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Super Sealing

Super sealing SANDVIK Mining and Construction has signed a global partnership agreement with Canadianbased drill-rig manufacturer Cubex for the distribution of QXR surface drills.

Many factors play a critical role in the selection and placement of an effective annular seal. The ability of the contractor to assess the variables effectively, and determine the appropriate materials and procedures to create a continuous source of safe water is important, not only to the individual but also to the long-term viability of the water well industry itself.

Water well contractors work in an environment where failure is not an option and they do so with impressive results. The growing need for sustainable ground water demands that the choice of sealant be based on the geological/subsurface environment.
Bentonite Vs cement

Bentonite and cement are the two primary materials selected for the development of annular seals in water wells. Regulatory aspects may dictate the use of one material versus another in a given location. However, neither product offers an absolute solution for grouting. Each material has its strengths and weaknesses, and therefore represents a tool for contractors to use at their discretion or to their advantage. Several factors effect the design and ultimate success of an annular seal, including:



subsurface water chemistry,

historic land uses.

In addition, the type of well (open-hole completion or screened well) must be considered. Successful grouting and sealing operations are accomplished in three phases:

pre-spud planning;


zonal isolation.
Pre-Spud planning

In any well-drilling project, the geological conditions dictate the type of well that is installed, whether it be an open-hole completion or a screened well. Offset well data is a valuable tool that can be used to establish a basic knowledge of the lithology. This can give the contractor insight as to potential lost circulation zones, the need for surface casing, the appropriate drilling method and drilling fluid design indicators. Offset well data can be obtained from state and provincial databases by way of drillers’ logs, well diagrams and geophysical data. Many contractors also develop and maintain records with respect to problematic geological conditions, water quality and well production history. Certain governing agencies will dictate the requirement for specific well design and grouting materials based on the geographical area and, where appropriate, contractors should contact them to obtain more information before choosing materials.

Geology is the driving factor for the selection of annular sealing material. However, the extent to which it can effect the performance of an annular seal can vary, based on the latter’s orientation and the degree of saturation.

The vadose or unsaturated zone is dynamic and extremely variable. In many areas, it is minimal or non-existent, presenting little if any concern, while it can be extensive in more arid regions.

This upper portion of the geology not only fluctuates in moisture content, but also in geochemistry. The lack of constant saturation in the surrounding geology can have a negative impact on the ability of cement slurries and bentonite grouts to create a viable and sustainable seal within this interval.

Annular seals composed of sodium bentonite chips offer some advantage, but the seal produced can suffer from the loss of hydrated water to the unsaturated zone.

The placement of an annular seal initiating just above the producing interval and within a zone of complete saturation represents a ‘Best Well Construction Practice’ as the critical isolation point is established in very close proximity to the production zone. This practice effectively isolates the production zone and creates a competent point at which to begin grouting the remaining interval of the annular space.

Increasing concerns regarding the deterioration of water quality in upper aquifers magnify the issues related to those shallow aquifers mixing with deeper primary production aquifers. Prior knowledge of the location of the upper and lower aquifers, along with the associated confining layers is vital when determining appropriate casing programmes.

Knowledge of geological features like fractures, unconsolidated zones, mineralised sections, faulting, or extensive coal or lignite seams is also essential when preparing to place a functional annular seal. For example, if large sections of lignite or coal are present, bentonite grout would not be the best choice for the seal as lignin can produce low pH environments, and act as a natural thinner and dispersant to bentonite.

In comparison, zones of extreme porosity and permeability do not provide optimal conditions for the use of cement as the density and hydrostatic pressure exerted by a column of cement usually results in significant and continuous loss of material to the formation. These conditions would be better served through the use of a pumpable bentonite grout or sodium bentonite chips.

Pre-existing knowledge of subsurface water chemistry is always a valuable tool to help select the appropriate grouting material. This can be the result of natural geological factors or of historical land usage; e.g. mining or industrial applications.

These operations can create a variety of conditions such as low pH, elevated chlorides, increased levels of sulphates/ sulphides, and high levels of iron. The ultimate impact on bentonite and cement are cumulative in nature, and can be extremely detrimental. As the degree of geological complexity increases, it is often necessary to use both cement and bentonite separately to complete the task at hand.
Drilling phase

In many cases, the impact of drilling on the effectiveness of grouting operations is overlooked. Lack of attention is often the cause of a variety of problems that directly affect the long-term viability of the grout seal. Geological information acquired during pre-spud planning is vital to the selection of the drilling method and fluid system applied.

Continuous borehole stability is essential during drilling and the fluid system design has a critical role, helping the contractor to minimise the potential for washouts and the development of erosional features. Maintenance of borehole gauge is necessary for efficient grout displacement and uniform distribution of the grout surrounding the casing.
Grouting phase

A key issue for successful grouting is the size of the annular space: 5cm annular is recommended as this allows the contractor to choose from a larger number of grouting materials. In addition, the increased annular space allows the use of larger-diameter tremie lines, which reduce frictional pressure losses during grout placement.

Sodium bentonite chips are an effective tool for a number of completion applications – when regulations allow, the isolation of the filterpack or placement of the bentonite chips at the base of the intermediate casing string are excellent applications of the material.

Centralisation of the casing is also very important for efficient grout placement. If the casing is compressed prior to grout placement it might not sit concentrically within the borehole and can come into direct contact with the wall. This increases the probability of channelling and ineffective displacement, which can result in the non-uniform distribution of grout around the casing.

The use of physical centralisers is recommended to correct the orientation of the casing in the borehole. Another option is to pull the casing string into tension rather than leave it in compression. This adjustment will sit the casing concentrically in the borehole and improve the displacement efficiency of the grout.
Choosing a grouting material

Determining the appropriate grouting material for each application is based on the information acquired during the planning phase and confirmed during the actual drilling.

When a pumpable bentonite grout is selected for use, a few basic guidelines should be followed. Bentonite grouts are combined with fresh water to create the resulting seal, so the quality of the water used is critical to the final yield and performance of the resulting grout. The make-up water should always be pre-treated with soda ash to facilitate the removal of excess calcium, and bring about an increase in pH to a range of 8.5-9.5 prior to mixing and the placement of grout.

The optimal values for hardness of the make-up water for use in conjunction with bentonite grouts should be ≤100mg per litre. The mixing and placing of bentonite grouts is predominantly designed around the concept that the bentonite component should be added to a pre-determined volume of fresh water, and mixed at low shear until the material is ready for placement into the annular space.

The best method for ensuring a consistent solids content is through the volume of water required to create each batch of grout. Accurate measurement and marking of the grout tank to identify the proper volume of water needed prior to mixing will help the contractor to meet this requirement. Bentonite grouts should always be mixed in accordance with the manufacturer’s recommendations for consistency and optimal performance.

The use of cement also demands adherence to specific requirements for mixing and performance. Class A or Type I Portland cement used for the hydraulic cementing of wells calls for a water requirement of 46% by weight of cement. Regulations may dictate the amount of water to be used, and acceptable concentrations of bentonite and/or sand that may be added.

A critical property of cement that is often overlooked is the filtration rate of the resulting slurry. Common cement slurry essentially has no filtration control and can easily lose the majority of its water phase before curing is completed. This effects the ability of the cement to bond and achieve isolation of the zone of interest. Filtration-control additives exist that can address this issue and dramatically improve the performance of the cement slurry.

It is important to remember that cement is often selected to establish a seal in the presence of contaminants. The continued practice of increasing the water content often reduces the resistance of the cement to chemical attack and reduces the bonding characteristics of the grout. If a cement solution is selected to counteract the presence of a contaminant, it is vital that strict adherence to appropriate water requirements be followed.

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