Fracture’s Impact on the Recovery of Hydrocarbon from Low-Permeability Rock’s Pores: New Insights from 1H Nuclear Magnetic Resonance Experiment

Abstract

Summary Successful examples of hydraulic fracturing (HF) have led to a general consensus that fractures enhance hydrocarbon production as it connects isolated pockets filled with oil. However, the fracture’s impact on the hydrocarbon recovery from rock’s pores, which can account for a significant amount, is not well studied but experimentally investigated here. Uniquely, we fill our samples' fractures with a slime made of guar gum and heavy water (D2O). Such slime cannot penetrate into pores, and the heavy water does not generate nuclear magnetic resonance (NMR) signals detectable by our apparatus. Using such slime as a fracture filling material allows us to isolate and eliminate the NMR signals from fractures. Subsequent huff ‘n’ puff (HnP) experiments show that fracture results in a decline of the total HnP recoverable oil. We reasoned that fracture reduces the treatment gas’ sweeping efficiency. This issue can be partially mitigated by injecting N2 instead of CO2; N2 can enter rock’s pores more efficiently as a treatment agent. Nevertheless, N2 HnP still suffers a noticeable reduction in the total recovery for samples with smaller pores. Our experiments provide an important new insight into the development of unconventional hydrocarbon resources. Maximizing fracture intensity in the field HF operation may negatively impact the later HnP oil recovery. At last, this observation is only possible with our unique experiment design; a misleading and opposite finding will be reached if one directly compares the results from before and after fracturing the samples. We reinforce that pore structures are changed during the fracturing process in our experiment (and possibly other researchers’ work); such change needs to be properly accounted for to assess fracture’s impact on pore fluid movement fairly.