Abstract
To date, commercial production from light oil, organic-rich shales in the Permian Basin has largely come from a solution-gas drive recovery mechanism as a result of horizontal drilling and multi-stage hydraulic fracturing. These onshore, capital-intensive developments feature steep production declines and low expected ultimate recoveries. This study involved laboratory experiments introducing miscible gases into core samples to investigate Enhanced Oil Recovery (EOR) mechanisms for Permian Basin shales to provide information to design field tests for a Huff-and-Puff (HnP) recovery process.
Fluid and core samples were collected from a new well completed in the nano-Darcy organic-rich shale zone in the Permian Basin. PVT tests, including swelling tests and minimum miscibility pressure (MMP) measurements, were conducted for three different gases (CO2, methane, and field produced gas). All three gases show miscibility at initial reservoir pressure conditions, but CO2 was the most efficient solvent, with first contact miscibility at the lowest tested pressure. An equation-of-state (EOS) model was generated based on the PVT tests. Special laboratory experimental setup and procedures were tailored to investigate the efficacy of a novel EOR process using shale core plugs. The core experiments, conducted with CO2 at reservoir conditions, showed favorable results, including good oil recovery and CO2 utilization in up to seven consecutive Huff-and-Puff cycles. Hydrocarbon composition change was also observed during the different cycles. Furthermore, the significant oil saturation reduction after the HnP process, as detected by the non-invasive NMR (nuclear magnetic resonance) technology, confirmed the extraction efficiency of this process.
Introduction
With the application of hydraulic fracturing and horizontal drilling technologies on organic-rich shales and other unconventional tight resources, US crude oil production has been on an upward trend since the mid-2000s [1]. The average recovery factor in the unconventional resources is typically less than 10% with very steep decline rates [2][3], indicating enormous potential for EOR in these unconventional resources. In recent years, there have been research efforts and a few field pilots of unconventional EOR reported in the Bakken and Eagle Ford shales [2]-[8]. Most research and pilot projects focused on miscible gas (either CO2 or produced gas) injection, while others also investigated water-based chemical injection [3]. This paper provides EOR fluid and core analyses in the Permian Basin organic-rich shale, which is another unconventional hydrocarbon growth play in the US with different geological/rock quality and fluid properties than the Bakken and Eagle Ford. The experimental results from this paper were used to calibrate Occidental's unconventional EOR reservoir simulation and field pilot design [9].
