Zhengwen Zeng

Experimental Study of Overburden and Stress Influence on Non-Darcy Gas Flow in Dakota Sandstone

A series of laboratory experiments were conducted to investigate the influence of overburden and in-situ stresses on non-Darcy gas flow behavior in Dakota sandstone. Nitrogen was flooded through cylindrical core in a triaxial core holder under specific condition of temperature at 100oF, with core outlet pore pressure at 500 psi, axial and radial stress from 2,000 to 10,000 psi, and. nitrogen reservoir pump pressure at 2,000 psi with pump flow rates from 25 to 10,000 cc/hr at 80oF. Permeability and non-Darcy coefficient were determined using Forchheimer’s method. It was found that with the increase of overburden and in-situ stresses, permeability decreases while non-Darcy flow coefficient increases. Average effective normal stress and shear stress were used to quantitatively express the influence of overburden and in-situ stresses. It was found that average effective normal stress has a good linear relationship with both permeability and nonDarcy flow coefficient. In contrast, average shear stress did not appear to influence the permeability and non-Darcy coefficient.

Experimental Observation of Injection Rate Influence on the Hydraulic Fracturing Behavior of a Tight Gas Sandstone

It has been widely accepted in hydraulic fracturing that the higher the pressurization rate, the higher the breakdown pressure. However, linear elastic fracture mechanics (LEFM) suggests the opposite. In an effort to clarify this “injection rate paradox,” three controlled laboratory hydraulic fracturing experiments were conducted using different injection rates. These tests showed that the higher the injection rate, the lower the breakdown pressure. Further investigations indicated that the elastic- and poroelastic- models were not able to properly predict the observed breakdown pressures. Using Griffith’s energy balance concept, an LEFM-based breakdown pressure model was derived which gives satisfactory predictions.

Adsorption Kinetics of Surfactant Used in CO2-Foam Flooding onto Berea Sandstone

This paper reports adsorption/desorption kinetics and equilibrium onto Berea sandstone for a surfactant used as a CO2–water foaming agent. Also, foam stability of the surfactant is compared before and after adsorption/desortion has occurred. Results show that this surfactant’s adsorption onto and desorption from Berea sandstone took several days to reach equilibrium. Both adsorption and desorption were characterized by a short period of rapid adsorption/desorption followed by a longer period of slower adsorption/desorption. Both adsorption and desorption processes were best fit by a pseudo-second-order kinetic model. Coefficients of adsorption and desorption rate were determined. A method to predict equilibrium adsorption density using adsorption process data over a relative short period is proposed, which saves considerable time in determining complete adsorption isotherms. Surfactant concentration and temperature affect equilibrium density. Foam stability is affected by selective surfactant adsorption onto the rock.

Comparison of Non-Darcy Flow of CO2 and N2 in a Carbonate Rock

This work presents the non-Darcy behavior results of carbon dioxide (CO2) compared to the previous work using nitrogen (N2) and is based on 85 series of high-velocity gas flooding experiments under high-pressure and high-temperature. It was found that pore pressure has more influence on permeability in CO2 flooding than that in N2 flooding. In contrast, temperature has definite and consistent influence on both permeability and non-Darcy flow coefficient in N2 flooding, but the same influence in CO2 flooding was not observed. Mechanism behind these differences is attributed to physical property differences of the two gases. Much of the work was near the CO2 critical point or liquid regions. Other anomalies are attributed to thermal effects caused by expansion cooling of the CO2. Field data indicates that this phenomenon could be responsible for productivity loses in high flow rate CO2 wells. Accordingly, attention should be paid to avoid flowing CO2 at conditions near its critical point.

Effects of Geometrical-Size of Cylindrical-Shell Transducer on Acoustic- Beam Steering Efficiency for a Slim-Hole Acoustic-Logging Tool

The slim-hole acoustic-logging tool is often used for measurement while drilling and horizontal well logging. The source and receiver are generally thin cylindrical-shell piezoelectric transducers. The radius of the drilling-collar limits the size of the cylindrical-shell transducer in the logging tool. The smaller the radiation area of the transducer, the smaller the radiated acoustic energy, and the smaller its radius, the higher the frequency of the radiated acoustic signals. Besides, the attenuation for higher frequency wave propagating in the medium is higher. Due to these reasons the amplitude of the measured acoustic signal by using the slim-hole logging tool is usually much smaller than that by using the conventional logging tool. Therefore, the acoustic-beam steering technology is important for enlarging the amplitude of the received acoustic signal during logging with a slim-hole logging tool. The geometrical-size of the cylindrical-shell transducer influences the acoustic-beam steering efficiency of the logging tool with a line-array source and a receiver. In this paper, in the frame work of the acoustic-logging transmission network model with the concept of directivity-weighted coefficient, we have carried out the calculation and analysis of the effects of geometrical-size of the transducer on the acoustic-beam steering efficiency of the slim-hole acoustic-logging tool with a line-array source and a receiver. The calculated results are useful for optimizing the design of the slim-hole acoustic-logging tool with a line-array source and a receiver.

Identifying Solid-Fluid Parameters of the Subsea Tunnel Based on Evolutionary ANN

How to determine the fluid-solid geophysical parameters of subsea tunnel is the basement of calculation and construction. Aiming at the problems of long time of forward numerical calculation and locally optimal solution, the paper proposed an identification method for fluid-solid parameters based on difference evolution (DE) arithmetic and artificial neural network (ANN). This paper adopts orthogonal experimental design and numerical simulation to produce learning samples, utilizes the nonlinear reflection of ANN and global optimization of DE, then establishes the evolution ANN model relating the solid-fluid parameters and character indices. Furthermore, we use the relative error between predictive and observed character indices as fitness of DE, searches the solid-fluid parameters in agreement with observation. This method can utilize the large engineering software. The calculation case study states that the method is feasible and can get satisfactory result.

Imaging the Initiation of Asymmetrical Hydraulic Fractures in Laboratory Experiments

Asymmetrical hydraulic fractures have been observed in the field to reduce the efficiency of such stimulation treatments. Efforts have been made to investigate asymmetrical hydraulic fractures by imaging their initiation using microseismic imaging technology developed in-house. Images of the fracture initiation were generated using the located sources of microseismic events. Analyses of these images indicate that the initiation of an asymmetrical hydraulic fracture is a complex process. While asymmetrical fractures were generated under asymmetrical stress fields along the length direction, selective propagation along the height direction was also observed, as well as non alignment and reorientation of the fractures near the borehole. Scarce distribution of microseismic events near the borehole may indicate poor connection between the far-field fracture and the borehole. Dense distribution of microseismic events near the borehole probably results from the coupled effect of fluid leakoff and stress-induced tensile fracturing.