Elsevier

Fuel

Volume 103, January 2013, Pages 905-914
Fuel

Simultaneous oil recovery and residual gas storage: A pore-level analysis using in situ X-ray micro-tomography

https://doi.org/10.1016/j.fuel.2012.06.094Get rights and content

Abstract

We imaged sandstone cores at residual gas saturation (Sgr) with synchrotron radiation at a nominal resolution of (9 μm)3. We studied two three-phase flooding sequences: (1) gas injection into a core containing oil and initial water followed by a waterflood (gw process); (2) gas injection into a waterflooded core followed by another waterflood (wgw process). In the gw flood we measured a significantly higher Sgr (=20.6%; Sgr in the wgw flood was 5.3%) and a significantly lower residual oil saturation (Sor; Sor in the gw flood was 21.6% and Sor in the wgw flood was 29.3%). We also studied the size distribution of individual trapped clusters in the pore space. We found an approximately power-law distribution N  sτ with an exponent τ = 2.02–2.03 for the residual oil clusters and τ = 2.04 for the gas clusters in the gw flood. τ (=2.32) estimated for the gas clusters in the wgw process was significantly different. Furthermore, we calculated the surface area A–volume V relationships for the clusters. Again an approximate power-law relationship was observed, A  Vp with p  0.75. Moreover, in the gw flood sequence we identified oil layers sandwiched between the gas and water phases; we did not identify such oil layers in the wgw flood. These results have several important implications for oil recovery, carbon geo-sequestration and contaminant transport: (a) significantly more oil can be produced and much more gas can be stored using a gw flood; (b) cluster size distributions for residual oil or gas clusters in three-phase flow are similar to those observed in analogue two-phase flow; (c) there is a large cluster surface area available for dissolution of the residual phase into an aqueous phase; however, this surface area is significantly smaller than predicted by percolation theory (p  1), which implies that CO2 dissolution trapping and contamination of aquifers by hazardous organic solvents is slower than expected because of reduced interfacial contact areas.

Highlights

• Waterflooding a reservoir prior to gas injection leads to significantly lower oil recovery. • More oil can be produced by directly injecting gas into a virgin oil reservoir. • Waterflooding a reservoir prior to gas injection leads to significantly lower gas storage capacities. • More residual gas can be stored by directly injecting gas into an oil reservoir. • Residual oil and gas clusters have a large cluster size distribution and surface area.

Introduction

With a growing global population and fast economic development coupled with dwindling fossil fuel resources – and the fact that world energy consumption is currently mainly based on fossil fuels (they account for more than 80% of the total world’s energy consumption [1]) – it is important to develop advanced technologies that can recover additional fossil fuel. Another challenge concerns the carbon dioxide (CO2) emissions associated with burning fossil fuels and the changes to global climate that may result. One technology to deal with this problem is CCS – Carbon Capture and Storage – where CO2 is collected from fossil-fuel burning power stations and other industrial sites, transported and injected deep underground into saline aquifers or depleted oil or gas fields [2].

Crude oil is the most important fuel; in 2008 it contributed 41.6% (equivalent to an energy of 3505 Mtoe) to the world’s total final consumption [1]. Crude oil, which is not produced by primary production or natural drive mechanisms such as solution gas drive, water influx or gravity drainage, can be produced by enhanced oil recovery (EOR) methods [3]. EOR processes include miscible or partially miscible gas flooding, thermal stimulation [3], surfactant flooding [4] or polymer flooding [5]. Gas injection EOR (GEOR, with natural gas, carbon dioxide CO2, or nitrogen) is usually employed to displace and recover residual oil that remains in the reservoir after natural depletion and waterflooding.

In a GEOR process three fluid phases flow: oil, gas and brine; three-phase flow also occurs in carbon geo-sequestration (CCS) in depleted oil or gas reservoirs [6], [7]. CCS can be combined with GEOR. The objective is to simultaneously maximise CO2 storage and hydrocarbon recovery [8]. Gas is injected either as a secondary process, into oil and initial water, or as a tertiary process into residual oil and water after waterflooding. For carbon dioxide storage it is valuable to trap the CO2 as a residual phase, and so both gas injection sequences can be followed by further waterflooding. We will compare these two processes in this paper.

Several researchers have investigated three-phase flow at the meso (centimetre) scale, mainly with the focus on oil recovery [9], [10], fluid distributions [11], relative permeability [12], [13], [14], [15], or capillary pressure measurements [16]. Pore-scale displacement studies have also been conducted [10], [17], [18], [19], [20], [21], [22], [23]. These pore-scale studies employed 2D models, which are, however, not necessarily representative of reservoir flow conditions as the connectivity of the pore network cannot be captured correctly (for example the percolation threshold for 3D lattices is significantly lower than for 2D lattices [24]). In addition such 2D models typically use strongly simplified artificial materials – not reservoir rock – which may not be representative of reservoir conditions. Furthermore, three-phase trapping has been measured in rock samples [25], [26], [27], which is important for CCS risk and capacity assessments and related residual trapping capacity predictions [28].

To optimise GEOR, reservoir flow models are required that can predict the efficiency of oil recovery and associated time scales. However, because of the complexities of rock pore morphology, fluid–fluid and fluid–solid interactions, theoretical understanding is currently limited to simple models which only have limited predictive capabilities with scant physical foundation, based on pore-scale displacement processes. To overcome this, we analyse three-phase flow (oil, brine, gas) in a sandstone at the pore-scale (micrometre scale) in 3D with micro-computed tomography (μ-CT), and we compare two GEOR flooding sequences.

Section snippets

Experimental methodology

We compared two GEOR flooding sequences:

  • (1)

    gas flood of a virgin oil reservoir; gas was directly injected into a core at connate water saturation (Swc) followed by a chase brine injection (gw sequence), and

  • (2)

    gas flood of a waterflooded oil reservoir; where gas was injected into a waterflooded core at residual oil saturation (Sor) followed by chase brine injection (wgw sequence).

For these experiments we selected a clean, well-sorted relatively homogenous sandstone outcrop (Clashach, a quarried

Fluid saturations – residual oil and gas saturations

Fluid phase saturations measured from the μ-CT images are listed in Table 2 for both flooding sequences. In addition several meso-scale literature values are added for comparison. It is clear from our datasets that much more gas can be stored and more incremental oil can be produced if gas is directly injected into a virgin oil reservoir (gw flood sequence).

Furthermore, we have previously studied similar two-phase (brine and oil) flow processes at the micro- and meso-scale [30], [37], [38], [39]

Conclusions

We have investigated the efficiency of two different three-phase flood sequences at the micrometre pore-scale level with micro-computed tomography in terms of their oil recovery and gas storage potentials. Our results demonstrate that significantly more oil can be produced by directly gas flooding a virgin oil reservoir (gw process, Sor = 21.6%) as compared to gas flooding a waterflooded oil reservoir (wgw process, Sor = 29.3%) under non-spreading conditions. In addition, our results indicate that

Acknowledgements

We would like to acknowledge our sponsor Shell under the Shell Grand Challenge on Clean Fossil Fuels and the Elettra Synchrotron Light Source Facility in Trieste, Italy, for providing beamtime and technical support.

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