Sanding: A Rigorous Examination of the Interplay Between Drawdown, Depletion, Start-Up Frequency and Water Cut
Authors: Vaziri Hans H. , Allam Robert D. , Kidd Gordon A. , Bennett Clive L. , Grose Trevor D. , Robinson Peter A. , Malyn Jeremy 
Published: December 2007

Factors and mechanisms leading to sanding are described within an integrated-rock and soil-mechanics framework. While the conventional sanding models generally consider a single-mechanism for sanding, namely the critical depletion resulting in rock disaggregation, the proposed approach considers the interplay of several mechanisms that can lead to the rock breakup and sand transport. One important difference is that rock disaggregation is not seen to represent the onset of sanding, because the sand mass can offer significant resistance from frictional properties, interlocking of sand grains, and arching. The approach presented here can be used to explain why sanding in the field tends to be episodic, and how depletion, which is a major factor in rock breakup, can be highly effective in holding broken-up sand grains together and, in fact, become a sand-stabilizing agent.

The proposed approach is used in discussing sanding at several wells in two different fields. These wells have been in production for several years and show that sanding cannot be linked to just one unique mechanism (e.g., depletion). However, once all mechanisms for sanding are incorporated, a more consistent analysis can be used by completion and production engineers to make more objective and pragmatic decisions in managing sanding while maximizing production over the life of the well.

Introduction

While a great deal of work has been done in the general area of sand production (Veeken et al. 1991; Weingarten and Perkins 1995; Risnes et al. 1982; Morita 1994; Sanfilippo et al. 1995, 1997; Tronvoll and Halleck 1994; Tronvoll et al. 1997; Papamichos and Malmanger 1999; Morita and Boyd 1991; Bradford and Cook 1994; Van den Hoek et al. 1996; Vardoulakis and Papanastasious 1988; Willson 1996; Morita et al. 1996), most of the approaches used for practical applications are on based on the assumption that the onset of sand production is represented by the failure of the perforation-tunnel wall, which is generally determined using the thick-wall-cylinder (TWC) strength test. Such an approach is well suited to predicting the maximum depletion in relatively competent rocks, particularly if they have brittle behavior. But what about weak-to-totally unconsolidated rocks having an almost-zero TWC strength, yet remaining stable under reasonably high drawdown (DD) and, in fact, showing an increase in stability with depletion (field examples presented later)? How should the DD strategy in terms of rate of change and magnitude be adjusted as the rock undergoes a structural change from a cemented formation to a totally disaggregated sand mass?

Strictly speaking, the conventional techniques for sanding prediction, which are based on Geertsma's (1985) equations, disclose the increase in confining pressure required to fail a perforation tunnel or the wellbore cavity. In practice, the approach provides an indication of the depletion that can be sustained before the weakest perforation tunnel undergoes a significant deformation, leading to its disaggregation (normally referred to as the critical bottomhole reservoir pressure). This single-case-solution scenario, which is not coupled with fluid flow, does not provide options to make objective assessment of the risks at different stages in the well's life and the most effective contingencies to mitigate such risks [see Vaziri et al. (2002a) for a full discussion of the past work in this area, formulations used, and some of the limitations). For the base case of no active sand control, operators would like to know, at any stage, how much sand will be produced (rate and duration) for a given production strategy (e.g., maximum DD, and rate of bean-up/shutdown frequency) and other changes in the reservoir conditions, such as water cut (WC). By better understanding the roles of multiple variables, one is enabled to choose the optimal completion method over the life of the well. For a more comprehensive discussion of these issues see Vaziri (2004).