life of the drilling fluid.

By completely removing the problematic cuttings from just a fraction of the total drilling fluid volume, the unit is able to actively maintain the desired average mud weight/density of ~1.02-1.04SG. Unlike conventional solids control equipment that often runs all shift, the System 360 unit only needs to be operated when regular mud weight checks indicate a steady rise in density. Mud weight is used to set thresholds at which users would be required to run the unit.

System 360 is also equipped with a high-performance mud-mixing system that, coupled with a suitable above-ground storage tank, can provide a complete closed-loop, pitless drilling-fluid system.

The potential benefits of pitless drilling for both drilling contractors and operators are significant. Contractors can expect to see improvements in drilling performance with improved borehole stability reducing re-drills; increased rates of penetration; easy operation and on-site assembly; and the ability to keep drilling when additional water is not available. Operators can see significant reductions in water usage (~80%) and drilling additive usage (30%), slurry disposal and haulage costs; improved safety by reducing haul-road traffic; and reduced cost of digging and rehabilitating ground pits.

How System 360 works 

The System 360 unit treats the drilling fluid in three steps:

Step 1: drilling fluid from the sump is pumped through 2in x 2.5in (5.08cm x 6.35cm) hydrocyclones, separating beneficial from non-beneficial solids. The overflow (usable drilling fluid) is returned to the active system. As less dense overflow fluid is mixed with the active system, the average mud weight of the circulating system is maintained;

Step 2: the underflow, which contains the damaging non-beneficial solids, is mixed with System FLOC-360 polymer flocculant, resulting in rapid flocculation. In less than three seconds ‘free’ clear water is observed separating from flocculated solids;

Step 3: the ‘free’ water and flocculated solids mixture is discharged directly into a porous bag situated above a collection tank. The flocculated solids are captured while the clear water drains into the tank. The flocculated material is compacted using a manual press into a ~20kg bag for safe, simple handling and disposal. Up to 80% of the original water content of the treated liquid can be reclaimed for re-use in making additional drilling fluid. 

This article was written by Andy Bilton, field service representative, Baroid Industrial Drilling Products. For more information:
Courtesy Geo Drilling

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