Measured Crude Oil MMPs with Pure and Mixed CO2, Methane, and Ethane, and Their Relevance to Enhanced Oil Recovery from Middle Bakken and Bakken Shales

Abstract

Enhanced oil recovery (EOR) processes using CO2 in tight unconventional plays like the Bakken Formation are expected to be very different from the processes which control EOR in conventional reservoirs. During CO2 EOR in conventional reservoirs, CO2 flows through the permeable rock, and the minimum miscibility pressure (MMP) is an important operational parameter for achieving a successful "miscible" flood. In contrast, in tight fractured systems like the Bakken, CO2 flow may be dominated by fracture flow, and not by CO2 flowing through the rock matrix as in a conventional reservoir flood. Since fracture-dominated CO2 flow could essentially eliminate the "flushing" mechanisms responsible for increased recovery in conventional reservoirs, operation at or slightly above MMP may or may not be relevant for the success of an EOR flood in such tight fractured reservoirs. To investigate this concept, capillary-rise vanishing interfacial tension (VIT) was used to measure MMP values for a typical Bakken crude oil (API gravity 41.5) with CO2, methane, and ethane at 110°C (230°F) typical for the reservoir. The effect of these different fluids, as well as the effect of pressures at, above, and well above MMP on recovering crude oil hydrocarbons was determined for small rock core samples from the productive Middle Bakken laminated zone as well as from Upper and Lower Bakken samples. Compared to the MMP value for CO2 of 2520 psi, MMP with methane nearly doubled at 4510 psi, but was nearly cut in 1/2 for ethane at 1360 psi. The recovery of crude oil hydrocarbons from both the Middle Bakken and Lower Bakken shale samples with 24-hour exposures to these fluids at reservoir pressures showed efficiencies that parallel the MMPs determined with each fluid; i.e., ethane yielded faster and more efficient recovery of the crude oil than CO2, but both CO2 and ethane were much more efficient than methane at recovering the crude oil from the 11-mm round rod rock samples. Although hydrocarbon recoveries from the rock samples paralleled each injectant's respective MMP values, extractions with CO2 at the MMP, and at ca. double and triple the MMP pressure showed much more efficient crude oil recoveries at higher pressures from both the Middle Bakken and Lower Bakken shale, demonstrating that EOR pressures much higher than the MMP could substantially increase oil recoveries in tight unconventional systems like the Bakken.

Molecular Simulation Study on the Volume Swelling and the Viscosity Reduction of n-Alkane/CO2 Systems

“…While the minimum miscibility pressure (MMP) between Bakken crude oil and CO 2 as well as CO 2 flow in tight Bakken rocks were studied, ,, more work needs to be done on the solubility of CO 2 in Bakken crude oil, the swelling factors of the crude oil/CO 2 system, and the crude oil viscosity reduction. In this study, five main n -alkanes (octane, decane, dodecane, tetradecane, and hexadecane) in Middle Bakken crude oil were selected, and a series of configurational-bias Monte Carlo (CBMC) , simulations were performed to study the impacts of pressure, temperature, and carbon chain length on the solubility of CO 2 in different n -alkanes and the swelling of n -alkane/CO 2 systems.…”

Experimental and computational evaluation of cyclic solvent injection in fractured tight hydrocarbon reservoirs

“…The previous laboratory studies represent a step change in the thinking around experimental evaluation of corebased HNP process through suggesting alternative flow geometries 7,20 , use of large-scale samples/fractures 7 and combining 'outside-of-coreholder' drilling and advanced core-based hydraulic fracturing under stress inside the coreholder to induce fractures 8 . However, the current work is unique due to (1) the proposed methodology used for creating fractures under stress and (2) its integrated experimental and simulation approach in which a compositional model is calibrated to laboratory data (porosity, matrix/relative permeability, etc.).…”

“…The majority of previous laboratory studies have been performed on core plug samples with permeability values down to the microdarcy/millidarcy range 12,13 until now, very few laboratory-based studies 8,14-17 have been conducted on tight hydrocarbon systems with permeability values down to the nanodarcy range (a common permeability range for unconventional hydrocarbon reservoirs). Further, the laboratory methodologies are primarily focused on intact ('unfractured') core plug samples using either the 'flow-through-matrix' 12,18,19 or 'flow-around-matrix' 1,20,21 schemes, which do not realistically represent the fracture-matrix contact or sequences associated with a typical cyclic solvent injection (HNP) process. While seminal works have been conducted to mimic HNP process on fractured cores 7,8 , the fractures were created outside of the coreholder either fully using saw 7 or partially using drilling bits 8 (combined with hydraulic fracturing under stress inside coreholder).…”

Huff-n-Puff Test For Minimum Miscibility Pressure Determination For Heavy Oil

“…But in the most cases, researches assume that heavy oil is not miscible (Cuthiell et al 2006;Quoc et al 2017) because MMP in heavy oil reservoir is usually much higher than reservoir pressure. Since carbon dioxide (CO2) gas tends to be almost the best miscible gas (Hawthorne et al 2017), and it has the most interest in scientific studies, field implementation. This paper presents some problems correlated to unsuccessful CO2 MMP determination of heavy oil by slim tube test (STT), then description of novel solution for MMP determination via Huff-n-Puff test for heavy oil, intermediate results of Huff-n-Puff Test and MMP calculation via MMP correlation for heavy oil for future comparison with experimental results.…”