APPLICATION OF WATER ALATERNATING GAS (WAG) INJECTION  FOR HEAVY OIL RECOVERY IN NIGER DELTA

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ABSTRACT

The world continues to rely heavily on hydrocarbon resources for energy. While the demand for these resources is steadily rising, the discovery of new reserves is becoming more challenging. Therefore new ways of enhancing recovery from matured and producing reservoirs must be found in order to recover more oil from these reservoirs. Recently, there has been greater interest in enhanced oil recovery techniques that can improve overall recovery by increasing both the displacement efficiency and the sweep efficiency.

This study seeks to investigate, at laboratory conditions, the improvement in ultimate oil recovery when immiscible water alternating gas (WAG) injection is use as an enhanced recovery method. Synthetic brine simulating formation water from offshore Niger Delta was prepared and three WAG injection tests each preceded by either water or gas injection were carried out on three Wallace sandstone core plugs in the laboratory. The test runs were performed using the Benchtop Relative Permeameter. The results from the experiment shows that using WAG injection after secondary water or gas injection leads to additional recovery of up to 21% of original oil in place (OOIP).

CHAPTER 1

INTRODUCTION

Substantial quantities of oil normally remain in the reservoir after primary and secondary recovery. A significant portion of this residual oil can be economically recovered through Water

–Alternating-Gas injection (Shahverdi, Sohrabi, & Fatemi, 2013). Water alternating gas injection (WAG) also referred to as combined water and gas injection (CGW) is an enhanced oil recovery (EOR) method where water and gas injection are carried out alternately in a reservoir for a period of time in order to provide both microscopic and macroscopic sweep efficiencies and reduce gas override effect (Mahli & Scrivastava, 2012). The alternate injection of gas and water slugs increases mobility control and stabilizes the displacement front (Stenby, Skauge, & Christensen, 2001). Displacement of oil by gas has better microscopic efficiency than by water and displacing oil by water has better macroscopic sweep efficiency than by gas. So WAG injection improves oil recovery by taking advantage of the increased microscopic displacement of gas injection with the improved macroscopic sweep efficiency of water flooding. Compositional exchanges between the oil and gas during WAG process can also lead to additional recovery (Stenby et al., 2001).

WAG injections are mainly divided into miscible and immiscible processes and the gases used are divided into two types; namely hydrocarbon and non-hydrocarbon gases. The hydrocarbon gases are the paraffins of lower molecular weight (e.g. methane, ethane, propane, and butane) and the non-hydrocarbon gases are carbon dioxide and nitrogen. If the gas injection happens above minimum miscibility pressure (MMP), the process will be miscible WAG and injection of the gas below MMP is called immiscible WAG. Both miscible and immiscible WAG C have been successfully applied with different gases worldwide particularly in USA, Canada,
Russia and North Sea.