Alkali Surfactant Polymer Flooding Using Ammonia for Offshore use
Authors: van den Pol Esther , T. van Rijn Carl H. , van Batenburg Diederik W. , Southwick Jeffrey G. , Mastan Ahmad A. , Abdul Manap Arif A. , Razak Sitimazura A. 
Published: May 2016

One of the issues when applying alkaline surfactant polymer (ASP) flooding offshore is the large amount of chemicals needed. Typically over 50% of the weight of the chemicals is the alkali, normally sodium carbonate. The possibility of replacing sodium carbonate by ammonia has been studied in this paper. For offshore use or in remote locations, ammonia has potential advantages since 6 times less mass is needed to provide the same alkalinity and it can be delivered as a liquid.

It was found that ammonia can perform well together with a surfactant blend of alkyl propoxy sulfate and internal olefin sulfonate (APS/IOS). In dynamic adsorption tests, reduced surfactant retention was observed compared to using no alkali. Without calcium present in solution, the surfactant retention was similar to sodium carbonate. However, because of ion exchange from clays calcium will be present in solution. With increasing calcium concentration the surfactant retention was found to increase because ammonia does not precipitate calcium ions. This behavior was, at least partly, explained by zeta potential measurements performed on crushed rock in ammonia solutions with different calcium concentrations. The zeta potential was observed to become less negative when the calcium concentration increased; therefore the repulsive forces between the negatively charge rock and surfactant reduce.

Even though calcium was present, in an oil displacement coreflow test similar performance was observed for ammonia and sodium carbonate indicating a good potential for ammonia. In this case the calcium tolerant APS/IOS surfactant system was used. For higher temperature reservoirs often sulfonate surfactants are used that are intolerant to calcium. Indications were found that in that case a hybrid system could work with a small amount of sodium carbonate added to the ammonia.

Introduction

In chemical enhanced oil recovery, alkali surfactant polymer (ASP) flooding is often used because of reduced surfactant retention compared to surfactant polymer (SP) flooding (Nelson et al. 1984, Solairaj et al. 2012, Hirasaki and Zhang 2004, Bae and Petrick 1977). Sodium carbonate is often used as the alkali since it combines low cost, buffered alkalinity and precipitation of divalent ions. However, for offshore use, space and weight are limiting factors and the quantity of sodium carbonate needed for a full field scale chemical flood will present logistical challenges. In that case, anhydrous liquid ammonia might be a better candidate for the alkali, since ammonia has a molecular weight which is 6 times lower than sodium carbonate (Southwick et al. 2014). This makes it preferred from a logistical point of view because 6 times less mass is needed to produce the same number of moles of alkalinity. Moreover, it is a liquid (at a modest pressure of 15 bar, or below the boiling point of -33oC) which can be pumped from support vessels and also directly diluted into an injection stream with minimal mixing.

Since ammonia does not precipitate calcium ions from solution, like sodium carbonate does, it should be used with calcium tolerant surfactants. Another solution is using a hybrid alkali with sodium carbonate. The amount of calcium present in solution depends on the water compositions but also on the rock properties. Because of ion exchange, rocks with a high cation exchange capacity (CEC) cause a large amount of calcium to be present in solution after injecting an ASP solution.