Fluid Flow Simulations of a Large-Scale Borehole Leakage Experiment

Borehole leakage is a common and complex issue. Understanding the fluid flow characteristics of a cemented area inside a borehole is crucial to monitor and quantify the wellbore integrity as well as to find solutions to minimise existing leakages. In order to improve our understanding of the flow behaviour of cemented boreholes, we investigated experimental data of a large-scale borehole leakage tests by means of numerical modelling using three different conceptual models. The experiment was performed with an autoclave system consisting of two vessels bridged by a cement-filled casing. After a partial bleed-off at the well-head, a sustained casing pressure was observed due to fluid flow through the cement–steel composite. The aim of our simulations is to investigate and quantify the permeability of the cement–steel composite. From our model results, we conclude that the flow occurred along a preferential flow path at the cement–steel interface. Thus, the inner part of the cement core was impermeable during the duration of the experiment. The preferential flow path can be described as a highly permeable and highly porous area with an aperture of about $$5\,\upmu \mathrm{m}$$ 5 μ m and a permeability of $$3 \cdot 10^{-12}\,\mathrm{m}^{2}$$ 3 · 10 - 12 m 2 (3 Darcy). It follows that the fluid flow characteristics of a cemented area inside a borehole cannot be described using one permeability value for the entire cement–steel composite. Furthermore, it can be concluded that the quality of the cement and the filling process regarding the cement–steel interface is crucial to minimize possible well leakages.

Impact of Heterogeneity on the Transient Gas Flow Process in Tight Rock

There exits a great challenge to evaluate the flow properties of tight porous media even at the core scale. A pulse-decay experiment is routinely used to measure the petrophysical properties of tight cores including permeability and porosity. In this study, 5 sets of pulse-decay experiments are performed on a tight heterogeneous core by flowing nitrogen in the forward and backward directions under different pressures under pore pressures approximately from 100 psi to 300 psi. Permeability values from history matching are from about 300 nD to 600 nD which shows a good linear relationship with the inverse of pore pressure. A preferential flow path is found even when the microcrack is absent. The preferential path causes different porosity values using differential initial upstream and downstream pressure. In addition, the porosity values calculated based on the forward and backward flow directions are also different, and the values are about 1.0% and 2.3%, respectively, which is the primary novelty of this study. The core heterogeneity effect significantly affects the very early stage of pressure responses in both the upstream and downstream but the permeability values are very close in the late-stage experiment. We proposed that that there are two reasons for the preferential flow path: the Joule–Thomson effect for non-ideal gas and the core heterogeneity effect. Based on the finding of this study, we suggest that very early pressure response in a pulse-decay experiment should be closely examined to identify the preferential flow path, and failure to identify the preferential flow path leads to significant porosity and permeability underestimation.

Recharge of River Water to Karst Aquifer Determined by Hydrogeochemistry and Stable Isotopes

The Jinan Karst Spring System in Shandong province, China, has suffered to maintain groundwater level and spring flowing for decades. Recharge of river water to karst aquifer in Jinan is important for the outflowing of four large karst springs in the city center. Field investigations were conducted for two times in May and October, 2015, respectively and water samples were collected for hydrogeochemical and isotopic measurements. Results showed that (a) the water type was predominantly Ca-HCO3-SO4 for karst groundwater, and Ca-Mg-SO4 for river water; (b) the concentration of HCO3− and NO3− in karst groundwater were higher than that in river water, in contrast, the concentration of SO42− and K+ in karst groundwater were lower than that in river water; (c) the δ2H and δ18O values with average of −51.2‰ and −6.6‰ for river water is more enriched than the values in groundwater samples (−59.1‰ and −8.3‰), in that river experienced evaporation in the upstream reservoir; (d) Based on the distribution pattern of δ18O, groundwater near river bank was found to be recharged from river water and found a preferential flow path in karst aquifer situated from Dongkema to Manzizhuang near the river bank. This study provides useful information for understanding of the hydraulic connection between river water and karst aquifer, and benefit the protection and management of water resources.

Changes of preferential flow path on different altitudinal zones in the Three Gorges Reservoir Area, China

Liu, M., Du, W. and Zhang, H. 2014. Changes of preferential flow path on different altitudinal zones in the Three Gorges Reservoir Area, China. Can. J. Soil Sci. 94: 177–188. Preferential flow in soil macropores plays an important role in runoff control and soil and water conservation. The aim of this study was to investigate the distribution of preferential flow paths in the soil profile of various altitudinal belts, analyze its variation among different soil horizons, and define the cause of soil macropores. A dye tracer method combined with photographic analysis was conducted for four hillslope sites in the Three Gorges Reservoir Area of China (TGRA). The results show that stained area proportion, as well as its vertical distribution in soil sections, presented varied patterns due to changes of forest vegetation and soil type with altitude. Stained area ratio of soil profiles increased, while stained depth decreased with increasing altitude. For soil sections in the subalpine belt, mid-mountain belt, and low-mountain belt of TGRA, stained area ratios were 62, 42, and 45%, and stained depths were 52.4, 56.4, and 69.5 cm, respectively. For brown earth covered with subalpine temperate deciduous broad-leaved forest, stained area ratios were the largest, but dyed patches were concentrated in the humus horizon. For yellow earth covered with low-mountain warm coniferous forest, stained depth reached 69.5 cm, and stained patches existed in the total soil profile. Compared with forest soil, stained depth and stained area ratio of abandoned farmland in low-mountain belt were lower, and the depth of dye infiltration was even shallower.

New Insights Into the Propagation of Emulsified Acids in Vuggy Dolomitic Rocks

Summary Carbonate formations are very complex in their pore structure and exhibit a wide variety of pore classes, such as interparticle porosity, moldic porosity, vuggy porosity, intercrystalline porosity, and microporosity. Understanding the role of pore class in the performance of emulsified-acid treatments and characterizing the physics of the flow in the acid propagation are the objectives of our study. The study was performed by use of vuggy-dolomite cores that represent mainly the vuggy-porosity-dominated structure, whereas the homogeneous cores used represent the intercrystalline pore structure. Coreflood experiments were conducted on 6 × 1.5-in. cores by use of emulsified acid formulated at 1 vol% emulsifier and 0.7 acid volume fraction. The objective of this set of experiments is to determine the acid pore volume (PV) to breakthrough for each carbonate pore class at different injection flow rates. In this paper, we examine whether the heterogeneities observable at the thin-section scale have a significant influence on the results of the emulsified-acid coreflood experiments. The heterogeneities were characterized by use of thin-section observations, tracer experiments, scanning electron microscopy (SEM), and resistivity measurements. Thin-section observations provide means to study the size of vugs and their distribution and connectivity, and to explain the contribution of the pore class in the acid propagation. Also, the rotating-disk experiments of emulsified acid with dolomite were related to our coreflood experimental results. The acid PV to breakthrough for vuggy-porosity-dominated rocks, ranging from 0.1 to 0.3, was observed to be low when compared with homogeneous carbonates (intercrystalline pore structure) with PVbt ranging from 2.5 to 3.5. Also, the wormhole dissolution pattern was found to be significantly different in vuggy rocks than that in homogeneous ones. Thin-section observations, tracer results, and the coreflood experiments indicate that the vugs are distributed in a manner that creates a preferential flow path, which can cause a rapid acid breakthrough and effective wormholing rather than those with a uniform pore structure. The rotating-disk results also showed that the reaction kinetics played a role in determining the wormhole pattern.