Gao, Rui (RIPED, CNPC& Key Laboratory of Reservoir Stimulation, CNPC) | Wang, Xin (RIPED, CNPC& Key Laboratory of Reservoir Stimulation, CNPC) | Yang, Zhen (Planning Department, CNPC) | Zhan, Qiang En (Planning Department, CNPC) | Zheng, Wei (RIPED, CNPC& Key Laboratory of Reservoir Stimulation, CNPC) | Liu, Ying (Planning Department, CNPC) | Yang, Li Feng (RIPED, CNPC& Key Laboratory of Reservoir Stimulation, CNPC) | Liu, Zhe (RIPED, CNPC& Key Laboratory of Reservoir Stimulation, CNPC) | Wang, Zhen (RIPED, CNPC& Key Laboratory of Reservoir Stimulation, CNPC)
Tight oil and gas reservoirs are characterized by strong heterogeneity, poor physical properties, low single well production, difficult development and others. The volumetric stimulation fracturing technology has become a key technology for the effective utilization of tight reservoirs. In the current fracturing optimization design, there are some limitations in simulating the true pattern of fracture propagations because the geological model is relatively simple and it is not necessary to consider the heterogeneity of reservoir plane. At the same time, the effect of large-scale and large-volume injection of fracturing fluid on formation permeability field cannot be neglected in the volume stimulation, and the coupling relationship between fracturing fluid loss and reservoir seepage is not considered in conventional productivity simulations so that the effective stimulated reservoir volume (SRV) cannot be calculated accurately. In this paper, a numerical simulation technology of fracturing based on rock deformation is introduced through theoretical analysis and field application. The effective SRV is analyzed quantitatively, and the optimization simulation method of volume stimulation parameters with the effective SRV as the evaluation objective is formed preliminarily, which guides the fracturing design of volume stimulation in tight oil blocks.
Hydraulic fracturing has been widely used for unconventional reservoirs including organic-rich carbonate formations for oil and gas production. During hydraulic fracturing, massive amount of fracturing fluids are pumped to crack-open the formation and only a small percentage of the fluid is recovered during the flowback process. The negative effects of the remaining fluid on the formation such as clays swelling and reduction of rock mechanical properties have been reported in literatures. However, effects of fluids on source rock properties, especially the microstructures, porosity and permeability, are scarcely documented. In this study, microstructure and mineralogy changes induced in tight carbonate rocks by imbibed fluids and corresponding changes in permeability and porosity are reported.
Two sets of tight organic-rich carbonate source rock samples were examined. One sample set was sourced from the Middle East field and the other was an outcrop from Eagle Ford Shale that is considered to be analogous to the one from the Middle East field. Three fracturing fluids, namely 2% KCl, 0.5 gpt slickwater and synthetic seawater, were used to treat the thin-section of the source rock and core samples. Modern analytical techniques such as SEM and EDS were used to investigate the source-rock morphology and mineralogy changes prior and after the fluid treatment at micron-scale level. Porosity and permeability as a function of confining pressures were quantified on core samples to investigate changes in flow properties due to the fracturing fluids treatments.
The SEM and EDS results prior to and after fracturing fluid treatments on the source rock samples showed the microstructural changes in all three fluids. In 2% KCl and slickwater fluid, reopening of some mineral-filled natural fractures was observed. The enlargement of micro-fractures was slightly more noticeable for samples treated with 2% KCl in comparison to slickwater at the micron-scale level. In one sample, dissolution of organic matters was captured in slickwater fluid treated rock sample. Some mineral precipitation and new micro-fractures generation were observed for samples treated with seawater. The new micro-fractures generation and mineral dissolution through the fluid treatment would result in the increases in both porosity and permeability, while the mineral deposition would result in permeability decrease. The overall increase in absolute gas permeability was quantified by the experimental measurements under different effective stresses for the core plug samples. This effect on absolute gas permeability increase has an important implication for hydrocarbon recovery from unconventional reservoirs.
This study provides experimental evidences at different scales that aqueous-based fracturing fluid may potentially have positive effect on gas production from organic-rich carbonate source rock by increasing absolute gas permeability through mineral dissolution and generation of new or re-opening of old microfractures. This observation will be beneficial to the future usage of fresh and seawater based fluids in stimulating gas production for organic-rich carbonate formations.