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Offshore reservoirs requiring sand control pose a major completion challenge because of extremely high cost and risk involved in remedial treatments, particularly in sub-sea completion and/or deep-water environments. It is therefore of utmost importance to ensure sand control without sacrificing flow conformance, recoverable reserves and well deliverability throughout the expected life of the completion. A major trend in these environments is towards open-hole, horizontal, gravel-packed completions. Although gravel packing stabilizes the wellbore, it can also entrap the filter-cake formed by the reservoir drilling fluid, potentially resulting in high drawdown requirements (flow initiation pressures) and/or low production rates (retained permeabilities). The cleanup procedures in the industry have varied significantly from no cleanup at all to complicated two-stage breaker treatments involving post-completion coiled tubing intervention, with no guidelines existing in the literature. In this paper, we present experimental results and field cases involving filter-cake flow-back through gravel packs with and without cleanup. Effects of various parameters, including gravel size (40/60, 20/40, and 12/20), formation permeability, drill-solids type (clays, quartz) and concentration, and the type of cleanup fluid have been investigated. Flow initiation pressure and retained permeabilities during flow back are reported as a function of these parameters. The experimental results show that the flow initiation pressure is a strong function of gravel size and the type of drill solids. It is concluded that, in clean (low-to-no clay content) formations of large grains and high permeabilities (~ several darcies) requiring large gravel sizes (e.g., 12/20), an enzyme or an oxidizer treatment is sufficient based on laboratory results and productivity predictions. This conclusion is also supported by several field applications as shown. In lower permeability (~ 100–250 md) formations of small sand sizes requiring smaller gravel (e.g., 40/60) elimination of both the fluid loss control agent (starch) and bridging agent (CaCO3) is necessary based on high flow initiation pressures and low retained permeabilities. In intermediate permeability (~ 500–800 md) formations of medium size sand-grains typically requiring 20/40 gravel, the results depend strongly on the type of drill solids: in clean formations (no clays in drilling fluid), an enzyme or an oxidizer treatment is sufficient, while in dirty formations removal of both CaCO3 and starch is necessary. These results are also supported by field case histories presented in the paper. Introduction Gravel packing has been gaining wider popularity in open-hole horizontal completions where sand control is required, particularly in sub-sea completion and/or deep-water environment. The cost of intervention in such cases makes risk mitigation a much more pronounced task. Until recently, a large majority of horizontal sand control completions have utilized standalone screens. However, because a substantial fraction of these wells have failed prematurely (either productivity loss due to screen plugging or loss of sand control due to screen erosion),1 many operators have changed their primary completion technique in these wells from standalone screens to gravel packing. This is particularly true in formations containing a large fraction of non-pay (shale, mudstone/siltstone) and/or have a wide particle size distribution.2

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In order to improve drilling mud design to cater for specific well situations, a more comprehensive knowledge and understanding of filter cake failure is needed. This paper describes experimental techniques aimed at directly probing the mechanical properties of filter cakes, without having to take into account artefacts due to fluid flow in the substrate. The use of rheometers allows us to determine shear yield stress and dynamic shear modulii of cakes grown on filter paper. A new scraping technique measures the strength and moisture profiles of typical filter cakes with a 0.1 mm resolution. This technique also allows us to probe the adhesion between the filter cake and its rock substrate. In addition, dometer drained consolidation and unloading of a filter cake give us compression parameters useful for Cam Clay modelling. These independent measurements give similar results as to the elastic modulus of different filter cakes, showing an order of magnitude difference between water based and oil based cakes. We find that these standard cakes behave predominantly as purely elastic materials, with a sharp transition into plastic flow, allowing for the determination of a well-defined yield stress. The effect of solids loading on a given type of mud is also studied. Introduction Filter cakes formed on wellbore walls during the drilling process have, among other roles, the task of protecting the formation from the invasion of damaging drilling fluid. In many wells, the requirement from the operator is to be able to produce the well with the minimum number of interventions and treatments, ideally by just reversing the pressure drop between the wellbore and the formation1. Depending on the amplitude of this pressure drop, the permeability of the reservoir rock and the nature of the drilling mud employed, it has been shown that the filter cake can fail in two very dissimilar fashions. The cake either detaches in large slabs from the rock surface ("liftoff" mode), or it "pinholes", with the bulk of the cake essentially remaining in place but developing many small erosion channels through which the oil can flow. If the cake ruptures by lifting off with no cleanup treatment, there is a risk of potentially clogging the completion put in place (i.e. slotted liner or gravel pack). The challenge is thus to design a drilling fluid which will consolidate into an impermeable filter cake (so as not to damage the formation during the drilling phase), but weak enough to allow back flow of oil under a draw-down reversal of minimal amplitude.

Over the years the industry has seen major changes in drilling fluids technology, especially in the field of organic-phase fluids (OPF) such as diesel, mineral oil and synthetic hydrocarbon base fluids. Environmental concerns have driven the development of ‘traditional’ oil-based fluids away from diesel and through to the less toxic, more biodegradable synthetics such as esters and olefins. Many companies are now considering the overall picture regarding the disposal of wastes and are looking for alternative uses for drilling by products, thus turning wastes into useful raw commodities. While organic-phase fluids evolved, research into water-based fluids (WBF), which are generally considered less harmful to the environment, concentrated on duplicating the technical performance of oil-based fluids, the absence of oily discharges being the environmental benefit. A more holistic view of the overall impact of WBF discharges and concerns about the persistence of some WBF chemicals has now focused development on alternative ways to further reduce the impact of water-based fluids on the environment and accelerate recovery of the impacted areas. The key to reducing the environmental impact of drilling fluids is typified by the standard waste management hierarchy. The areas to consider are source reduction, recycling/re-use of the product, recovery of useful or valuable materials and treatment prior to disposal. The focus of this paper is how new technologies can be used to bring about these changes and to discuss the various ways in which the amount of drilling fluid by-products can be reduced. The paper also describes ways in which new drilling fluid developments such as salt-free drilling fluids or the use of colloidal weighting agents can be designed to optimize waste management and reduce the amount of waste. These technologies also facilitate the re-use and recycling of drilling fluids and their components. Introduction To significantly reduce the environmental impact of drilling operations, the process of drilling a well needs to be viewed holistically and environmental benefits need to be tied to financial savings. For example, which is more advantageous, drilling quickly with an "expensive" fluid that saves four days rig costs and thus reduces overall CO2 emissions or using a "low-cost" fluid which does not perform as well as the expensive one and results in increased rig costs and emissions. It is equally important to ensure that the fluid used to drill the reservoir section maximizes recovery of the available hydrocarbons and reduces the need to drill more wells. This paper uses some of the concepts of life cycle analysis1 to consider the total impact of the various stages of drilling. Factors that should be considered when trying to optimize the overall waste management strategy include total materials used as well as solid, liquid and gas emissions. One of the key points of an effective waste management strategy is the waste management hierarchy. Figure 1 ranks the desirability of each stage of the hierarchy. The best solution is to avoid producing the waste, but if this is unavoidable, then the amount of waste produced should be minimized. Steps should then be taken to maximize the recovery, re-use and recycling of materials before reducing the amount of material for final disposal2. Avoid - Reduce Well Intervention Reduction of the total number of operations/interventions required to extract hydrocarbons will result in a reduction of both the total energy budget and the overall environmental impact. It is also important to recover the maximum amount of hydrocarbons (or energy) from the reservoir. This means that in addition to drilling to the reservoir quickly and efficiently, it is also necessary that drilling practices do not reduce the productivity of the reservoir causing an increase in the overall energy costs.