Abstract

Field engineers and reservoir modelers often question if equilibrium conditions prevail downhole and when do super(sub)saturation conditions exist. This is especially critical in designing seawater injection for pressure maintenance due to serious barite scale problem in barium containing formation. The objective of this paper is

  1. to discuss the experimental research on rock-brine interaction to determine when equilibrium condition prevail and when kinetics prevail; and

  2. to develop realistic seawater/inhibitor injection schemes for scale control during seawater injection.

We have examined the question from three points of view:

  1. thermodynamic equilibrium;

  2. mass transport kinetics; and

  3. experimental testing of flow through columns with or without scale inhibitors.

If the reaction direction is from undersaturated toward saturation, then equilibrium normally occurs rapidly. Equilibrium is observed within about 5 min. When the reaction is going from supersaturated to equilibrium direction, such as during seawater injection into a barium containing formation, the precipitation reaction is often slow, i.e., equilibrium is not observed after 10 min of contact time. Both barite dissolution and precipitation rates on clean core material are consistent with those reported in literature with a second order rate constants for dissolution (˜ 30,172 l2·mol-1·m-2·sec-1, 100 ºC) and precipitation (= 938 l2·mol-1·m-2·sec-1, 100 ºC).

The kinetics of barite formation can significantly slow down in the presence of scale inhibitors and the sulfate tolerance can be increased. The kinetics of both barite dissolution and precipitation are poorly understood at the present time. It is proposed that combining sulfate reduction and scale inhibitor application in intelligent engineering design can significantly reduce the problems and costs associated with seawater injection. Equations for engineering design of such treatment were derived from nucleation kinetics, inhibition efficiency, and inhibitor adsorption and transport. Sulfate tolerance in the presence of scale inhibitors is measured and compared with the prediction from nucleation inhibition theory with excellent agreement. An innovative inhibitor treatment via core flood has been done as proof-of-concept and will be discussed.

Introduction

In oil and gas production, seawater, produced water or fresh water is often injected into the oil bearing formation for pressure maintenance. Due to the high sulfate concentration in seawater, barite scale formation can be a serious problem when seawater is injected into a barite containing formation 1, 2. Typically, the scaling problem is most severe only when the seawater/brine mixing zone is near the wellbore or if seawater breaks through and mixes with incompatible brine from a different zone or different wells at a subsea manifold or due to temperature and pressure changes 3, 4. Many precipitation problems are transient, i.e., the problem can be avoided by delaying the precipitation by a short period with scale inhibitors. Since the formation is vast, it serves as an infinite sink for barite. Therefore, scaling may even be advantageous if brine/seawater mixing occurrs deep in the formation. One may even want to engineer barite stripping deep in the formation to remove sulfate in-situ, only if such process poses little to no risk. In order to better manage the seawater injection issues, an understanding of the kinetics of barite precipitation and dissolution under realistic oilfield condition is important.

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