Improving Oil Recovery by Use of Carbon Dioxide in the Bakken Unconventional System: A Laboratory Investigation

Abstract

Summary Compared with a conventional reservoir, the ultralow permeability in the Bakken Formation makes it very challenging to perform normal waterflooding or gasflooding operations. “Permeability-jail” effects cause low injectivity and prevent injected fluids from sweeping oil out of the matrix efficiently. Two distinguishable flow regimes have been identified in fractured, hydrocarbon-rich shale formations: viscous flow in high-permeability fracture networks and diffusion-dominated flow in the low-permeability matrix with high oil saturation. Improving hydrocarbon transport (and technically recoverable resources) in unconventional reservoirs relies on our ability to enhance diffusion-dominated flow from the oil-saturated matrix to the natural- or induced-fracture network, which is the focus of this study. To unlock the unproduced Bakken and Three Forks oil, high-pressure carbon dioxide (CO2) may be used to enhance the diffusion-dominated flow in the matrix and keep the viscous flow in the fractures under reservoir temperature and pressure conditions (e.g., 230°F and 5,000 psi). Core samples were collected from two Bakken wells, including all oil-bearing intervals: Upper Bakken (UB), Middle Bakken (MB), and Lower Bakken (LB) Members and the Three Forks (TF) Formation. Detailed core analyses were performed to measure petrophysical properties and characterize these units. Ten samples were selected for pore-size-distribution measurement and 21 samples (11-mm-diameter rods) were used for 24-hour CO2 exposures and hydrocarbon-recovery experiments. These experiments were conducted as CO2 “bathing” at reservoir conditions (rather than “flow through” tests) and were aimed at increasing our understanding of the microstructure and diffusion-dominated-flow ability within these tight geologic formations. CO2-exposure and hydrocarbon-extraction experimental results clearly showed the improvement of diffusion-dominated flow in all the Bakken members. The UB and LB samples, characterized by generally high total-organic-carbon (TOC) content (10–15 wt%) and small pore size (approximately 3–7 nm), yielded approximately 60% of the present mature hydrocarbon at the end of the 24-hour exposure. The MB and TF samples, characterized by lower TOC content (<0.5 wt.%) and moderate pore size (approximately 8–80 nm), provided more-favorable flow conditions for CO2 and hydrocarbons, yielding approximately 90% of the mature-hydrocarbon content. Because all experiments were conducted at reservoir conditions, the results demonstrate that diffusion plays a significant role in the mobilization of oil in tight reservoirs. CO2 greatly enhances the diffusion process to improve hydrocarbon transport in the tight matrix. This observation is especially useful for densely fractured shale-oil formations (high surface-area/volume ratio) where CO2 has greater areal contact with the reservoir, enabling CO2 diffusion into the matrix and hydrocarbon diffusion out of the matrix to occur more efficiently (increasing recoverable reserves), and where the fracture networks assist in alleviating potential injectivity challenges.

Methods for Enhancing Recovery of Heavy Crude Oil

“…The oil transportability in the small pores improves, then the CO 2 can extract oil from the unconventional core samples by diffusion. The result proves that by injecting this gas, it is able to recover up to 99% of oil samples in 24 hours under reservoir condition exposure, and it summarizes that the CO 2 could be injected to highly fractured tight reservoirs through fractures to recover oil [35,36].…”

The integrated feasibility analysis of water reuse management in the petroleum exploration performances of unconventional shale reservoirs

“…• Shale field-1 water reuse In this field, produced water has generally had higher levels of TDS, low amounts of TSS, and moderate scaling tendency; that is to say, that, in this field, the volume of water reused and treated by membrane treatment techniques is relatively 8% of the total amount of water which is used for drilling and hydraulic fracturing operations. However, water reuse treatments play a significant role in production and drilling operations; logistical and economical performances impose specific restrictions in the administration of large volume of water reuse in this field (Jin et al 2017;Mantell 2011).…”

Experimental evaluation of ASP flooding to improve oil recovery in heterogeneous reservoirs by layered injection approach

“…Because CO 2 can be dissolved in oil, which can significantly reduce interfacial tension and oil viscosity, can improve the fluidity ratio of CO 2 flooding, and has good injectivity and displacement, CO 2 flooding is becoming more and more important in the development of low‐permeability reservoirs 22 . Some scholars have studied CO 2 flooding in low‐permeability reservoirs; Jin et al 23,24 carried out experimental and theoretical research on CO 2 flooding of dense Bakken shale. The results show that CO 2 injection has good diffusion capacity and accompanying injection capacity and is an effective method for enhancing oil recovery, which can be used continuously after initial oil production.…”