Summary
Many of the problems associated with the use of water-based fluids in drilling and completion operations are caused by incompatibilities between the fluids and the shales. Such incompatibilities may result in washouts, increased drilling costs (e.g., solids handling, rig time, dilution fluids), and shale sloughing during the drilling operation and after displacements to completion fluids or during gravel packing. One of the most important factors leading to an undesired result (either a premature screenout, thus a potential sand-control failure, or a higher skin) in water-packing of open holes is the presence of reactive shales in the interval to be gravel packed.
Although there is a substantial amount of literature on shale inhibition with water-based drilling fluids, the importance of shale inhibition and the problems associated with shale reactivity during gravel packing remain largely unexplored. Furthermore, shale-inhibitor selection often relies on a comparison of the results from bottle-roll tests using shale samples in candidate fluid/inhibitor pairs (drilling or completion fluid) and on tests measuring the degree of shale swelling. While these tests are highly functional, they can provide information only on the relative performance of fluids, and their relevance to gravel packing is questionable because these tests do not simulate the conditions experienced during such treatments.
This paper presents guidelines on selection methodology of shale inhibitors for use in gravel-packing applications on the basis of the data available in our respective companies, including a comparison of results from conventional bottle-roll tests to those from flow through predrilled holes in shale core samples. Recommendations are made depending on brine type and density, type of shale, temperature, fluid exposure history, and environmental considerations.
Introduction
Openhole-horizontal completions have emerged as a cost-effective means of exploiting deepwater reservoirs, many of which require sand control. Gravel packing is the preferred sand-control technique for such environments where remedial treatment costs are prohibitively high (Price-Smith et al. 2003). Two techniques have been employed for gravel packing open holes with varying degrees of success: alternative path and water packing. The focus of this paper will be to address one of the problems considered to be a key risk factor in successful implementation of water-pack treatments.
The risks associated with openhole water packing completions can be summarized asSwabbing, which has been addressed through the development of antiswab-tool systems (Vozniak et al. 2001)Exceeding fracturing pressure--during the beta-wave, which has been addressed with the development of beta wave attenuators (Coronado and Corbett 2001) or use of low-density gravel, allowing lower pump rates without the concern for gravel settling in the work string (Pedroso et al. 2005)--during the alpha wave in environments with narrow-frac window, that in some cases may be addressed through the use of low-density gravel (Pedroso et al. 2005)Filter-cake erosion, (the conditions under which this becomes a risk remain to be determined) (Gilchrist et al. 1998)Reactive shales that may either collapse/slough or disperse in the carrier fluid; the former may lead to a premature screenout because of blockage of the annulus, and the latter may result in a low-permeability gravel pack because of shale and gravel intermixing (Gilchrist et al. 1998; Corbett and Winton 2002; Mathis et al. 2000; Murray et al. 2003)
Shales are characterized by high clay content, low quartz content, and low permeability (a byproduct of the small-clay size). On the basis of numerous factors, shale can react catastrophically when exposed to some aqueous fluids. These factors include downhole-stress states, native-fluid composition, mineralogical composition, and interaction with the completion-fluid chemistry and properties. It is important to note that these factors also determine the time a shale will take to fail when exposed to a given completion fluid, and hence, a shale that has survived the drilling process may still fail during the post drilling activities leading to the gravel pack (Dickerson et al. 2003).
It is possible to minimize and even eliminate this adverse reaction by selecting a suitable completion fluid. This selection may involve choosing the correct brine type and additives to increase the inhibitive qualities of the completion fluid.
The literature on the subject of shale compatibility with muds is vast. The reactivity of shales to aqueous muds with various additives has been well studied (Chenevert 1970; O'Brien and Chenevert 1973; van Oort 1997). However, the effect of completion fluids has not been studied extensively. The purpose of ensuring proper shale inhibition with drilling mud is to address shale reactivity concerns such as cuttings disintegration, wellbore instability during drilling, and bit balling (van Oort 1997). On the other hand, a completion fluid must be formulated to inhibit shale to maintain wellbore stability after drilling (e.g., during mud displacements or gravel packing) in reactive-shale sections (Gilchrist et al. 1998; Mathis et al. 2000) and to prevent erosion of weakened shales during gravel packing (Ali et al. 1999).
Various testing techniques have been proposed in the literature to characterize the inhibitive properties of drilling fluids (Roehl and Hackett 1982; RP 131 2004; Bailey et al. 1994; Mondshine 1973). Because these tests were designed specifically for drilling applications, their direct applicability to water packing, subsequent to water-based drilling, is questionable. Of these testing techniques, the wellbore-simulator tests first described by Darley (1969) and further developed by Bailey (1994) and Gaylord (1983) are more useful for evaluating inhibitor effectiveness in gravel-pack applications, as is also suggested by Corbett and Winton (2002).
By exposing various fluids to boreholes drilled in shale cores, Darley (1969) showed the different modes of failure and correlated them to the effect of tectonic stresses, mineral content, age of shales, and flow of mud through the shale borehole. Gaylord developed this testing to look further at the effect of fluid-mechanical parameters on borehole erosion and concluded that erosion will be most pronounced if the particular shale/fluid system is reactive. In addition, hole erosion increases with increasing shear stress and is exacerbated under turbulent conditions. The tests done by Bailey (Price-Smith et al. 2003) look only at the effects of reactivity by shale but corroborate the mechanism of weakening of the shale and subsequent erosion by flow. This is evident in their tests through increased wellbore diameter resulting from erosion. On the basis of this information, a similar test will be used in this work to evaluate completion-brine inhibition.
It is the objective of this paper to provide guidelines on selection methodology of shale inhibitors for use in gravel-packing applications. The paper is organized as follows. First, a brief description of the typical critical stages in a gravel-packed completion is given. This is followed by a discussion of the current testing methodology typically employed in the industry. Next, the experimental techniques and materials used in this study are presented, followed by the results from hot-roll and drilled-core experiments. Finally, conclusions are drawn.
