Abstract

Experience of sand production from the Statfjord field has been systemized to be able to predict the effect of drawdown and depletion on sand production. The aim of the study is to determine how reservoir pressure depletion will influence the sand production and to determine how sand production will influence the maximum liquid production rates. Most wells produce according to a maximum acceptable sand production criterion. Field data based on more than 300 maximum acceptable sand rate tests have been systemized by fitting those to an empirical criterion. For a better determination of the parameters, sand production histories of four wells have been analyzed in detail.

To understand more about the mechanisms underlying the sand production, the field results are compared with four different models. This work concludes that the behaviour of sanding wells at Statfjord can be described by a concept of two sand production regimes, one controlled by shear failure and the other controlled by rate.

Finally the empirical model is used to extrapolate the field experience to low reservoir pressures related to the planned deep depressurization. The results show that sand production will limit well production rates unacceptably for most formations and that downhole sand control is needed to secure high production rates during the late-life development of the field.

Introduction

The Statfjord field is a large oil field located in the Northern part of the Viking Graben and straddles the border between the Norwegian and UK sectors. The field has been in production since 1979 and has utilized both water flooding and gas injection for secondary recovery. To date over 65% of the STOOIP has been recovered. The producing reservoirs (Brent and Statfjord) consist of weak to intermediate strength sandstone. Some historical data is summarized in Fig. 1. The operator has for many years followed a strategy whereby sand produced with the wellstreams has been handled topside prior to on-site disposal. Producers have therefore been completed naturally with a perforated liner and many of these have been producing sand in a controllable manner from the moment they came on stream. For late life development of the field, a deep depressurization is planned to produce solution gas[1]. The aim of this study is to determine how the planned reservoir pressure depletion will influence sand production and consequently field production.

Sand management has been an important part of the production strategy on Statfjord for more than a decade. The yearly sand production on each platform is estimated to be 50 to 100 tons[2]. At present, most producing wells are restricted by an acceptable sand rate production criterion. Typically wells are production tested about once a month for allocation purposes. During the acceptable sand rate (ASR) test the production rate is determined at which no more than 0.015 liter of sand is produced in the sand trap during two hours of testing. This test is called a MSFR (maximum sand-free rate) test at Statfjord, but since some low level of sand is actually permitted, we call it an acceptable sand rate (ASR) test in this paper. This criterion is based partly on the measured efficiency of the sand trap and historical reasons. Since many producers at the Statfjord field are operated by an ASR criterion, the field production is strongly related to sand production. If the planned reservoir depressurization were to lead to significantly more sand production, then this would have unacceptable consequences for the late-life production of the field.

The main objective of this study is to determine how reservoir pressure depletion will influence the sand production and to determine how sand production will influence the maximum liquid production rates. Field measurements are used as much as possible, but mechanistic models are required in order to be able to extrapolate the field experience towards deep depressurization. This is a challenge since sand production can be regarded as a three-step process:

  1. failure of the rock matrix,

  2. erosion of the failed material and

  3. transport by the fluid flow through the well.

These steps are controlled by cohesion breakdown, drag forces and well hydraulics, respectively.

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