Sedimentary Cobalt Protoporphyrin as a Potential Precursor of Prosthetic Heme Group for Bacteria Inhabiting Fossil Organic Matter-Rich Shale Rock

This study hypothesizes that bacteria inhabiting shale rock affect the content of the sedimentary cobalt protoporphyrin present in it and can use it as a precursor for heme synthesis. To verify this hypothesis, we conducted qualitative and quantitative comparative analyses of cobalt protoporphyrin as well as heme, and heme iron in shale rock that were (i) inhabited by bacteria in the field, (ii) treated with bacteria in the laboratory, and with (iii) bacterial culture on synthetic cobalt protoporphyrin. Additionally, we examined the above-mentioned samples for the presence of enzymes involved in the heme biosynthesis and uptake as well as hemoproteins. We found depletion of cobalt protoporphyrin and a much higher heme concentration in the shale rock inhabited by bacteria in the field as well as the shale rock treated with bacteria in the laboratory. Similarly, we observed the accumulation of protoporphyrin in bacterial cells grown on synthetic cobalt protoporphyrin. We detected numerous hemoproteins in metaproteome of bacteria inhabited shale rock in the field and in proteomes of bacteria inhabited shale rock and synthetic cobalt protoporhyrin in the laboratory, but none of them had all the enzymes involved in the heme biosynthesis. However, proteins responsible for heme uptake, ferrochelatase and sirohydrochlorin cobaltochelatase/sirohydrochlorin cobalt-lyase were detected in all studied samples.

Postdiagenetic Bacterial Transformation of Nickel and Vanadyl Sedimentary Porphyrins of Organic-Rich Shale Rock (Fore-Sudetic Monocline, Poland)

Nickel and vanadyl porphyrins belong to the so-called fossil geo- or sedimentary porphyrins. They occur in different types of organic matter-rich sediments but mostly occur in crude oils and their source rocks, oil shales, coals, and oil sands. In this study, we aimed to understand the process of bacterial transformation of geoporphyrins occurring in the subsurface shale rock (Fore-Sudetic Monocline, SW Poland). We studied these transformations in rock samples directly obtained from the field; in rock samples treated with bacterial strain isolated from shale rock (strain LM27) in the laboratory; and using synthetic nickel and vanadyl porphyrins treated with LM27. Our results demonstrate the following: (i) cleavage and/or degradation of aliphatic and aromatic substituents of porphyrins; (ii) degradation of porphyrin (tetrapyrrole) ring; (iii) formation of organic compounds containing 1, 2, or 3 pyrrole rings; (iv) formation of nickel- or vanadium-containing organic compounds; and (v) mobilization of nickel and vanadium. Our results also showed that the described bacterial processes change the composition and content of geoporphyrins, composition of extractable organic matter, as well as nickel and vanadium content in shale rock.

Experimental Investigation on the Initiation of Hydraulic Fractures from a Simulated Wellbore in Laminated Shale

Understanding the formation mechanisms of complex fracture networks is vitally important for hydraulic fracturing operations in shale formation. For this purpose, a hydraulic fracturing experiment under a core-plunger scale is conducted to investigate the impact of the bedding plane angle, borehole size, and injection rate on fracture initiation behaviors of laminated shale rock. The results on rock properties demonstrate that the anisotropic characteristics of shale rock are reflected not only in elastic modulus but also in tensile strength. The results of fracturing experiments show that the bedding plane dip angle and borehole size have significant effects on fracture initiation behaviors, in that fracture initiation pressure (FIP) decreases with the increase of those two factors. The impact of injection rate, by contrast, has no obvious variety regulation. The above data is further used to validate our previously proposed fully anisotropic FIP model, which shows better agreement with experimental results than those using other models under various parameter combinations. Finally, a postfracturing analysis is performed to identify the fracture growth patterns and the microstructures on the fracture surfaces. The results show that the hydraulic fractures (HFs) always grow along mechanically favorable directions, and the potential interaction between HFs and bedding planes mainly manifests as fracture arrest. Meanwhile, the roughness of fracture surfaces is physically different from each other, which in turn results in the difficulties of fluid flow and proppant migration. The findings of this study can help for a better understanding of the fracture initiation behavior of laminated shale rock and the corresponding fracture morphology.

Sliding behavior of silica ball-shale rock contact under polyacrylamide aqueous solutions

The tribological properties of proppant particle sliding on shale rock determine the shale gas production. This work focuses on investigating the impacts of sliding speed on the coefficient of friction (COF) and wear of the silica ball-shale rock contact, which was lubricated by water or different types of polyacrylamide (PAM) aqueous or brine solution. The experimental results show that both boundary and mixed lubrication occur under specific speed and normal load. COF and wear depth of shale rock under water are higher than those under PAM solution due to superior lubrication of PAM. COF of shale rock under PAM brine solution increases and the wear of the rock is more serious, attributed to the corrosion of shale rock and adverse effect on lubrication of PAM by brine.

Experimental Research of the Influence of Microfracture Morphology on Permeability of Shale Rock

Hydraulic fracturing is currently one of the main technical methods of shale gas exploitation. The permeability variation of shale gas reservoir after fracturing is inevitable, while the influence of fracture length and fracture width on permeability and seepage characteristics of shale rock is a mystery. Besides, the stress sensitivity characteristics of shale rock, derived from different initial permeability, with the same permeability after fracturing are also ambiguous. To this end, a series of seepage characteristic experiments related to different fracture parameters are carried out with the black shale of the Longmaxi Formation in Sichuan gas field as the research target. The results show that the fracture length and fracture width have a good exponential relationship with the corresponding permeability of the reformed shale rock, and the contribution of the fracture width to shale permeability is much greater than that of the fracture length. In addition, the nonlinear seepage characteristics of shale rock are gradually significant with the reduction of fracture length and fracture width. Taking the primitive effective stress (10 MPa) as a critical point, the permeability of shale with large initial permeability decreased by 26.4%, which is about twice as much as that of shale rock with small initial permeability (14.9%) in the selected pressure loading stage, owing to the difference of fracture width inside the shale rock. The permeability of the shale rock with a large initial permeability is restored by 14.7%, while the shale rock with a small initial permeability is only recovered to 5.2% in the pressure unloading stage, which is attributed to the closure of fractures, especially the loss of fracture width. This research can provide some new insights for the production prediction of shale gas reservoir after fracturing.