Assessing the Efficiency of Saturating Shale Oil Cores and Evaluating Hydrocarbon Gas Huff ‘n’ Puff Using Nuclear Magnetic Resonance

Abstract

Summary Shale oil formations have a very low primary yield despite advances in multistage hydraulic fracturing and horizontal drilling. In so doing, gas huff ‘n’ puff (HnP), among other improved oil recovery methods, is implemented to recover more liquid hydrocarbons. Gas HnP has proven to be an effective recovery process in shales taking into account the fracture properties, fluid-fluid interactions, and gas diffusion controlled by matrix properties. However, a laboratory-scale understanding of the gas HnP mechanism proves challenging. At this scale, measuring saturation before and after HnP tests in a nonintrusive and nondestructive manner, and understanding rock properties that affect diffusion is essential. In addition to ascertaining how the multiscale pore systems and varying mineral composition of shales affect its evaluation. The nuclear magnetic resonance (NMR) is considered a suitable tool for estimating fluid content in shales and understanding rock/fluid interactions. Generally, synthetic oil samples are used on either outcrop core plugs or crushed reservoir samples for NMR measurements, which may not be representative of rock/fluid interactions in bulk shales. This study is focused on carrying out NMR tests with dead oil on reservoir core plugs at relatively different depths to determine an effective means of saturation and understand oil production due to gas HnP. Gas HnP experiments were performed at reservoir conditions (3,500 psi and 125°C) on representative rock types from the Lower Eagle Ford (LEF) interval. Low field NMR measurements were subsequently carried out on the LEF core plugs at different states: as-received, saturated, and after gas HnP. The results show that oil recovery due to gas HnP occurred mainly in the organic pores (OPs) and inorganic pores (IPs) and ranged from 48 to 56% of the oil-in-place with indications of adsorbed/trapped methane (CH4) and remaining heavier components. This plays a vital role in evaluating the HnP process to know the extent of invasion and remaining oil components. In saturating the core plugs, the optimum saturation period was found to be 2 weeks for the LEF shale at current conditions. This presents an idea of how long to saturate a shale oil core effectively before it is tested for gas HnP. On the basis of the impact of varying mineral composition on the recovery mechanism, we observed the LEF core plug with the highest clay content to have the least recovery. This is in line with a high T1/T2 ratio alluding to reduced mobility of fluids in the presence of clay minerals with relatively small sizes of clay porosity and adsorptive surfaces.