(274e) Green Solvent for Enhanced Recovery in Unconventional Resources

In the past decade, the oil and gas industry has made tremendous improvements in horizontal drilling and well stimulation techniques, yet recovery rates for unconventional oil and gas reservoirs continue to remain quite low. Typically, less than 10% of the original oil in place (OOIP) is recovered. A key challenge for oil recovery in unconventional formations is accessing the oil that resides in the low-permeability, low-porosity rocks. While induced fracture networks certainly help to connect pore networks and create pathways to enable oil production, significant volumes of oil remain trapped in the unfractured reservoir matrix.

In certain reservoirs, such as the Bakken and Eagle Ford oil plays, pore spaces are clogged by immobile organic phases that impede the flow of oil and gas. The immobile phases are typically separated into two classes based on solubility: kerogen (the insoluble component) and bitumen (a soluble component in standard organic solvents). Kerogen exists as a solid and hosts the nanoscale pore network partially responsible for hydrocarbon storage and transport. Bitumen exists as a semi-solid or viscous fluid similar to shoe polish with a high viscosity that renders it essentially immobile. This viscous (but soluble) organic phase clogs pores, swells kerogen and ultimately, impedes production from certain formations where bitumen is abundant.

Bitumen is relatively abundant in lower thermal maturity shales (i.e. shales that have not been subjected to much heat in the subsurface), and the loss of oil recovery due to bitumen can be dramatic. For example, the Upper and Lower Bakken intervals have relatively high volumes of OOIP; however, these zones have so much bitumen that none of the oil can be economically recovered. Instead, essentially all the production from the Bakken system comes from the Middle Bakken and Three Forks intervals, which have lower volumes of OOIP but also much lower bitumen content. The same is true in the Eagle Ford; many operators produce from zones of low bitumen content, but zones that are richer in bitumen are not produced.

One potential EOR method to produce these bitumen-rich zones is to apply a chemical treatment that removes the soluble bitumen from the formation. In laboratory analyses, it is common practice to extract bitumen from source rock by washing the sample with an organic solvent. In field applications however, the organic solvents used in the laboratory (such as methylene chloride or chloroform) are not practical because they are expensive and toxic. Ongoing research at Schlumberger has led to the development of a green solvent that is environmentally and economically acceptable to be deployed at large scale in the subsurface.

The composition of bitumen varies based on factors such as source and maturity. Thus, the optimum formulation for bitumen removal also varies from region to region. The proposed green solvent for bitumen dissolution is a blend of three separate solvents. The effectiveness of removing bitumen is assessed on a ternary phase diagram, with the pure solvents at the apex of this diagram. Each experimental trial is represented as a point on the plot, with the color/size of the point representing the effectiveness of the blend in removing bitumen. A color scale Is used to represent solvent efficacy relative to methylene chloride.

Initial laboratory tests demonstrate that this green solvent extracts bitumen as efficiently as the toxic solvents and generates similar enhancement in the porosity of the source rock. Furthermore, these alternative solvents are readily available today, are inexpensive, and are environmentally permissible for subsurface applications.