Detection of intervals / layers in sections of the wells with anomalous areas of drilling mud filtrate contamination according to the well logging (with negative test results of horizons)

The zone of infiltration of the drilling fluid filtrate into the reservoir rock creates significant difficulties for the study by logging methods and during further testing of the formation. Due to the penetration of filtrate, significant contamination of the near-wellbore zone occurs. The porosity and filtration characteristics of reservoir rocks are changing. There is a possibility of blockage by filtrate in the invaded zone of oil or gas flow from the formation to the well. As a result of the studies carried out using well logging data, it was found that the presence and distribution of a mud cake on the borehole wall opposite the reservoir is an important factor influencing the process of filtration of the drilling fluid into the layers. On the examples of the Yablunivske oil and gas and Kolomatske gas fields of the Dnieper-Donets basin, it is shown that the absence of a mud cake on the borehole walls leads to the formation of maximum, anomalous zones of filtrate invasion. The determining of the diameter of the invaded zone was carried out according to the data of electrical logging methods. In addition, the diameter of the invaded zone was calculated as a solution direct task equation for the case of direct filtration without blocking by the mud cake. Comparison of the results of determining the diameter of the invaded zone by two methods made it possible to draw certain conclusions. An important conclusion is that even partial absence of mud cake on the reservoir wall in the well leads to horizontal and vertical filtration of the drilling fluid from the well into the formation. As a result, the invaded zone may be so deep, that the gas flow rate is absent even at high values of porosity, permeability and gas saturation

ANALYSIS OF THE INFLUENCE OF GEOLOGICAL FACTORS ON THE DEPTH OF THE FILTRATE INVADED ZONEAT THE PRIMARY DISCLOSURE OF GRANULAR RESERVOIRS ACCORDING TO THE WELL-LOGGING DATA

Possibilities of using the well-logging data for revealing the factors of the geological nature that influence the formation of invaded zone of a drilling mud filtrate at oil and gas wells drilling are considered. Electrical logging data were used with probes of different sizes and different types for adequate calculation of the relative diameter of the invaded zone. 5 wells from the gas condensate field were selected for analysis. The terrigenous section of the wells is represented by the alternation of argillites, siltstones and sandstones. Rocks reservoirs of granular type; the layers with thicknesses from 3,4 to 18,2 m were selected for analysis. The results of statistical analysis (cluster and factor analyzes) revealed 3 groups of rocks, the characteristic features of which are significantly differentfrom the invaded zone, layer thickness and porosity and gas saturation coefficients. It is established that for terrigenous sections with reservoir rocks of granular type (Serpukhovian) for one field on the example of 5 wells there is a maximum direct correlation between the value of the relative diameter of the invaded zone and the thickness of the layers. The conducted researches allow making prognostic estimations concerning the approximate distributions of diameters of an invaded zone in terrigenous cuts in case of accident-free drilling with observance of technological conditions.

Abundance and composition of denitrifiers in response toSpartina alterniflorainvasion in estuarine sediment

Nitrite reduction is regulated by nitrite reductase encoded by nirK and nirS genes. This study aimed to investigate the abundance and composition of nirK- and nirS-containing denitrifiers in response to Spartina alterniflora invasion at the Jiulong River estuary, China. The sediment samples (depth: 0–5.0 and 5.1–20 cm) were collected from 3 vegetation zones, 1 dominated by the exotic plant S. alterniflora, 1 dominated by the native plant Kandelia candel, and 1 dominated by the native plant Cyperus malaccensis, and from an unvegetated flat zone. nirK- and nirS-containing denitrifier population sizes were lower in the invaded and nonvegetated zones than in those dominated by native K. candel and C. malaccensis, which were impacted by depth – vegetation species interaction. The ratios of nirS to nirK abundance ranged from 42.10 to 677.27, with the lowest ratio found for the upper layer in the invaded zone. The nirK-containing denitrifier compositions showed a 35% similarity between invaded zone and others. Most of the sequences of nirK genes recovered from the S. alterniflora zone were specific and distinct from those of nirK genes recovered from other vegetation types; nirS genes in the invaded zone were highly divergent. These results reveal that S. alterniflora invasion has a significant effect on the abundance and composition of both nirK- and nirS-containing denitrifiers, and nirS-containing denitrifiers were less responsive to invasion than nirK-containing denitrifiers.

Simulation of Drilling Fluid Filtrate Invasion Near an Observation Well

Summary We used a commercial reservoir simulator to study, first, the dissipation of aqueous drilling fluid filtrate invasion around a cased observation well in an oil-saturated formation under the action of capillary pressure and, second, the interaction of a waterflood front with the cased well and remaining invaded zone. Hysteretic behavior of the capillary pressure and relative permeabilities is critically important to these processes and is taken into account by the use of the Carlson model, with the various bounding drainage and imbibition curves computed from a pore network model. Filtrate invasion into a hydrocarbon formation influences the readings of well-logging tools. Although this phenomenon has been known, and corrected for, for many years, uncertainty remains with regard to the long-time behavior of invasion around observation wells where no flow in or out of the formation occurs after completion, and with regard to the influence of formation wettability. We find that after sufficient time, the invaded zone dissipates completely in a water-wet formation, but some invasion always remains in the oil/mixed-wet case. Nonwetting-phase trapping, manifested through relative permeability hysteresis, is the cause. Because trapping affects the values and the endpoints of the relative permeability curves, a waterflood front passing across an observation well is more distorted in the oil/mixed-wet case. The simulation results allow us to understand how logging-tool measurements made in cased observation wells are influenced by drilling-fluid invasion and will therefore lead to improved interpretation. This study shows strong links between the wettability of the formation and the persistence of invaded zone saturation and between invaded zone saturation and the distortion of subsequent flood fronts.

Gas Recovery From Tight Sands: Impact of Capillarity

Summary Hydraulic-fracturing operations carried out by injecting large volumes of water cause invasion of the injected water into the formation and create a water block. The flow of gas toward the wellbore/fracture during production will result in the removal of the water block through viscous displacement, as well as evaporation, that occurs because of gas expansion over a long period of time. However, some observations from the field show that the productivity of hydraulically fractured tight gas wells improves after a period of shut-in, leading to a speculation as to whether capillary suction is responsible for the cleanup of water block that eventually leads to productivity improvement. In this work, we use laboratory-scale experiments and modeling to find that capillary-driven transport is an important mechanism that helps redistribute water within the tight gas rock sample. Without capillarity, the model underpredicts the effective gas relative permeability recovery in the laboratory sample. We also find, using simulations, that capillary transport has the effect of enhancing the overall evaporation rate of water from the rock core. The model for calculating saturation changes and the effective gas relative permeability is complete with regard to all the mechanisms, such as displacement and evaporation. This is unlike previous studies, which did not include one or the other. Field-scale-simulation study of gas flowback using the new integrated model shows that the effective gas relative permeability of the invaded zone is significantly affected by capillary suction. In the absence of capillary suction, displacement and evaporation proceed as usual, but the invaded-zone water saturation does not dissipate quickly enough. The fracture-face skin, which is a function of the effective gas relative permeability, decreases faster as the invaded zone water is redistributed because of capillary suction. The simulations show that the evaporation of water from the invaded zone is very slow because of the low gas-flow rates in the tight rock matrix. In comparison to evaporative removal of water from the invaded zone, capillary-suction removal is significantly higher and faster. A sensitivity study on fracture-face skin shows that capillary suction has a significant effect on the cleanup at low drawdowns and smaller invasion depths. At complete shut-in conditions, the invaded-zone saturation continues to dissipate because of capillary suction. This confirms the general observation and anecdotal evidence that tight-sandstone wells produce at greater gas-flow rates after a period of shut-in. The methods described in this study can be adapted to perhaps determine the duration of such shut-in periods. Additionally, the models can be used to rigorously predict gas-production rates from a fractured well, including capillary effects, without resorting to averaging concepts such as fracture-face skin.