Jianchun Guo

New flow model for the triple media carbonate reservoir

A new flow model in triple media carbonate reservoir is established. There exists a triple total system including the matrix, fracture and vug subsystem, and the three subsystems are relatively independent in physical properties; in the process of oil flow, the inter-porosity flow of the vug subsystem to fracture subsystem would occur and the inter-porosity flow of the matrix subsystem to fracture subsystem would also occur and ignore the inter-porosity flow between the matrix subsystem and vug subsystem. Compared with the traditional model (the inter-porosity flow of vug to fracture is pseudo-steady), the new model considers the unsteady inter-porosity flow, based on the hypothesis that the shape of vug is spherical. The new model is illustrated and solved, and the standard type curves are drawn up, so the process and characteristics of flow are analysed thoroughly, and it is found that the new-style type curves in shape and characteristics are evidently different from the type curves of traditional model. The research would not only deepen the understanding of flow law but also enrich the theoretical models for carbonate reservoir. The research results on this new model could be applied to a real case study.

New modelling of transient well test and rate decline analysis for a horizontal well in a multiple-zone reservoir

The no-type curve with negative skin of a horizontal well has been found in the current research. Negative skin is very significant to transient well test and rate decline analysis. This paper first presents the negative skin problem where the type curves with negative skin of a horizontal well are oscillatory. In order to solve the problem, we propose a new model of transient well test and rate decline analysis for a horizontal well in a multiple-zone composite reservoir. A new dimensionless definition of rD is introduced in the dimensionless mathematical modelling under different boundaries. The model is solved using the Laplace transform and separation of variables techniques. In Laplace space, the solutions for both constant rate production and constant wellbore pressure production are expressed in a unified formula. We provide graphs and thorough analysis of the new standard type curves for both well test and rate decline analysis; the characteristics of type curves are the reflections of horizontal well production in a multiple-zone reservoir. An important contribution of our paper is that our model removed the oscillation in type curves and thus solved the negative skin problem. We also show that the characteristics of type curves depend heavily on the properties of different zones, skin factor, well length, formation thickness, etc. Our research can be applied to a real case study.

Engineering geological characteristics and the hydraulic fracture propagation mechanism of the sand-shale interbedded formation in the Xu5 reservoir

In the Xu5 formation the sandstone reservoir and the shale reservoir are interbedded with each other. The average thickness of each formation is about 8u2009m, which increases the difficulty of the hydraulic fracturing treatment. The shale thickness ratio (the ratio of shale thickness to formation thickness) is 55–62.5%. The reservoir is characterized by ultra-low porosity and permeability. The brittleness index of sandstone is 0.5–0.8, and the brittleness index of shale is 0.3–0.8. Natural fractures are poorly developed and are mainly horizontal and at a low angle. The formation strength is medium and the reservoir is of the hybrid strike-slip fault and reverse fault stress regime. The difference between the minimum principal stress and the vertical stress is small, and the maximum horizontal principal stress is 20u2009MPa higher than the minimum horizontal principal stress and vertical stress. A mechanical model of a hydraulic fracture encountering natural fractures is built according to geological characteristics. Fracture mechanics theory is then used to establish a hydraulic fracturing model coupling the seepage–stress–damage model to simulate the initiation and propagation of a fracture. The hydraulic fracture geometry is mainly I-shaped and T-shaped, horizontal propagation dominates the extension, and vertical propagation is limited. There is a two to three meter stress diversion area around a single hydraulic fracture. The stress diversion between a hydraulic fracture and a natural fracture is advantageous in forming a complex fracture. The research results can provide theoretical guidance for tight reservoir fracturing design.

Unsteady-state diffusion modeling of gas in coal matrix for horizontal well production

In this work, an unsteady-state diffusion model was developed to describe gas transport in coal structures for horizontal well production. The model assumes unsteady-state diffusion of gas through the matrix according to Fick’s law, Darcy flow through the cleat network (natural fractures), and that gas adsorption can be described by the Langmuir equation. Then the model using Laplace integral transformation was solved. Using the new model, dimensionless-type curves for pressure-transient and rate-decline analyses were used to analyze transient transport characteristics. The differences for unsteady-state diffusion between horizontal and vertical well models and the differences for horizontal well production between unsteady-state diffusion and pseudosteady-state diffusion were specifically analyzed. Several scenarios confronting real coal beds were studied and discussed fully through simulating well production under different conditions. The research results showed that the unsteady-state diffusion model would be another choice with which to analyze well tests.

Experimental Analysis of Proppant Embedment Mechanism

Proppant embedment in hydraulic fracturing can reduce remarkably the fracture width and thus damage the fracture conductivity. This paper describes a study of the proppant embedment mechanism. Two groups of experiments were carried out. A proppant embedment mechanism was proposed. An analysis showed that proppant embedment is not an elastic process but could be a plastic or even failure process depending on the surface microstructure of the formation core.

A New Device and New Methods of Studying the Stress Sensitivity of Capillary Pressure

A new experimental device for studying the stress sensitivity of capillary pressure is presented. The components of the device are introduced in detail and a protective device for the semipermeable plates is explored. The device offers several advantages. Precise and high drainage pressure are achieved with the use of a pressurized measurement system and high drainage pressure is achieved, while a thermostat maintains a constant temperature throughout the entire process, eliminating the effect of temperature on the stress sensitivity of capillary pressure. The effect of temperature can be studied by varying the parameters of the thermostat. Experiments with a single rock type under varying confining pressures, but with constant drainage pressure are conducted. Based on the analysis of the experimental results, it is established that capillary pressure is affected by the overburden pressure.

Multilayer stress field interference in sandstone and mudstone thin interbed reservoir

General fracturing and separate layer fracturing play an important role in sandstone and mudstone thin interbed (SMTI) reservoirs, where one of the main issues is to control the excessive height growth of fracturing. The fracture propagation at the interface depends on the induced stress produced by the hydraulic fracturing construction. This paper employed a poroelastic coupled damage element with the cohesive zone method (CZM) to establish a 2D fracture quasi-static propagation model. A parametric study was performed under different fracture height, fracture width, pumping rate, fluid viscosity, in situ stress, elastic modulus and tensile strength with this model. General fracturing and separate layer fracturing are compared with each other through fracture morphology and induced stress. The simulation results show that the absolute value of induced stress increases with the decrease in matrix stress near the fracture tip. As a result, the propagation of the fractures is much easier due to the weakened degree of compression. The growth of fracture height and width, the increase in pumping rate and the excessively large or small value of fluid viscosity lead to larger induced stress on the interface. Higher in situ stress, lower elastic modulus, and higher tensile strength of the interlayers can control the excessive height growth of fracturing. The simulated results also show that the fractures are more likely to be overlapped with each other in general fracturing compared to that in separate-layer fracturing. Results of the simulations suggest that lower pumping rates, the proper value of fluid viscosity, separate layer fracturing and interlayers with higher in situ stress, lower elastic modulus and higher tensile strength tend to limit fracture height. Finally, the proposed model was applied to a practical oil field case to verify its effectiveness.