Integrating Petrographic and Petrophysical Analyses with CO2 Permeation and Oil Extraction and Recovery in the Bakken Tight Oil Formation

Abstract

Over 40 rock samples were obtained from six Bakken wells which penetrate through the major oil pay including two shale intervals: Upper and Lower Bakken, and two tight intervals that are the targets for drilling: Middle Bakken and Three Forks. Detailed petrographic and petrophysical analyses were performed on the samples to better correlate the extraction results with the physical and geochemical properties of the rocks. Round rods (11.2-mm diameter X ca. 30–40 mm long) drilled from each of the 40 samples were individually exposed in a "bath" of CO2 for 24 hours at reservoir temperature and pressure of 5000 psi and 230°F (34.5 MPa, 110°C), and the recovered crude oil hydrocarbons were collected periodically and analyzed to determine the rates and efficiencies of oil recovery. For the 26 Middle Bakken and Three Forks rocks, hydrocarbon recovery upon CO2 exposure averaged 86% after 7 hours, and 99% after 24 hours. Recoveries of the crude oil (not including kerogen) from the 15 Upper and Lower shales were surprisingly high with an average of 30% recovered after 7 hours, and 50% recovered after 24 hours. While the Middle Bakken and Three Forks TOC values were ca. 0.3 wt.% (similar to their crude oil content), TOCs for the Upper and Lower Bakken shales were typically 10 to 15 wt.%, with ca. one-tenth of that organic content being crude oil hydrocarbons as opposed to kerogen. The Upper and Lower shales also had significantly smaller pore throat sizes (averaging ca. 3 nm) than the Middle Bakken and Three Forks samples (which averaged ca. 10–26 nm). Additional studies are being performed to determine whether the small pore throat sizes (which approach molecular dimensions) and/or the sorption of crude oil hydrocarbons onto the kerogen in the Upper and Lower shales are responsible for the slower hydrocarbon recovery than that achieved from the Middle Bakken and Three Forks rocks under CO2 exposure. Currently, the main targets for horizontal drilling are Middle Bakken and Three Forks, where thousands of multistage hydraulically fractured wells have been drilled in the past decade. The high oil recovery factor observed in cores from these intervals, especially when compared to the 7% average recovery in the field, indicates the huge potential for oil recovery factor improvement in these units by increasing oil production based upon supercritical CO2 extraction.

A Literature Review of CO2, Natural Gas, and Water-Based Fluids for Enhanced Oil Recovery in Unconventional Reservoirs

“…Many articles and conference proceedings have emerged in recent years that report investigations regarding CO 2 extraction of hydrocarbons from tight and shale cores. Experiments are designed to identify the effects of injection pressure, ,,,,, rock lithology, ,,− soak time and number of huff-n-puff cycles, ,, water saturation, ,, and asphaltene deposition ,,− on oil recovery. Table lists all reports of CO 2 core flooding experiments that may be relevant to CO 2 EOR in unconventional reservoirs.…”

“…In this experiment, 80% of hydrocarbons were recovered in only 2–4 h. This result suggests that the amount of exposed surface area of the rock is critical for hydrocarbon recovery, especially for shale. , The size of the cores used for CO 2 core flooding experiments also affects the amount of surface area exposed to CO 2 , which in turn effects oil recovery. Figure shows a compilation of oil recovery results after 24 h of CO 2 huff-n-puff laboratory testing at near-miscible and miscible pressure conditions. − ,,,, The y -axis represents the percent of oil recovered from the sample, and the x -axis represents the ratio of the volume ( V ) of the piece of shale to the surface area ( A ) exposed to CO 2 during the “huff” phase ( V / A ). If the core is confined by a sleeve, then A is the area of one circular end of the core.…”

“…Compilation of oil recovery results after 24 h of CO 2 huff-n-puff laboratory testing at near-miscible and miscible pressure conditions for Bakken, Mancos, Eagle Ford, and other unspecified shale. − ,,,, Oil recovery (%) is plotted versus core volume/core surface area of shale exposed to CO 2 ( V / A ). Percent oil recovery was compiled at 24 h, regardless of cycle length and number of cycles (e.g., one 24 h cycle or six 4 h cycles).…”

“…The mechanism of oil recovery in unconventional reservoirs has been the subject of many experiments and simulations. Here, we introduce a proposed mechanism of miscible gas EOR in fractured ULRs as a summary of results of several research groups. ,− In the mechanism described here, CO 2 is used as an example of a miscible gas, but other miscible gases such as natural gas or ethane would operate by similar mechanisms. The mechanism of CO 2 EOR in fractured shale reservoirs can be divided into four steps (Figure , bottom):…”

Measurement of CO2 diffusion coefficients in both bulk liquids and carven filling porous media of fractured-vuggy carbonate reservoirs at 50 MPa and 393 K

“…Also, later ow back towards the production well occurs through fracture. 47 The effective CO 2 diffusion coefficient in both siltstone and fractured siltstone are compared in Fig. 7.…”