Matching reservoir characteristics and completion technology with optimum drilling fluid formulation is important to the economics of any field development. In recent years, this need has become even more important with the growing application of open hole completions in high angle or horizontal wellbores that are now commonly drilled and completed with the same drill-in fluid.
Openhole completions place several demands on the drilling fluids used in the payzone section. In order to maximise benefits, the properties of the fluid need to be optimised for the conditions prevailing in the reservoir. The drill-in fluid needs a rheology good enough to enable good hole cleaning, lubricity good enough to avoid frictional problems, good inhibition with respect to interstitial clays and interceded shales, and, very importantly, to be minimally damaging to the permeability of the formation. Other factors to be considered include completion design and proposed payzone clean up procedure.
The number of potential variables involved means it is very difficult to design a single fluid to cover all eventualities, but a range of alternative drill-in systems has been developed in which each fluid is designed to cover a limited set of circumstances. Most applications can be covered by at least one of the fluids. This paper will discuss four generic drill-in systems, each of which embodies a different approach to achieving the desired properties:
water-based polymer systems which may be based on brines of varying concentration and which may contain water or acid soluble bridging particles
all-oil systems, covering a wide density range
aqueous system based on a complex formed between bentonite and mixed metal silicate.
solids-free water-based polymer system
Formulations, properties and some examples of field performance are presented together with a discussion of relative advantages and disadvantages of each system type. The objectives of each fluid type are reviewed against actual field experience. Factors influencing selection of fluid type are discussed and guidelines are presented.
During the last decade the practice of completing wells with an openhole deviated or horizontal section has become established as a means of achieving improved productivity. Indeed the better reservoir draining and productivity possible from horizontal wells has had a major impact on the economics of some developments. In some cases reduction in the number of platforms required for offshore fields has been possible and fields have been developed which would not have been viable using other currently available techniques.
In parallel with the increased use of openhole technology has been the development of specialized drill-in fluids. This has followed from a recognition that the fluid used to drill the upper hole may not be appropriate for the payzone section; that for the reservoir a fluid should be devised which meets the specific characteristics of the reservoir and will be minimally damaging to the permeability. This last requirement is particularly important as in openhole completions there is no perforation step to enable communication to be established through any near well bore permeability damage.
There are other advantages besides limitation of permeability damage to be obtained from changing to a specific reservoir drill-in fluid. It means that specific aspects of the payzone, such as stabilisation of clays and questions of compatibility with the connate water, can be addressed directly. It is not necessary to compromise with the different requirements needed for the upper hole fluid.
The main factors which are important for drill in fluids can be listed as follows: